Neutron Activation, N-16, and pools

"Guessing before proving! Need I remind you that is how all important discoveries have been made?" - Henri Poincare

K-278 Komsomelets (Project 685 Плавник), Mike Class

K-278 was a unique and magnificent submarine.  Only one of this class of ship was built.  Because she was so unusual, construction took an unusually long 5 years.  Her construction began in April 1978, she was launched in May 1983, and commissioned on the last day of 1984.  The Soviet Navy rarely named their submarines, but this ship was special, and received the name of "Komsomelets" - which means "a member of the Young Communist League".

K-219 (Project 667A)

K-219 was a Soviet ballistic missile submarine.  She was a Yankee I class ship, which carried sixteen liquid-fueled nuclear missiles for use against land targets in the US.  The ship was propelled by two PWR reactors, and had two propellor shafts.

On a side note, the Soviet Navy gets credit for envisioning and building the first submarine to carry an ICBM, the Zulu Class submarine.  The Zulu was a conventional diesel-electric submarine that was modified with an extended sail to carry a single nuclear-tipped long-range missile.  It was also the first submarine to test launch a ballistic missile.

K-27, Project 645 (645 Кит-ЖМТ)

The Soviet and US Navies operated a large number of submarines, each generation improving in quality.  A silent covert game of cloak and dagger took place beneath the waves that most people were completely unaware of. 

The Soviets at one time had the largest fleet of submarines in the world.  In many respects, advanced Soviet submarines were superior to their US cold war counterparts.  Several Soviet submarine designs could dive to a greater depth than a standard US MK 48 torpedo!  

US submarines (from the arrival of Thresher) had held the advantage of stealth, and superior sonar equipment.  Soviet submarines, in contrast, held the advantage of survivability (due to double-hull construction and huge amounts of reserve bouyancy), weapon-carrying capacity, greater depth and top speed.

The US experimented with a variety of reactor/propulsion designs, but only one used a liquid-metal cooled reactor:

USS Seawolf (SSN-575) was the second US submarine (after USS Nautilus), and the only US submarine to have a liquid metal cooled reactor.  The reactor was cooled using liquid sodium, which of course would be problematic for the crew if  it ever leaked.  Seawolf also had steam superheaters, for added efficiency.  These were also problematic, and thus were seldom in service.  Because liquid metal is much more efficient than water at removing core heat, the propulsion plant was only 40% the size of Nautilus'.  Seawolf was eventually converted to a more typical S2W pressurized water reactor (PWR) with a saturated steam plant.  PWR and saturated steam plants in US submarine design continues to this day.

The Soviets' emphasis on submarine speed, depth and power of course led to more propulsion designs that used liquid metal cooled reactors.  Soviet reactors of this type used a Lead-Bismuth coolant that was far less hazardous than liquid sodium, at least from a fire hazard standpoint.  From a power-weight (and size) standpoint, the liquid metal cooled reactor is far superior to a light water cooled reactor.  From a safety standpoint, not so great.

Recall that liquid metal cooled reactors are Fast neutron reactors, or sometimes intermediate speed reactors.  All liquid metal cooled reactors have a positive void coefficient of reactivity.  That means that if the coolant inadvertantly boils in the core, reactor power will increase.  Which will boil more metal, and increase power even more.   This happens rapidly, and core damage (meltdown!) is fairly common with this type of reactor.

So with that background, lets talk about the Soviet submarine K-27, or Projekt 645. 

The Soviet's first class of nuclear attack submarines was called the November class.  They used dual 70 Megawatt PWR reactors for propulsion.  13 of these were built before technology allowed creation of superior designs.  Even so, they were superior to the USS Nautilus, in speed, depth, and stealth.  One could also argue that Nautilus was really an experiment to prove that nuclear propulsion could work on a submarine, rather than a true nuclear attack submarine, however, and not be wrong.

Profile of a November-Class Submarine:



Back to K-27.  This was a unique single-ship design by the Soviets, just as Seawolf was for the US Navy.  K-27 was a November-Class submarine with a unique power plant.  Rather than two 70 Megawatt PWRs, the Soviets used two VT-1 liquid metal cooled reactors, with an output of 73 MW. The advantage of smaller footprint and weight of the metal-cooled reactors allowed more weapons to be carried.

She was laid down on June 1958 and launched in April 1962.  She was commissioned October 1963 after full-scale builders sea trials and official tests.  She performed well (although with heavy maintenance for the new metal-cooled reactors) until a reactor accident in the port (left) reactor happened in May 1968.

The ship was making a full speed submerged run, when a reactor automatic control rod withdrew itself.  Boiling occured, and reactor power plummeted from 83% to 7% in about 90 seconds, as the core melted.  Unfortunately for the crew, poor decisions made after the initial accident would cost many of them their lives. 

The main purpose of cladding U-235 in a reactor with Zircaloy or Stainless steel is to keep the highly radioactive freshly split atoms from getting into the coolant and spreading.  When the fuel assemblies melt down, these radioactive atoms mix in the coolant, and get outside the heavily shielded reactor vessel. 

Unknown to the crew, the captain had the radiation alarms disabled.  Radioactive gases were released from the fuel, which the crew were exposed to.  Another captain might have surfaced the ship and ventilated it with the massive air blowers all submarines are equipped with.  The ship limped home on the starboard reactor and was laid up for several years.  Five sailors who worked in the propulsion plant died within a week of the accident, while 30 more died between 1968 and 2003.  Quite a high death rate for a crew of young, healthy men.

K-27 was brought into shipyard, and the starboard reactor coolant was kept liquid by steam piped in at the shipyard while the radioactivity in the port side reactor died down.  In 1973 the decision was made that repairing or replacing the reactor in the aging ship was not worthwhile, and the ship was decomissioned in February 1979.

Her disposal was... interesting.  Rather than remove the melted down mess that remained of the port side reactor, the Soviets decided to fill her reactor compartment with a solidifying agent.  Next they towed her, not out to sea, but very close to land.  In 1982 they sunk her in just 100 ft of water, just offshore of Novaya Zemlya. Google Earth Coordinates Here

She didn't want to sink, however, so they ended up having to ram her.

K-27 refusing to be scuttled: 

There is now a great deal of urgency in re-floating K-27 and removing her radioactive coolant system and fuel.  This is an environmental hazard that will eventually become a serious problem, and quite close to shore.  Where it was disposed of is the Island of Novaya Zemlya, a harsh glacier-scoured island that has been a nuclear testing and dumping ground for generations. 

Interestingly there is equipment available to de-fuel this unique ship that was used on many other liquid-metal cooled ships at the end of the cold war.  However, this now-unused de-fueling equipment will not remain in optimum condition forever, so the race is on.  Hopefully someone is interested in recovering this ship before it becomes a big environmental mess.

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...

Background:
In a Pressurized Water Reactor (PWR), the control rods are completely removed from the core after the reactor is up to full power, and dilute Boric Acid (a strong neutron poison) is carefully added to control reactivity.  

The purpose of this process is to achieve a flatter neutron flux profile throughout the core. 

A flat neutron flux is desirable for a couple of reasons:  It helps to even out fuel burnup and it helps to reduce hot and cold coolant channels.  When control rods are partially inserted into an operating reactor core, neutron flux is depressed near them, since they absorb neutrons.  Therefore fewer fissions occur near the control rods.  This localized reduction of fission causes uneven fuel burn and creates cold zones due to reduced fission near the rods.  For a given power output, other sections of the reactor core away from the control rods must now run hotter to compensate.

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 is possible 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.

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 and closure head, however, are not.  Due to their size, it is impractical to make the entire thing from stainless steel.  Instead, the reactor vessel and head are made from carbon steel, and their 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 contact with the Boric Acid in the primary coolant.
 
The Event:
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 hadn't been noticed - in a million gallon system, a minor leak can go undetected almost forever.  Any water leaking from the primary coolant would also be quite hot and would flash to steam immediately, so no water puddling would occur.


Davis-Besse had a tiny coolant leak, however.  One which they were unaware of.  In 2002 the plant shut down for a refueling outage, and performed an inspection underneath the insulation on the reactor vessel head.  This was done after other plants of the same design had uncovered minor leakage.  They found a little problem...

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.

In the end it took a couple of years to manufacture a replacement head and get the plant back online.  

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 7.37 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.