K-431 (Project 675)

K-431 was an Echo II class submarine.  Construction was started in January 1964, and she was commissioned in September of 1965, in a very rapid construction process.  The Echo II class was a twin reactor ship which carried cruise missiles for attacking surface ships.  Not very pretty.

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.

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.

Odd Quirks About Nuclear Reactors - Criticality Accidents

One quirky thing about nuclear reactors:  If you are not careful - particularly with Plutonium - it is possible to create a reactor unintentionally.  That is, you can inadvertently assemble enough fissile material to start a chain reaction, outside of the safe confinement of a shielded reactor vessel.

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.