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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.