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Wednesday, December 25, 2013

Another excellent documentary on Cold War Submarines

This is an episode of Nova that interviews a lot of Navy and intelligence insiders, as well as the authors of the book "Blind Man's Bluff".  Nice write up.  Merry Christmas, too ;)

Speaking of Christmas, my two worst were in the Navy. 

The second-worst Christmas was being underway on station, when the ship that was supposed to relieve us arrived several days late.  It's one thing if you know and expect to be at sea during Christmas.  It's another thing entirely to be ordered to stay put at the last minute.  On the bright side, it wasn't much different than any other day at sea, and everyone accepted the bad deal with grace.

My worst-ever Christmas was at Nuclear Power School in Orlando, Florida.  School let out for a week or two, and we were encouraged to go home and enjoy the holidays.  I had no home other than the Navy, so I pretty much had to stay put.  All my classmates and friends went home to spend Christmas with their parents.  I hung out in a dark and empty base, stood lonely watches, and ate bland chow hall food for Christmas dinner.


Tuesday, December 24, 2013

Documentary on Submarines during the Cold War

I found this cool documentary on submarine operations during the Cold War. 

At time 22:40 they have a segment on the NR-1, the worlds smallest nuclear submarine, which I nearly re-enlisted for. 

The segment right after that is about Navy divers doing special operations.  Having been in the submarine service during that era, I can say that the documentary comes close to describing a couple of situations my ship found itself in.  I was NOT involved in any of the specific incidents mentioned in the show.

Cool video below on how subs and the cold war were deeply connected.

The last segment on Sweden is interesting.  It suggests that US submarines may have been creating fake "Soviet periscope sightings" to swing a neutral Sweden toward NATO.  A false-flag operation.

For a really entertaining book on this subject, I recommend this: Blind Man's Bluff:  The Untold Story of American Submarine Espionage.  Disclaimer: I cannot confirm or deny anything... ;)

Thursday, December 19, 2013

Overhauling a Steam Locomotive

Saw this cool video on Boing Boing and had to share it.

This describes the process of overhauling a British steam locomotive.  It's fascinating to watch the craftsmen going about their work.  Obviously steam trains require a tremendous investment in man-hours.  According to the video, this entire process had to be repeated every 130,000 miles.  Ouch!

I don't care though.  They are still incredibly cool machines :)

Wednesday, December 18, 2013

O Christmas Tree

The road I live on is pretty rural.  We live out in the woods, and the house sits pretty far back off the road.  Pine and Fir trees crowd the road, and during the winter months, it's a pretty dark drive.

This year for Christmas, I decided to give our road a little color, by lighting one of the smaller trees at the entrance to our driveway.  Now when I say "smaller", that doesn't mean that it is small.  It's still about 20ft tall. 

What I didn't realize was how difficult this project would turn out to be!  At first I drove my truck to the end of the driveway next to the tree.  I stood in the bed of the truck and attempted to hang lights on the tree, by dangling them from a broom.  I didn't have enough height, by a long shot.

So I drove the truck back to the house, got my step-ladder and headed back to the tree.  Because I hate leaning off ladders, I drove the truck right up against the tree.   It was still a pain in the butt to string the lights using the broom (they don't go where you want when you use a broom), and it was a little dangerous as well.

Not my rig, but this was the setup.  Safety first!

Eventually I got the lights strung down low enough to work from the ground, and so I finished up.  I had used six or seven 100ft strands.  Next I strung out three 100ft extension cords, so I could get power to the tree from the house.  When I plugged in the tree, I found out that several of the strands didn't work :(

I ended up going back out to the tree with the truck and the step ladder, and replacing a few strands... although at this point I was nearly as ready to give up as to finish the job.  I bagged all the connections so there would be no electrical faults. 

It ain't exactly how I would have liked it to turn out, but it's not too bad - especially since it was done when the weather wasn't very nice.  In any event, I have the best (the only) lighted tree on a 5 mile stretch of forest road.

Tuesday, December 17, 2013

Sledding Raven

I saw this video and had to put it up.  This raven is sledding (or snowboarding) using what appears to be the lid from a jar. 

I was never very fond of ravens, but I changed my mind after meeting one when I lived in the high desert.  This raven had been tamed by a friend, and was a pretty cool pet.  The raven had a collection of shiny stuff - coins, pull-tops from beer cans, aluminum foil, etc.  When you would give him a piece of dry dog food, he would go to his treasures and bring you back something that he valued in exchange. 

How cool is that?  Too bad people can't understand this basic exchange!

Thursday, November 28, 2013

Rickenbacker and Hammond/Leslie sounds

I love Rickenbacker guitars.  They were part of the 60's rock sound (along with Hammond Organs), and that jangle and chime sound is unique to the guitar.  It's an electric semi-hollow body with single coil pickups, and it really shreds in the treble range.

I have been pretty sure for a LONG time that the opening guitar riff to this song was performed on a Rick, just by the sound.  However I only found the video today and verified it :)

The Byrds - a classic Rick band. Roger McGuinn is playing the Rickenbacker at the right. This was the first successful band to fuse folk music with rock music.

Another 1960's band.  What's that John has in his hands?  A Rickenbacker!

And now for the sound of a Hammond B3 or C3 organ, coupled with a Leslie phase shifting speaker.  Think Deep Purple, Booker T. and Keith Emerson (Emerson, Lake, and Palmer)

First of all, the guts of the Leslie speaker:  Below is a video of someone playing an organ into a Leslie 122.  The top part of the cabinet contains a tweeter/midrange speaker and a rotating horn.  Only one side of the horn is hollow and allows sound from the tweeter to escape.  The other is a dummy and just there for rotational balance.  You get the vibrato sound as the opening moves toward you and away from you. 

The same is true of the bottom woofer, but in this case, a rotating cylinder is used, with a port cut in it.  Again, the vibrato sound is created as the opening rotates toward and away from the listener. 

The speaker has a small vacuum tube amplifier inside, and a two speed motor.  The sound is awesome!!

Next up, the Hammond B3/C3 Organ coupled to a Leslie, in the hands of a skilled musician.

These organs have a characteristic growl that is unmistakeable.  Here is Jon Lord doing "Smoke On The Water".  You can easily tell when the Hammond joins the chorus with the guitar :)  Awesome sound!  Solo starts at about 3:00.

Here is Three Dog Night performing a Hammond-y "Out in the Country"  The organ solo starts at about the 2:15 mark.

Here is another classic 60's/70s band that used a Hammond/Leslie rig.  The Greg Allman organ solo starts at 3:45, but the entire song is pretty cool, and has a lovely Hammond-filled sound, as well as that awesome dual-guitar sound that the Allman Brothers pioneered. 

Wednesday, November 27, 2013

What type of music don't I like?

On a more eccentric note, I have been having a blast watching forgotten old music videos.  There are a raft of music videos that everyone has seen.  Then there are those eclectic videos that are nearly forgotten, those are the ones I've been enjoying watching and listening to.

I love pretty much all music.  Classical, Big Band, Punk, Blues, etc etc.  This is just a hodge-podge of fun videos that I wanted to stick up here for the moment.

New Wave!  Devo - one of their less well-known cover songs.  Blows the Rolling Stones away!  "Can't Get me no..." Love the robotic movements!

Punk.  The Dead Kennedys.  Holiday in Cambodia.  The lyrics to this song crack me up :)

More Punk.  Bad Religion.

Roman Holliday - A little early-1980's swing.

Oingo Boingo.  Yep that's Danny Elfman, the acclaimed movie soundtrack producer, back when he was frontman for an offbeat little band.

Billy Preston.  I grew up listening to this awesome musician on Top 40 AM radio.   I miss funk music!!!

More funk.  The Commodores.   That's Lionel Ritchie on the keyboard :)

Blues.  IMHO, Foghat does the best-ever version of "Sweet Home Chicago".  The studio version is absolutely awesome, but this live version comes pretty close!  I am a huge fan of slide guitar...

Below is Elmore James doing "Dust my Broom".  You can hear the riffs that Foghat used in "Sweet Home Chicago", above. 

Another awesome slide guitar video.  Roy Rogers.

Spanish Guitar also works!  Paco de Lucia and Al DiMeola.

Booker T. and the MG's.  Time is tight.  Just offstage you can see the members of Creedence Clearwater Revival rocking out :)  A couple of these guys were in the Blues Brothers movie.

Dave Brubeck.  Early Modern Jazz

Joe Satriani. 

Lindsey Buckingham of Fleetwood Mac soloing Big Love. Awesome accoustic guitar work.

Miserlou, the classic version

Miserlou, the surf-rock version

Happy (almost) Thanksgiving

...and I'm working, of course.  Haven't had a major holiday off in a couple of years now.  It's not that easy being the newest guy at a facility.  Reminds me of when I was in the Navy, and all the married guys would get cut loose over the holidays.  All the single guys would get stuck babysitting the ship.

A couple of things have come up since my last post.  I read an amazing blog post by a lady who describes the hopelessness of being poor.  Day after day, year after year.  She is highly intelligent and self-aware, and happily, her post has gone viral.  She is hoping to write a book soon, and I would certainly want to read and buy it.

Here is her post.  Go read it now.  She is trying to do good things with her 15 minutes of internet fame.  Awesome lady.

Let's also be thankful for what we have.

Wednesday, November 20, 2013

The Viola Organista

The Viola Organista was an invention dreamed up by Leonardo da Vinci.  In 1993 and 2004 a couple of these were made.  Now we have some great video of a man who built one and performed a concert.  Below is the original drawing that Leonardo made over 500 years ago.

A Polish master instrument maker used the drawing as a basis for making the third ever Viola Organista.  It uses rotating drums, wrapped with horse hair, as sort of an endless violin bow.  The strings are pressed down onto the rotating drum when the key is pressed. 

The thing sounds like the entire string section of an orchestra!  It's really neat. 

Music starts at about 4:00, and English subtitles are available by clicking "CC" at the bottom right, if you are interested in the interview part.

Below, the first time this instrument was ever heard in public.

Sunday, November 17, 2013

Belated Veteran's Day Post

My daughter's school once again held a Veteran's Day assembly, where any veteran they knew was invited to attend, and honored.  Again, it was nice to have the service of country be acknowledged.

The transition from military to civilian can be pretty easy, or it can be difficult.  Coming out of the submarine service, I had a little bit of difficulty.  I missed the intensity of running casualty drills, of the camraderie, and also with the return from isolation from the public.

My own service was not what I wanted to post about for Veteran's Day though.  This guy is.

Friday, November 08, 2013

The Northern Garage

A family member recently moved nearby, and I promised that we would insulate and drywall the garage for them, if they would pay for the materials.  The garage was quite cold and drafty, as the eaves are vented into the attic space on three of the walls.

I bought and installed a new garage door opener as a gift.  I also purchased (but have not yet installed) a couple of fluorescent fixtures to replace the bare 100W bulbs.

Having now completed the work, I can state with authority that I would rather be unemployed and hungry than be a drywall installer.

Below are two pictures taken of the garage before we started the work.

Below are pictures I took of our handiwork afterwards.  Fortunately it was "just a garage", so the finished look wasn't too much of an issue.

I added an access hatch to the garage attic, although I wasn't sure that it was necessary.

 The garage door opener is a belt-drive type.  It is unbelievably quiet!

 Below is a video comparing chain and belt drives.  I think I will be installing these in my own garage pretty soon :)

Thursday, October 24, 2013

Motorcycle Racing

I am a huge fan of motorcycle racing.  There is a great history of racing, going clear back to the wooden-tracked velodromes in the early part of the last century.

I am fascinated by the technology that is created to get a little extra speed.  Below is a Honda six-cylinder Double Overhead Cam motorcycle with only 125cc of displacement!

The other fascination I have is just how fast these guys go in the modern era.  Today there are two major types of road racing that I enjoy watching. 
The first type of race, MotoGP (Previously known as Grand Prix), takes place on custom-made race tracks, with prototype motorcycles.  These are the fastest race bikes on the planet.  No part of the motorcycle is allowed to have anything in common with a street bike.  The tracks in MotoGP are as safe as they can be made, having plenty of run-off room, and no obstacles to collide with.  In addition, the tracks are usually pretty short, allowing the riders to learn them pretty quickly and master them.
Because MotoGP bikes are prototypes, they are allowed to use electronic traction control, wheelie control, and specialized tires, all of which contribute to insane lean angles and fast lap times.  To a certain extent, a rider can open the throttle, and the onboard computer will decide whether to allow the engine to make more power based on rear wheelspin and cornering angle.
It's impressive to see what these machines are capable of.  Motorcycle action starts at about the 0:30 mark in the video below.

The other type of road race... is the Road Race!  These guys are just crazy.  I believe this type of racing is only still done in Ireland.  Bear in mind that if these guys crash, it's likely that they will hit a tree, block wall, curb, or other unfriendly object.  Needless to say the injury and death rate for this type of race is quite a bit higher than for MotoGP.  Not just for the riders, but for the fans.  Crowds are allowed right up to the race course.  I am sure it's very intense to be a spectator.

Possibly the most famous true Road Racing course is the Isle of Man TT, which is a six lap run around a 26 mile-long circuit on an island.  The Isle of Man isn't quite as crazy as the above race, because for the Isle of Man, they use a staggered start, and takes each rider's lap time. 

However because the Isle of Man is such a long course, utter concentration is required every moment of every lap, or the rider can easily be killed.

I will do some other posts about the role of technology in the bikes over the decades, and how each invention has helped squeeze a little more speed (or improve braking and handling) to get where we are now.

Sunday, October 20, 2013

TRIGA - an amazingly safe nuclear research reactor

So far we have talked about natural reactors, fast neutron reactors, plutonium production reactors, and thermal power plant reactors.

There is one cool reactor that I would be remiss in discussing, and that is the TRIGA reactor.  I worked at a facility where two of these were located, and was licensed to operate them both.  One was the very first TRIGA reactor ever built, rated at 250 KW (thermal), and the other was a MK IV model, rated at 1.5 MW (thermal).  These reactors are swimming-pool reactors, and so they don't generate steam or electrical power using steam turbines.

In fact TRIGA stands for (T)raining, (R)esearch, (I)sotope production, (GA) General Atomic, the manufacturer.

In the late 1950's there was a desire to promote "Atoms for Peace".  This was Eisenhower's attempt to invoke the power of the atom for peaceful purposes.  The world was understandably horrified by the images of Hiroshima and Nagasaki, as well as the possibility that the budding cold war might turn into a hot war at some point.

The invention of TRIGA reactors went a long way toward fulfilling that vision.  Unlike other reactors, TRIGA reactors have a solid moderator that is cast into the fuel itself.  Therefore there is a homogenous blend of solid moderator and fuel.  The moderator is Zirconium Hydride, and as you would expect, the hydrogen atoms do the moderating. 

Because about 60% of the moderator is solid and homogenous with the fuel, this reactor has what is known as a "prompt negative temperature coefficient of reactivity".  In other words, the very instant a runaway nuclear reaction begins and starts causing fuel temperature to increase, the solid moderator temperature also instantly increases, which in turn reduces the available thermal neutrons.  This provides a VERY rapid damping of the runaway nuclear reaction. 

If you recall in an earlier post, it is important (in all reactors except TRIGA) to never allow the reactor to be critical on prompt neutrons alone, because each generation of neutrons only last 10^-14 seconds.  There is no way to control a reaction that proceeds so quickly, so the small fraction of delayed neutrons are what allow us to control reactors.  Chernobyl, SL-1, and the Borax Experiment were each prompt criticality events that ended badly.

TRIGA reactors though, can easily tolerate a prompt critical event.  Doing this is called "Pulsing" the reactor. Any reactor can be pulsed, but only a TRIGA can do it more than once ;)  In fact, the record reactivity insertion into any reactor was  TRIGA, at 5.22 times the value needed to be prompt critical.  Because the moderator heats up as rapidly as the fuel, it shuts the reactor down just as soon as heat is generated, in a few thousandths of a second, without operator intervention.

TRIGA reactors ended up being sold around the world.   Being low-power, they weren't practical for making weapons, and the solid UZrH moderator was incredibly difficult to extract from the fissionable fuel, so using the fuel for making weapons was not possible.  Even so, currently manufactured TRIGA fuel has been reduced from 20% U-235 down to 7% to prevent proliferation.

Here is a video of a TRIGA reactor being pulsed to 2.5 x prompt criticality.  Any other reactor would vaporize the fuel and create a steam explosion, blowing water upwards out of the tank!

Below is an image of a TRIGA reactor at the bottom of the pool, while not in operation.  As you can see, it's quite simple.  The fuel elements rest on a bottom grid plate, and are kept vertical by the upper grid plate.  The fuel can be grabbed by a long-handled pole with a ball-type coupler at the end, similar to how modern hydraulic couplings work.

The rods sticking down into the core are just aluminum shafts that connect the drive motors to the control rods (which are partially out of the core).

The inner ring around the core is a lazy susan.  Samples can be dropped into a number of holders in this dry ring.  When the reactor is in operation, the ring rotates to ensure each sample is exposed to equal amounts of neutron flux.  This is useful for performing neutron activation analysis on several samples at once.

The outer ring is a graphite reflector/moderator, which reduces the amount of fuel needed.  The cans outside the reflector are neutron detectors, for determining what power level the reactor is at.  The little lanyard at the bottom is attached to a neutron source (usually Americium/Beryllium).  This makes sure there are enough neutrons available to start the reactor up.  Also its a daily test to pull it and stick it next to each neutron detector and make sure they work OK before you start the reactor up.

The reactor with a hole in its head...

In 2002 the US came very close to having a massive loss of coolant accident that probably would have led to major core melting (melt-down), and a release of offsite contamination.  Everyone knows about the meltdown at Three Mile Island, but not many people know about this...

Recall in a Pressurized Water Reactor that the control rods are completely removed from the core after it is up to full power, as dilute Boric Acid (a strong neutron poison) is added.   This is done to achieve a flatter neutron flux profile across the core in order to even out fuel burnup.  By completely removing the control rods, it is possible to prevent hot and cold zones near control rods, which is caused by depressed neutron flux near the control rods. 

Below, a side-view of a reactor core.  The solid line indicates neutron population in this reactor at steady-state power, with a control rod partially inserted to control reactor power.  Because there are very few neutrons in the area adjacent to the control rod, for a given power level, other areas of the core have to produce more fissions.  The spots on the solid line marked "A" are those places where we might see excessive fissions, overheating, and possible fuel element failures.

The dashed line indicates neutron population with the control rod removed (which you can do if you add Boric Acid as a virtual liquid control rod).  As you can see, neutron population is more consistent throughout the reactor, and therefore there won't be any excessive localized fission and heating, as you see at the points marked "A"

With borated primary coolant, neutrons are depressed equally throughout the core, and so power generation is more evenly distributed, and the reactor can be run closer to its thermal limit, because there is no need to account for hot and cold zones due to tilting of the neutron flux.

However you would hope that there is a better way to keep a flat neutron flux profile throughout a reactor core than using very hot diluted boric acid in the primary coolant loop. 

In 2002, Davis-Besse nuclear power plant in Ohio discovered they had a very minor primary coolant leak. The coolant was leaking out along a penetration in the reactor vessel head, where a Control Rod Drive Mechanism (CRDM) was mounted.  The CRDM is what pulls the control rods out of the core and allows the reactor to start and shut down.  The coolant leak was so minor that it wasn't even noticed.  Remember that any water leaking from the primary coolant was quite hot and would flash to steam immediately, so no water puddling would occur.

As mentioned above, the primary coolant in these PWR reactors contains Boric Acid, which is a mild acid.  For this reason, the piping, pumps, valves, etc are all made of high chrome steel (stainless steel).

The reactor vessel, however, is not.  Due to its size, it is impractical to make the entire thing from stainless steel.  Instead, the reactor vessel and head are made from carbon steel, and the interior is clad with a sheet of 3/8" thick stainless steel.  In this way, the carbon steel, which is not resistant to acids, is protected from the Boric Acid in the primary coolant.

Davis-Besse had a tiny coolant leak, however.  One which they were unaware of.  At the time it was discovered, the acid had eaten away a hole the size of a football completely through the reactor vessel head in the area of the leaking CRDM penetration.  The only thing that was holding the 2500 psig primary coolant in place was 3/8" of stainless steel cladding, which was bulging outwards from the pressure. 

Below is a picture of a Babcock & Wilcox design PWR primary coolant loop, with the reactor vessel head highlighted.
 Below is a cut-away of a B&W reactor vessel head, with a zoom-in on the point of the leak.

Below are images of the hole in the reactor vessel head.

The photo below gives you an idea how thick engineers designed the reactor vessel head to keep 2500 psig of primary coolant in place.  It's astonishing (and wonderful) that thin piece of 3/8" of stainless steel was able to keep the coolant from blasting out.

This would not have been merely a primary coolant leak.  This would have been a major accident.  Here are some of the potential consequences of such a massive leak, had the cladding ruptured:
  1. A massive steam/water jet would have blasted out of this hole, certainly damaging this control rod, but possibly adjacent ones also.  Could the ability of the reactor to shut down been compromised?
  2. The steam/water blast probably would have ripped tons of insulation loose.  This would have fouled the intake of the emergency water re-injection system, which takes suction from the floor of the containment building.
  3. Major coolant leak accidents are typically modeled for weaker points in the primary coolant system - pumps, steam generators, drain lines, etc.  These are equipped with remote-operated valves to isolate these leaks from the reactor core.  This leak was was directly above the reactor core, and not isolable from it.  Continuous water injection directly into the core would be required for several months to prevent decay heat from melting it.
  4. It is very likely that with the reactor fill system compromised due to ingesting insulation, and an unisolable leak right above the fuel, that core damage would have occurred.

Saturday, October 19, 2013

Odd Quirks About Nuclear Reactors - Criticality Accidents

One quirky thing about nuclear reactors:  If you are not careful (especially with Plutonium), it is possible to create one unintentionally.  That is, you can inadvertantly put together enough fissile material to create a nuclear chain reaction, outside the intended safe confinement of a shielded reactor vessel.

Friday, October 18, 2013


Last autumn, I had an electric garage heater installed.

The reason I thought that we needed a garage heater is because we keep a second refrigerator in the garage.  I wasn't sure what would happen if the outside of that refrigerator dropped below freezing.  I suspect that the cold would have seeped in and frozen everything inside. 

For that reason we put in the garage heater.  It keeps the garage above 40 degrees, and now we are able to store canned goods out there in cabinets as well.  However...

One part of the garage wasn't insulated, and that was the garage doors.  I am not sure what the r-value of 1/16" sheet metal is, but it probably isn't very good :).  We went through last winter without insulation on the garage doors, and the heater ran quite a bit.  Electricity is cheap here, but it's still wasteful to let all that heat escape.

This season I decided to insulate the garage doors, and found some cool styrofoam panels to do the job.  The panels are made by a company called Matador.  The front side is covered with a vinyl cloth, and the back side is scored so that you can break it off at the desired height.  Below is a picture of the back side of a panel.

I got to work, and it was pretty easy.
Here is how it turned out.  It took about an hour and a half to insulate the doors of a 3 car garage.  In this case, each panel had to be cut down somewhat to fit.  I am pretty pleased with the fit and finish.  Also, as a bonus, the doors open and close *much* more quietly! 

The biggest pain in the butt was clean-up.  It was impossible to get all the little styrofoam boogers cleaned up. They have a lot of static cling, and were still hanging from the inside of the garage doors when I rolled them up.

I ended up sweeping up as much as possible, then I used a leaf blower to knock them off the doors.  Afterwards I used the leaf blower to blow them out of the garage, because by then they had gone everywhere :)

The garage holds heat into the night-time quite a bit longer than it did before, and so I expect that the heater will be running a lot less often. 


Taking a brief hiatus from nuclear stuff...

I had a couple of days off, and decided to make some hard cider.  I thought it would be fun to try, so I will be giving it a shot.

The ingredients are:
5 gallons apple juice from concentrate (I am too cheap to buy 5 gallons of actual apple cider)
4 small cartons of Aspen Cider Spice.  This stuff is awesome; its loaded with sugar, cinammon and orange, that should ferment and add some needed flavors.
4 additional cups of white sugar
1 tsp of baking yeast, dissolved in tap water to activate it.

This *should* be ready around Thanksgiving.  Not having done an apple-juice based fermentation before, I am not certain how long it will take, nor how it will turn out.  I am looking forward to finding out though!

If it doesn't end up going down the drain, I think it will be called "Mark's Hard Cider" :)
UPDATE:  Interestingly, when you ferment apple juice, no krausen is produced.  Krausen is that foamy substance that floats up out of fermenting beer.  Below is a photo fermenting beer on day 2.  Note the foamy krausen floating on top.
Below are a couple of photos of fermenting cider on day 2.  No thick krausen, just bubbles. 


Monday, October 14, 2013

Odd Quirks About Nuclear Reactors...

There are a couple of things that all nuclear reactors do that make them behave quite differently than other, more mundane, heat sources.  One is just odd, and the other is a little scary.

Unusual Reactors - Fast Neutron Reactors

In earlier posts I alluded to Fast Neutron Reactors.  Here and Here

In a Fast Neutron Reactor, there is no moderator.  Neutron speed is allowed to remain at 49 million miles per hour.  Because the neutrons are traveling so fast, the likelihood of them interacting with a Uranium-235 nucleus and causing a fission is very much reduced.   To counter this, the enrichment of the fuel must be increased, typically to 20-30% U-235.

Sunday, October 13, 2013

Nuclear Reactors and Nuclear Weapons - There is a difference!

I know in a lot of people's minds, there is a notion that a nuclear power plant could detonate like a nuclear weapon.  It's not possible, and I will use the previous few posts to explain why that is.

Recall that neutrons from fission have a very high energy level, about 2 MeV, which equates to a speed of 45 million miles per hour.  Neutrons traveling this fast do not interact with Uranium atoms as often as they do after they have been moderated (slowed down) to thermal speed, about 4900 miles per hour.

Nuclear weapons require an absolutely uncontrolled fission reaction. The more fissions that occur before the core is vaporized, the better.  This requires that the entire thing take place in microseconds, which also means the neutrons must be fast neutrons.  It also requires a VERY dense concentration of fissile material, because fast neutrons don't interact well with atoms.

There are some other techniques used to improve nuclear weapon performance.  A neutron reflector is one.  It bounces fission neutrons back into the reaction, and makes more fissions possible before the whole thing vaporizes.  Another trick is using a tamper - a thick casing to hold the exploding device together for a couple more microseconds, allowing a couple more generations of neutrons to build up.

With a weapon, if the neutrons were thermalized, it would slow down the chain reaction to the point that the uranium or plutonium device would melt or vaporize, rather than detonate.  AKA, a "fizzle".

On the other hand, nearly all nuclear reactors are "thermal" reactors, meaning the neutrons are slowed down.  This provides fine control of the nuclear reaction, but more importantly allows us to use lower enrichment of Uranium than would be possible in a Fast Neutron Reactor.  Fast neutron reactors require enrichment that would be appropriate for weapons, which is why nobody really likes them. 

Typical Thermal Neutron Reactors, such as those in power plants, use just 3-5% enriched Uranium, which is too low for a nuclear explosion of the weapons type.

What can occur in a thermal reactor though is a runaway chain reaction that creates enough heat to vaporize metal in the core and create a steam explosion.  All nuclear reactors rely on "delayed neutrons" to achieve control.  Most fission neutrons are released within 10^-14 seconds following a fission.  These neutrons are called "prompt neutrons".  That reaction speed is far too fast for humans or electronics to prevent a runaway chain reaction. 

Fortunately for us, a small fraction of neutrons are produced by the neutron-rich fission fragments of the split atoms.  So... an atom fissions (splits) and releases 2-3 prompt neutrons.  Some of the split nuclei decay later and release more neutrons.  These are the "delayed neutrons", which can be generated up to 55 seconds after the initial fission event.

The key to controlling a nuclear reactor is to never allow the reactor to be critical on Prompt Neutrons alone.  If a reactor does become critical on Prompt Neutrons, the condition is called "Prompt Critical".  This is a dangerous uncontrolled reaction, which led to the violent explosions at both Chernobyl and SL-1.  It was also done intentionally for the SPERT experiment.

Each of these reactors experienced a prompt criticality event, just like a nuclear weapon.  However, because the neutrons were thermalized, which greatly slows down the increase of neutrons, none of these reactors detonated with a nuclear weapon type of blast.

Here is some interesting footage of the Prompt Critical SPERT reactor experiment:

Below is the account of the SL-1 accident, where the first operator fatalities took place.  Move it to the 1:00 minute mark to avoid the intro.  SL-1 might be the reason nobody allows the US Army to play with reactors any more :)

And lastly, Chernobyl.  The worlds biggest Dirty Bomb.

Saturday, October 12, 2013

Nuclear Weapons - Uranium Enrichment

As stated in the previous post, the Manhattan Project took a two-pronged approach to accumulating enough fissile material to build the first bomb: Uranium Enrichment and Plutonium Production.

We talked about Plutonium Production in that post.  This post is about Uranium Enrichment - increasing the isotope fraction of U-235 from naturally occuring Uranium, which is mostly U-238.

Nuclear Weapons - Plutonium Production

After the CP-1 Reactor successfully proved the concept of a self-sustaining fission reaction, the US accelerated its development of nuclear weapons.  The Manhattan Project took a two-pronged approach to generating enough material to build the first bomb: Uranium Enrichment and Plutonium Production.  This post is about Plutonium Production.

The post about Uranium Enrichment is linked here.

Friday, October 04, 2013

The CP-1 reactor

The first man-made nuclear reactor was built under the grandstand of Stagg Field, at the University of Chicago. 

The first reactor was called a "pile".  That may have been wartime jargon to hide its true nature from enemy spies, and it may have been descriptive.  In fact, it was a pile of uranium and graphite blocks.  It's designation was CP-1, or Chicago Pile #1.  CP-1 was part of the Manhattan Project, the US government's secret WW2 program to rush a nuclear weapon into production.  Its importance cannot be overstated.

The neutron had been discovered in 1932 by an Englishman named Chadwick.  In 1938, Lise Meitner, Otto Hahn, and Fritz Strassman collaborated to recognize and report on the fission of Uranium by neutrons.  By 1939 it was understood that excess neutrons from a split atom could maintain a reaction.  The next step was inevitable:  Make it happen.

Below:  The experimental setup that led to the discovery of nuclear fission.

Thus CP-1 was born - an industrial scale-up of what had previously been bench-top experiments in laboratories.  The techniques for increasing enrichment of a Uranium isotope with identical chemical properties had not yet been invented.  Therefore CP-1 used very pure refined metallic natural (unenriched) Uranium.

With CP-1 fueled by natural uranium, using light water for a moderator was not possible.  Light water would absorb too many excess neutrons, and prevent any reactor with such a low percentage of U-235 to maintain a chain reaction.  Light water is a good moderator, but it is also a mild poison.

For this reason, graphite was chosen for the moderator, as it absorbs neutrons 100x less often than light water does, and hence would be able to moderate fission neutrons without also creating losses through absorption.

To create CP-1, several other laboratory-level experiments also had to be ramped up to industrial levels.  At that point in time, the amount of metallic Uranium in the world was measured in grams.  Further complicating issues was the fact that nobody had ever bothered segregating trace amounts of Boron (A very powerful neutron poison) from the Carbon that the graphite moderator was made from.  After enough Uranium metal had been obtained, construction began.  Small cylinders of Uranium were interspersed between large blocks of graphite.

Enrico Fermi was in charge of the construction of the reactor.  Fermi was a brilliant physicist, and was also probably the only man in the world who could have assembled and controlled a nuclear reactor at that time. 

Below, a rendering of the CP-1 reactor.  The three cylindrical things dangling from a cable are neutron detectors.  The man standing is manipulating a control rod, which when removed, stopped absorbing neutrons and allowed a self-sustaining reaction to occur.

From Wiki:
However all these issues were overcome.  On 2 December 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, George Weil worked the final control rod while physicist Enrico Fermi carefully monitored the neutron activity. The pile "went critical" (reached a self-sustaining reaction) at 15:25. Fermi shut it down 28 minutes later.
Unlike most reactors that have been built since, CP-1 had no radiation shielding and no cooling system of any kind. Fermi had convinced Arthur Compton that his calculations were reliable enough to rule out a runaway chain reaction or an explosion. But, as the official historians of theAtomic Energy Commission noted, the "gamble" remained in conducting "a possibly catastrophic experiment in one of the most densely populated areas of the nation!"

With the proof at hand that a nuclear reaction could be made self-sustaining, the Manhattan Project would rapidly move forward toward a nuclear weapon. 

Thursday, September 26, 2013

Unusual Reactors - A reactor in nature!

So far, I've discussed BWRs and PWRs fueled with Low-Enrichment U-235 (typically 5% enrichment)

Lets talk about some other reactors.  How about a naturally occurring reactor that once occured in nature in a seam of Uranium?

I will simply copy and paste bits from the Wikipedia article linked above, since I cannot improve on it in any way.

From Wiki:

A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda.

The conditions found were very similar to what was predicted.

Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time 

My aside:  This enought to power about 1000 light bulbs of 100 Watts each.

In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the U-235 isotope. Normally the concentration is 0.720% while these samples had only 0.717%, a significant difference.

This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat à l'énergie atomique (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a U-235 concentration as low as 0.440%.

This loss in U-235 is exactly what happens in a nuclear reactor. A possible explanation therefore was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on September 25, 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.

Geological Situation in Gabon leading to natural nuclear fission reactors
1. Nuclear reactor zones
2. Sandstone
3. Ore layer
4. Granite
My aside:  Here is where the reactor physics gets interesting!
The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, the water returned and the reaction started again. These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported.

Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down to complete a 3-hour cycle.

A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope U-235 made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissile U-238.) Because U-235 has a shorter half life than U-238, and thus decays more rapidly, the current abundance of U-235 in natural uranium is about 0.7%. A natural nuclear reactor is therefore no longer possible on Earth without moderation using heavy water or graphite.

The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water and physical conditions needed to support the chain reaction was unique to the Oklo ore bodies.

Here are a couple of pictures I found on the web of the Oklo reactors.

10,000 pageviews

Just sayin'.  Probably most of them are mine :)

Wednesday, September 25, 2013

Moderators, Neutrons, and Enrichment, oh my!

I want to discuss a few other reactor designs besides the typical US power reactors, the PWR and BWR types.  Before I do that, it's important to understand a few things that force us down certain paths in designing reactor cores. 

We need to understand the relative abundances of U-235 and U-238, why we use neutrons to split atoms, and also how a moderator works, in a physical sense.

First off, about Uranium.  According to Wikipedia, naturally occuring Uranium consists of three major isotopes: U-239 (99.28%), U-235 (0.71%), and U-234 (0.0054%).  The stuff we need for fission (U-235) is less than 1% of naturally occuring Uranium.

There is an entire industry based around the enrichment of Uranium.  The process of enrichment is as fascinating as it is tedious.  I might do a post on that process at some point, but for now it will suffice to point out that it is very, very difficult to create fission with naturally occuring Uranium.  For this reason, in most reactors, the concentration of U-235 vs U-238 is increased, or enriched.

And we always accomplish large numbers of fissions by using neutrons.  Why neutrons?  A couple of reasons.  Since neutrons have no charge, they can wander at will through any part of an atom, and through many materials other than Uranium, as if that material weren't there.  Also, since fission produces more neutrons than it uses, they are ideal for initiating more fissions!

When a fission occurs, 2-3 neutrons are ejected, but they are moving at 2MeV, which is about 20% of light speed.  At this speed, they CAN interact with another Uranium nucleus and cause a fission, but it is unlikely.  In order to improve the likelihood of a fission, there are two things we can do:  Increase the number of U-235 atoms in the core, or moderate (slow down) the neutrons.

In practice, both methods are usually used.  Reactors using no moderator, and containing a core of highly enriched U-235 have been built and operated.  Since there is no moderator slowing the neutrons down, these type of reactors are called "Fast Neutron Reactors". 

The vast majority of reactors are "Thermal Neutron Reactors".  The reasons most reactors use thermal neutrons are two-fold.  Fast Reactors require fuel enriched above 20%, which can potentially be diverted and used for a nuclear weapon.  Additionally Fast Reactors have poor safety characteristics from a reactor physics standpoint, and thus very poor safety record.  Of the handful of Fast Reactors built, several  have suffered meltdowns.

More on moderators though.  The purpose is to slow neutrons down to the point where they are at thermal equillibrium with the surrounding material.  The way to accomplish this is by allowing them to impact (or "scatter") against atoms with low mass.  Think of the neutron as a ping-pong ball fired from a cannon.  To remove the most energy by collision, ideally it should run into another ping-pong ball, which will recoil and remove some speed.  If it collides with a heavy nucleus, like steel, it would be like our ping pong ball bounced off a boulder.  It would not lose as much energy to the boulder. 

Water is a decent, but not great, moderator.  It has two hydrogen atoms for neutrons to scatter against, and it is dense.  The oxygen molecule has the ability to scatter a neutron, but it due to its mass (16) it would take many more oxygen atom collisions to accomplish the task.  Water is also plentiful and cheap.  This is why in most applications, water is used as a moderator.

Water though, is not an ideal neutron moderator, and here is why:  Both the hydrogen and oxygen atoms have a small, but noticable possibility of absorbing a neutron, thus removing it from the process and making it unavailable to cause a fission. 

There are other moderators that are superior to light water from the standpoint of neutron absorption.  Even though a Fast neutron may require a larger number of collisions to reach thermal equillibrium, the lower likelihood of being absorbed make other materials better moderators.

Moderator         Number of collisions      Likelihood of neutron absorption
Hydrogen                   18                                             0.3326
Dueterium                  25                                             0.000519
Beryllium                   86                                             0.0076
Carbon                       114                                           0.0035
Oxygen                      150                                           0.00019
Uranium                    2152                                         7.57

Dueterium (in the form of heavy water) is probably the best moderator known.  This is water with hydrogen atoms that already have one neutron and one proton.  Normal hydrogen atoms have just one proton, and occasionally capture one, taking it out of use for the reactor.  Dueterium hydrogen atoms have already absorbed a neutron, and are unlikely to absorb a second one.

Dueterium is found in nature, but like U-235, is not abundant.  It also needs to be enriched at great difficulty and expense, to be available in sufficient quantities to be used as a moderator.

Beryllium is another excellent moderator, with very little likelihood of absorbing a neutron.  Its disadvantages are expense and high toxicity.

Carbon is an excellent moderator, evenwith the large number of collisions required, its low neutron absorption, and low cost make it a practical moderator, even if it is not a good material to remove heat.

The other two items on the list, Oxygen and Uranium are not moderators at all.  They just show how increasing the mass of the target atom increases the number of collisions required to moderate a neutron.  However on the right hand column, notice the likelihood of absorption for Naturally Occuring Uranium.  It does love to vacuum up neutrons :)

In the next post, using what we have learned here, I will describe a few unusual reactors that have been built, either for testing purposes, or for breeding additional nuclear fuel, for nuclear weapons production, and how a nuclear weapon itself works.

Boiling Water Reactors

Boiling Water Reactors (BWRs) are a fairly common design in electrical generation.  The advantage a BWR has over a Pressurized Water Reactor (PWR) is mainly the up-front cost.

The primary coolant loop in a PWR is kept at about 2200 psig (155 bar) to prevent boiling in the core.  This means the reactor vessel, pressurizer, and steam generators as well as all primary loop components, must be very robust.  In addition, they must be made from (or lined with) corrosion resistant materials, due to the addition of boric acid for reactivity control.

With a Boiling Water reactor, the primary coolant loop is also the steam loop.  There are no steam generators or pressurizer.  The operating pressure is about half that of a PWR, and no boric acid is used.  Thus a boiling water reactor can be built with less cost.  A BWR also has higher thermal efficiency than a PWR.

Below is a diagram of a BWR vessel.  While less expensive to build than a PWR, these are still pretty complex reactors. 

Below is a simplified overall diagram of an entire BWR steam/water circuit.

A few things you will notice that are different from a pressurized water reactor: 
  • The top of the reactor vessel is used as a steam drum, to separate water and steam.
  • Because the top of the reactor is a steam drum, control rods are moved to the bottom.
  • Inside the reactor vessel are circulation pumps (#4).  More on these later.
  • Steam directly from the reactor core flows through the entire steam system.
  • Not shown in the above image are the feedwater and steam shutoff valves.  In the event of a steam line rupture, it is important to keep water in the core, and also to prevent the release of radioactive steam outside of the containment structure.
  • Note the shielding required around the entire steam system. 
Some of the disadvantages of a BWR with respect to a pressurized and segregated reactor coolant loop become apparent. 
  • The control rods and circulating pumps penetrate the bottom of the reactor vessel.  Should a leak develop there, it can drain all the water from the reactor vessel.  Since the fuel must be kept submerged in water (even for several years after use), this is a potential hazard.
  • Water circulating in a power plant always contains minor levels of corrosion - flecks of iron from valves and pumps, and magnetite that falls from the inside of piping as temperatures expand and contract the pipe.  These corrosion particles can be carried through the reactor core and absorb a neutron.  They can become very radioactive, and then settle outside the reactor core.  This material is called CRUD, and is only detectable with a radiation detector.
  • In a PWR, CRUD is all contained within the primary loop and inside the containment structure.  In a BWR, CRUD is everywhere within the steam system - the steam and drain lines, the turbine, the condenser, the condensate pumps, and feedwater pumps.
  • There is an important nuclear reaction that occurs that involves the Oxygen in water molecules, H2O.  Oxygen-16, when hit with a high energy neutron, can absorb the neutron and eject\ a proton.  It then becomes radioactive Nitrogen-16.  This is a nasty isotope of nitrogen, mainly because it decays by emitting a gamma of 10 MeV.  That's a VERY hot gamma ray.  Nitrogen-16 has a half-life of 7.1 seconds, so that means it will be emitting these hot gamma rays while it is passing through the steam turbine, condenser, and settling in the hotwell.  Therefore massive shielding is required from all steam and feedwater systems during operation. 
  • Due to the short half-life of Nitrogen-16, it quickly decays away.  Shortly after the reactor is shutdown, the only serious radiological hazard in the steam system is due to CRUD, which is composed of metals that have longer half-lives.
 Below is the generator for a BWR.  Note the tan-colored radiation shield blocking the view of the steam turbine.  This is to protect personnel on the operating floor from Nitrogen-16 gamma radiation.
BWRs are a little more complex than PWRs from a reactor control standpoint.  A BWR, by its very description, has significant in-core boiling - primarily in the upper portion.  However a BWR, like the PWR, uses water as a moderator.  When water changes to steam, its neutron moderating properties change drastically - 1/1000 drop in density equates to about 1/1000 drop in moderating ability. 

With the phase change from water to steam there is an ongoing change in moderation and thermal neutron population, and therefore in the fission process.  It is therefore important to monitor neutron levels (flux) at many more points than would be necessary in a BWR, to ensure even power distribution.  Fuel burn-up (both axial and radial) is also difficult to calculate for the top end of a BWR, due to the dynamic two-phase activity in the core.

Interestingly, like the PWR, the BWR control rods are out of the core at full power, and reactivity is controlled by using the circulation pumps.  When the pumps are run at high speed, boiling occurs higher up in the core, and so more moderator is present, and more neutrons are available to create fissions.  Similarly, if reactor power needs to be reduced, the recirculation pumps can be slowed down, and boiling occurs lower in the core.  More steam in the core means less neutrons will be thermalized, and so fewer neutrons will be available to cause fission.

Lastly, BWR cores have to be somewhat larger than PWR cores for a given output, and that is mainly due to the fact that the upper part of a BWR core isn't generating much power, because steam is a poor moderator.

Sunday, September 22, 2013

Nuclear Reactors - How they work (part 2)

Previously we had a Pressurized Water Reactor (PWR) which was critical and adding heat to the coolant/moderator.  After the reactor reaches the point where it is generating heat, and the moderator becomes less effective, it becomes neccessary to withdraw the control rods again to increase power output.

Since this is a PWR, we are not supposed to have large-scale boiling in the core.  Steam doesn't transfer heat as readily as water does, and in a PWR, in-core boiling is a bad, bad thing.  For this reason the pressure in the primary coolant loop must be kept well above the boiling point of water for the temperature found in the core.  How do we do that?  We have a pressurizer!

The pressurizer is a dual-purpose vessel that does two things:  It acts as a surge volume for the primary coolant system, by having a compressible steam-filled void space.  Secondly, it keeps the system pressure high, so that boiling cannot occur in the core.  This is done by keeping a vessel half full of water and adding enough electrical heaters that the temperature in the pressurizer is maintained higher than that of the core.  All boiling will occur in the pressurizer, not in the core.

Below is a simplified diagram of the primary and secondary systems of a PWR.  The radioactive primary coolant (Red) is contained in a closed loop.  The reactor core is the heat source, the control rod drive mechanism positions the control rods to increase/decrease reactor power, the primary pump circulates the primary coolant, and pressurizer serves as a static surge volume.

The steam generator segregates the radioactive primary coolant from the steam/water system, but  allows heat to be transferred out of the primary coolant loop and generate steam for the turbine.  

Pressure in the primary system is increased by energizing submerged electrical heaters in the pressurizer, and reduced by spraying a small slip-stream of primary coolant from the reactor coolant pump through a spray head at the top of the pressurizer. This cools and condenses some of the steam in the pressurizer, reducing system pressure.

Temperature is gradually increased by withdrawing the control rods.  Remember in the previous post that the neutron population would increase with no control rod movement if the reactor were slightly supercritical?  That is no longer the case when power level is high enough for the reactor to generate heat.  After the core begins heating and the "negative temperature coefficient of reactivity" kicks in, withdrawing the control rods increases power, followed by a smaller drop in power as the primary coolant heats up.  So control rods are withdrawn in a series bumps, and the coolant temperature is slowly increased.

The primary coolant pump circulates primary coolant (very pure water) between the reactor core and the steam generator.  The reactor core adds heat created by fission.  The steam generator removes heat by boiling secondary water in a heat exchanger.  Thus, primary coolant is radioactive, but does not boil.  Secondary water is non-radioactive, and does boil.

In a PWR, excess fuel is loaded, more than what is required to bring the reactor up to power.  This is done so that the reactor can continue to run as the U-235 is expended, and fission products build up.  Many of the atoms that are created by fission are strong neutron absorbers (poisons), and eventually result in the inability of the reactor to maintain full power.  This is in spite of excess U-235 added at during refueling.  Only about 3% of the U-235 is used up before the accumulation of poison halts reactor operation.  This is why fuel recycling is so desirable from an industry standpoint.

Commercial PWRs are unique in that they add boric acid to the primary coolant.  Boron is a neutron absorber.  With the addition of boron to the coolant, the control rods may be entirely withdrawn from the core, using only boron to control neutron population.  This is desirable because in the vicinity of control rods, neutron population, and fissions will be depressed.  This forces other regions of the core to run hotter for a given power level.  With control rods out of the core, neutron distribution is more consistent and hot spots due to uneven neutron flux are less likely to develop.

At a certain point, on the secondary side of the steam generator, steam is at an adequate pressure to begin warming up the steam turbine.  This also has an interesting impact on reactor power.  As we create steam on the secondary side of the steam generator, we are also removing heat from the primary coolant.  This slightly cooler primary coolant returns to the reactor.  Since it is now more dense, it is a better moderator, and more neutrons stay in the core to cause fissions.  Reactor power now increases along with steam demand.  This is a very desirable feedback loop, because now there is little need to adjust the control rods.  We allow the steam turbine load to determine what power level the reactor will operate at.

The remainder of the steam plant operates in a very similar manner to a fossil-fueled plant.  One difference:  Since it is not possible to superheat the steam with this process, steam dryers are added between different stages on the steam turbine, to prevent mist from damaging the lower pressure sections.

Saturday, September 21, 2013

Nuclear Reactors - How they work (part 1)

I've always been fascinated by nuclear reactors.  Even as a child, I was mystified by the blue glowing water I would see in the National Geographic Magazine.  The whole notion of releasing enormous amounts of energy by gathering certain materials together in a small area is a bit mind-boggling.

So how does a reactor work?  First, a little chemistry, then a little physics. 

As an example of how nuclear energy is created, we can look at an atom of Helium-4, which contains 2 protons, 2 neutrons, and 2 electrons. 
Neutrons (blue) have no electrical charge.  Protons (red) have a +1 charge.  Electrons (yellow) orbit the atom at a pretty far distance and have a -1 charge.  Electrons and protons (and therefore charge) are balanced in typical atoms.

Because protons have a +1 charge, they repel each other. The repulsive electrostatic force is squared every time the distance is cut in half.  The closer these protons get to each other, the more they want to fly apart!  This is similar to pushing powerful magnets together with both north poles together.  However nature provides a way to hold these repelling protons together in spite of themselves :)

Back now to our physics...

The atomic mass of each component:

proton = 1.007766612
neutron = 1.00864160
electron = .000548892490

OK, funky numbers with lots of decimal places.  Bear with me here.  This is important.

Mass of all these individual components added together in a Helium Atom = 4.03391420898
Yet the actual measured mass of a Helium Atom is only = 4.002602  What the HECK!!!!???  
These numbers should be the same!

There is a difference in the atomic mass of 0.03131220898 between the individal components and the assembled atom.

This difference in the calculated mass and the actual mass is called the "mass defect".   Holding two positively-charged protons very close together requires a great deal of energy.  It also requires an incredibly powerful short-range force, called the nuclear force.  The nuclear force is so strong that it can overcome the repulsion of particles with like charges.  However, it requires so much energy to bind a nucleus together against the repulsive electrostatic force that there is a noticable change in the mass of the atom and its components.

Here is where we get to Einstein's awesome realization: Mass and energy are equivalent and interchangeble.

The difference in mass of an assembled Helium atom and its components is due to the large amount of energy required to hold that atom's nucleus together against the repulsive force of those positively charge protons that are trying to repel each other.  E=mc^2 is the equation that explains how much energy was needed to assemble the Helium atom from its components.  It's also how much energy would be released if they were taken apart again.  This energy is called Binding Energy.

Binding energy is the same exact thing as the mass defect, only from the viewpoint of energy, rather than mass.  E=mc^2 is the mathematical conversion between the two states (matter and energy).

To better understand how much energy, E we are talking about, we need to look on the right side of the equation.  The m (or mass) is a very tiny number, 0.03131220898.  However c is the speed of light, and that number is squared.  So the speed of light times the speed of light is a very, very, large number.  The energy that would be released if we break apart one helium nucleus into its components would be 28.3 Million Electron Volts.  To put this in perspective, an X-ray has energy at 40-60 thousand Electron volts, while visible light has energy at 2-6 electron volts. 

Unfortunately for our energy needs, Helium atoms are notoriously stable, and almost impossible to break apart.  However, nature has provided us with certain atoms that will split apart quite readily under the correct conditions. 

It turns out that certain high-mass atomic nuclei will split into two smaller fragments after absorbing (or capturing) a neutron.  More importantly, when these atoms split, they release many more neutrons, allowing a continuous reaction (or chain reaction) to be sustained.  Neutrons are the best subatomic particle to create large numbers of fissions, because they have no charge, and readily interact with the nucleus of an atom.  Protons have a positive charge, and therefore are repelled by the positively charged nuclei of atoms.

Below to the left, a neutron is absorbed by a fissionable nucleus. The nucleus splits into two smaller fragments, three additional neutrons, and a gamma ray. This is a pretty typical result of fission.

Atoms that will split (or fission) after capturing a neutron are called "fissionable".  There are a large number of atoms that will fission.   However not just any fissionable material will work in a reactor. 

To assemble a reactor core we need to find fissionable material with some specific properties: 
  • Some atoms which are fissionable don't readily absorb neutrons, so they won't work.  
  • There needs to be a sufficient supply.  If the fissionable material is rare, it cannot be used.
  • The fissionable atom must produce 2 neutrons when spit, or a reaction cannot be maintained. 
  • The fissionable atoms must have a long half-life, and not decay to some other element while in the reactor.
There is one other property that has to be met in our fissionable material.  It must also be "fissile". 

To understand the difference between fissionable and fissile, we need to understand about neutron temperature.  When a neutron is ejected from a split atom, it has enormous energy, about 2 Million Electron Volts (or 2 MeV). 

Physically, this is the speed of the neutron.  A 2 MeV neutron is traveling at about 45 million miles/hr.  Neutrons moving this fast don't often linger to induce fission in other atoms.  Statistically they are likely to leave the reactor core before an interaction occurs.  For this reason it is desirable to slow them down.  Neutrons are slowed by allowing them to bounce off lighter atoms and transfer their kinetic energy to the atoms.  This process is called "moderation", and slows the neutrons down to what is called "thermal" energy, .025 eV, or about 4900 miles/hr.  Depending on the mass of the moderating material, it may take from 10-2500 impacts to reduce the speed of a fast neutron from fission to thermal equillibrium.

Below is a table showing neutron energy (or straight line speed) of a neutron with its description.
Neutron energyEnergy range
0.0 eV-0.025 eVCold neutrons
0.025 eVThermal neutrons
0.025 eV-0.4 eVEpithermal neutrons
0.4 eV-0.6 eVCadmium neutrons
0.6 eV-1 eVEpiCadmium neutrons
1 eV-10 eVSlow neutrons
10 eV-300 eVResonance neutrons
300 eV-1 MeVIntermediate neutrons
1 MeV-20 MeVFast neutrons
> 20 MeVRelativistic neutrons

It is at thermal energy, speed, or temperature (pick one), that neutrons will more readily be absorbed by a nucleus and lead to another fission.

Now that we understand the process of neutron moderation, we can return to our need for Fissile atoms.  Fissile atoms are a small subset of fissionable atoms, that will readily split after absorbing a thermal neutron.  The list of candidates of fissile materials is small.  Uranium 233 and 235, Plutonium 239, and 241.  Only Uranium 235 is naturally occuring, while the others must be produced by excess neutrons from other materials inside a breeder reactor.

In another post I will describe other types of reactors, but for now we will stay with the basic thermal reactor, which is fueled with 5% U-235, both cooled and moderated by normal (light) water.

When an atom splits, it creates two highly radioactive atoms that must be contained, and not allowed to get into the coolant/moderator.  Otherwise that water will become very contaminated with radwaste.  For this reason, the uranium fuel is surrounded by zircaloy cladding.  Below is a photo of people inspecting new fuel assemblies for defects in the cladding.

The energy of a single split U-235 atom is about 10MeV, most of which is in the form of kinetic energy of the two new atoms.  They are moving at high speed, but due to their mass and charge, they quickly come to rest among other atoms of fuel.  The energy these atom fragments lose as they come to rest is converted to heat.

Back to the reactor core.  Water surrounds an array of these fuel assemblies, which is called the "core".  It takes a certain amount of fissile U-235 packed closely together, and moderated  neurtons for a continuous reaction to take place.  Below is a top-down view of a generic reactor core.

The reaction is started by withdrawing control rods.  Control rods contain strong absorbers of thermal neutrons.  These neutron absorbers are also known as "poisons", in that they slow or stop the nuclear fission process.  Typically a control rod contains cadmium, boron (or a borated alloy) for neutron control.

"Keff" is the number reactor physicists use to determine whether the reactor core is shutdown, steady-state, or starting up.  It's the neutron multiplier.  Keff is the likelihood that a single neutron will make it from birth to cause a fission in another Uranium atom.  Some of the other possibilities are that a neutron will escape from the reactor core, be absorbed in some material other than Uranium, or be absorbed by a Uranium atom, but not cause a fission.

If Keff is < 1.0 the reactor is "subcritical".  If Keff is = 1.0 the reactor is "critical", and if the Keff is >1.0 the reactor is "supercritical".  Nothing in any of these terms should be fear-inducing, they just describe a core reactivity condition for sustaining (or changing) the neutron population.

So... what happens when the control rods are removed?  I will skip the mathematics this time and give a simple description.  Neutrons are always present in U-235, because one of the natural decay modes for U-235 is spontaneous fission.  However reactors contain a strong artificial neutron source to ensure there are plenty of neutrons to get things going.  Think of the installed neutron source as sort of a pilot light for a gas furnace.

When the control rods are removed partially, neutron population begins to increase for two reasons.  The neutrons are no longer being absorbed in the poison, and so they begin splitting U-235 atoms, which generates even more neutrons.  The neutron population increases, even if Keff is less than 1, for the above reasons. 

Eventually though, if the control rods have been removed far enough, neutrons from new fissions will equal the losses, and Keff will be 1.  The reactor will then be critical.  However it won't be generating any heat.  Reactors typically are critical at an output of about 5 watts.  About the power required by a single "night light" bulb.  Power must be increased by a factor of 10^8 to get to 500 Megawatts.

After the reactor is critical, the neutron population must increase a great deal before any noticeable heat is generated.  So the rods are pulled out a little more, and each generation of neutrons has a few million more than the previous generation.  With this small increase in reactivity, there is no further need to withdraw the control rods.  Neutron multiplication will increase on its own.

Eventually the reactor reaches a point at which heat is generated, and this is where things get interesting.  The moderator (cooling water surrounding the fuel assemblies) heats up, and becomes less dense.  It is therefore less effective at slowing down neutrons. 

Because the moderator is less effective as temperature increases, more neutrons escape the core, which reduces neutron population, and reactor power stops rising.  We therefore have a heat-induced dampening effect on reactor power.  Its technical term is "negative temperature coefficient of reactivity".  Basically, more heating tends to reduce reactor power.  This is a desirable thing, because it prevents a runaway nuclear reaction.  Unfortunately not all reactors have this characteristic :(

So far, so good.  We have a critical nuclear reactor (Keff = 1.0) which is adding heat to the coolant!

I think that now is a good place to stop and think about the next post.  My brain hurts :)

Part 2 is here.