Fission Criticality In Cooling Ponds Threaten Explosion At Fukushima
Author: Joseph Trento, DC Bureau
Published on Mar 15, 2011 - 12:48:40 PM

Cutaway drawing of a typical Boiling water reactor (BWR) Mark I Concrete Containment with Steel Torus (suppression pool), as used in the BWR/1, BWR/2, BWR/3 and some BWR/4 model reactors. Photo: Sandia National Laboratories
March 14, 2011 - The threat of a fission explosion at the Fukushima power facility emerged today when the roof of the number three reactor exploded and fears that a spent fuel pool, located over the reactor, has been compromised. The pool, designed to allow reactor fuel to cool off for several years, was constructed on top of the Fukushima reactors instead of underground. As of 2010, there were 3450 fuel assemblies in the pool at the number three reactor. The destruction of the number three reactor building has experts concerned about whether the spent fuel storage pool, which sits just below the roof, could have survived intact the hydrogen explosion. The explosion was much more severe than Saturday's blast at the number one reactor.

As massive amounts of seawater are pumped by fire trucks into Fukushima's failing nuclear reactors and cooling ponds, the radioactive waste water, now laden with a variety of radioisotopes, is being flushed into the sea.

Just how much danger the spent fuel pool raises is made clear in a November 2010 powerpoint presentation from the Tokyo Electric Company detailing how fuel storage works at the huge complex.

The fuel inventory in the pool is detailed on page 9. According to TEPCO, each reactor generates 700 "waste" fuel assemblies a year, and there are 3450 assemblies in each pool at the Fukushima Daiichi plant, plus another 6,291 in a common pool in a separate building.

As shown in slide 10, the common pool building sits at ground level, with the pool itself above ground. The building also has windows on at least one side, and experts fear these were broken out by the tsunami which would have flooded the building.

According to Albert Donnay, a former nuclear engineer, "This means the common pool is now full of radioactive and corrosive seawater that will cause the fuel assemblies to fail and burst open, as they are doing inside the reactor cores that have been deliberately flooded with seawater. If the pool drains or boils away, the fuel will melt, burn and even possibly explode if the fuel collapses into a sufficiently critical mass."

This may explain why the Japanese government began adding boric acid to the reactor spent fuel pools at the facility shortly after the earthquake and tidal wave.

The Japanese government has not explained why it is adding boric acid and if the acid is being used to prevent criticality in the reactor or in the spent fuel pool. A spokesman for the Embassy of Japan, in Washington, D.C., said the boric acid was being only added as a "precautionary measure," but said the Embassy did not know why. Because the GE reactor's control rods are made of boron, and they were automatically inserted when the earthquake struck to end fission in the reactor, there should have been no need for additional boric acid. But if fuel rods had been compromised and the damaged fuel bundles were not properly separated, they can become critical and boric acid could be used to help prevent a far more serious meltdown in the spent fuel pools.

When the power was lost at the site, the cooling system for the pools would have run out of water in about a day. The water in these pools would heat up and evaporate to the point where the tops of the fuel bundles would be exposed about 24 hours after the cooling system shut down.

Experts fear the explosion rained debris into the pool that stopped natural cooling of the fuel bundles or knocked the bundles together, damaging them, sending the irradiated fuel chunks to the bottom of the pool where they could reach critical mass. "They got a one-two punch," said David Lochbaum, nuclear safety engineer of the Union of Concerned Scientists and a consultant to both industry and the Nuclear Regulatory Commission. Lockbaum told Roger Witherspoon on, "If it had just been the earthquake, or just the tsunami, we wouldn't even be talking about this. But the combination of nature was more than they could handle. It doesn't seem that they have lost control yet. But they have definitely run out of options.

"If those solutions – the sea water and the boric acid – don't work, there are no more arrows in the quiver. They have shot everything they have, they have run out of options and there is nothing left."

The problem for the Tokyo Electric Company engineers is water containing boric acid has to circulate in the pools to keep the bundles from going critical.

Both United States and Japanese governments have for decades allowed re-racking of the pools to reduce the originally-designed minimum safe distance between the assemblies so that more rods can be stored in each pool. Utilities complained they were running out of storage space on site at the reactors. The problem is if the spent fuel gets too close, they will produce a fission reaction and explode with a force much larger than any fission bomb given the total amount of fuel on the site. All the fuel in all the reactors and all the storage pools at this site (1760 tons of Uranium per slide #4) would be consumed in such a mega-explosion. In comparison, Fat Man and Little Boy weapons dropped on Hiroshima and Nagasaki contained less than a hundred pounds each of fissile material.

According to Donnay, "Several cores worth of spent fuel are usually stored in these pools until they are cool enough to transfer into dry cask storage. In comparison, the reactor itself contains only one core, and its total radioactivity is less than that in each spent core."

Nuclear Information Resource Service led a coalition of groups that petitioned the US Nuclear Regulatory Commission in 2005 requesting emergency enforcement action on the vulnerability of the Mark I and II elevated nuclear waste storage pool. The coalition's petition to the NRC was denied.

Another worry for engineers is that in 2009 plutonium-based mixed oxide fuel produced by the huge French nuclear power company AREVA was loaded into reactor number three.

Correspondent Celia Sampol spoke to AREVA and the company spokesman said AREVA will not make a specific statement on the issue or on the possible losses for its activities in Japan because "today the priority is for the Japanese authorities to save people and help victims". AREVA's employees in Japan were contacted on Friday, all are safe and some of them left Japan. Anne Lauvergeon "will talk about that in France soon".

Nathalie Bonnefoy, from the MELOX Division of AREVA La Hague, France, said, "Today, the type of fuel used in the reactor is absolutely not involved in the problems at the Fukushima facility...It's not a matter of the MOX fuel exploding; the problem is if you have a loss of cooling, you have a risk of fusion and the hydrogen released could generate difficulties in contact with air, but it is independent from the type of combustible used."

"In this site, all the MOX fuel has been already loaded in the reactor (it started in October 2010)," no MOX fuel is stored on site here. On others sites, a part of the MOX fuel is stored on site (every 18 months you have to renew one third of the MOX fuel because it has lost efficiency). According to Bonnefoy, four reactors in Japan are burning MOX fuel fabricated by AREVA; the first loading took place in December 2009; AREVA signed contracts with eight (out of eleven) Japanese electric companies to supply MOX fuel, but the French group has no reactors of its own in Japan. The company does have about 100 employees in Japan.

According to NIRS (Nuclear Information Resource Service) at"In the GE Mark I design, the irradiated fuel pool, containing billions of curies of high-level atomic waste, sits atop the reactor building, outside primary containment and vulnerable to attack, according to both NRC documents (2001) and the National Academy of Sciences (2005)."

The same diagram appears in the Sunday New York Times, pA11, with the uppermost rectangular chamber just to the left of the reactor top identified as the spent fuel storage pool, but the accompanying article does not discuss it.

Donnay said, "If these pools are breached (as could have happened in the explosions, Fukushima #3 looks worse than #1) and can no longer hold water, the spent fuel racked inside them will start to overheat, and eventually melt and burn. And since there is no longer any roof above these pools in reactors 1 and 3, all the radioactivity they contain is directly open to the atmosphere."

According to a Defense Department source, the cesium detected in the atmosphere around the plant could be coming from the spent fuel pools.

According to Donnay, there is an additional danger from used fuel being stored in casks: "I'm also worried about the dry cask storage pods that were on the site before the tsunami.

Full casks are very heavy and probably would not be carried away by the flood, but some were probably not full. Any that were only partially filled with spent fuel would have air locked into the unfilled chambers, making them able to float in water. Did the tsunami carry any of these casks away? Are they all still onsite? Before and after satellite photos should be able to show this clearly, but Google Earth is not showing after photos of the Fukushima plant.


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