Danger of Spent Fuel Outweighs Reactor Threat

Danger of Spent Fuel Outweighs Reactor Threat

Years of procrastination in deciding on long-term disposal of highly radioactive fuel rods from nuclear reactors is now coming back to haunt Japanese authorities as they try to control fires and explosions at the stricken Fukushima Daiichi Nuclear Power Station.

Some countries have tried to limit the number of spent fuel rods that accumulate at nuclear power plants — Germany stores them in costly casks, for example, while Chinese nuclear reactors send them to a desert storage compound in western China’s Gansu province. But Japan, like the United States, has kept ever larger numbers of spent fuel rods in temporary storage pools at the power plants, where they can be guarded with the same security provided for the power plant.

Figures provided by Tokyo Electric Power on Thursday show that most of the dangerous uranium at the power plant is actually in the spent fuel rods, not the reactor cores themselves. The electric utility said that a total of 11,195 spent fuel rod assemblies were stored at the site.

That is in addition to 400 to 600 fuel rod assemblies that had been in active service in each of the three troubled reactors. In other words, the vast majority of the fuel assemblies at the troubled reactors are in the storage pools, not the reactors.

Now those temporary pools are proving the power plant’s Achilles heel, as the water in the pools either boils away or leaks out of their containments, and efforts to add more water have gone awry. While spent fuel rods generate significantly less heat than newer ones, there are strong indications that the fuel rods have begun to melt and release extremely high levels of radiation. Japanese authorities struggled Thursday to add more water to the storage pool at reactor No. 3.

Four helicopters dropped water, only to have it scattered by strong breezes. Water cannons mounted on police trucks — equipment designed to disperse rioters — were deployed in an effort to spray water on the pools. It is unclear if they managed to achieve that.

Nuclear engineers around the world have been expressing surprise this week that the storage pools have become such a problem. “I’m amazed that they couldn’t keep the water in the pools,” said Robert Albrecht, a longtime nuclear engineer who worked as a consultant to the Japanese nuclear reactor manufacturing industry in the 1980s and visited the Fukushima Daiichi reactor then.

Very high levels of radiation above the storage pools suggest that the water has drained in the 39-foot-deep pools to the point that the 13-foot-high fuel rod assemblies have been exposed to air for hours and are starting to melt, he said. Spent fuel rod assemblies emit less heat than fresh fuel rod assemblies inside reactor cores, but the spent assemblies still emit enough heat and radioactivity that they must still be kept covered with 26 feet of water that is circulated to prevent it from growing too warm.

Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, made the startling assertion on Wednesday that there was little or no water left in the storage pool located on top of reactor No. 4, and expressed grave concern about the radioactivity that would be released as a result. The spent fuel rod assemblies there include 548 assemblies that were only removed from the reactor in November and December to prepare the reactor for maintenance, and may be emitting more heat than the older assemblies in other storage pools.

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Even without recirculating water, it should take many days for the water in a storage pool to evaporate, nuclear engineers said. So the rapid evaporation and even boiling of water in the storage pools now is a mystery, raising the question of whether the pools may also be leaking.

Michael Friedlander, a former senior nuclear power plant operator who worked 13 years at three American reactors, said that storage pools typically have a liner of stainless steel that is three-eighths of an inch thick, and they rest on reinforced concrete bases. So even if the liner ruptures, “unless the concrete was torn apart, there’s no place for the water to go,” he said.

At each end of a pool are 16-foot-tall steel gates with rubber seals, used to swing fresh fuel rod assemblies into a reactor and to swing out and store the spent assemblies. The gates are designed to withstand earthquakes, Mr. Friedlander said, but could have sprung leaks given the power of last Friday’s quake, now estimated to have had a magnitude of 9.0.

Even if water gushed out of the gates, there would still be about 10 feet of water left on top of the fuel rod assemblies.

When the water in a storage pool disappears, residual heat in the fuel rods’ uranium left over from their time in a nuclear reactor continues to heat the rods’ zirconium cladding. This causes the zirconium to oxidize, or rust, and even catch fire. This breaks the seal of the rods, and pressurized radioactive gases like iodine, which accumulated in the rods while they were in the reactor, suddenly spurt out, Mr. Albrecht said.

Each rod inside the assembly holds a vertical stack of cylindrical uranium oxide pellets. These pellets sometimes become fused together while in the reactor, in which case they may stay standing up even as the cladding burns off. If the pellets stay standing up, then even with the water and zirconium gone, nuclear fission will not take place, Mr. Albrecht said.

But Tokyo Electric said this week that there was a chance of “recriticality” in the storage ponds – that is to say, the uranium in the fuel rods could become critical in nuclear terms and resume the fission that previously took place inside the reactor, spewing out radioactive byproducts.

Mr. Albrecht said this was very unlikely, but could happen if the stacks of pellets slumped over and became jumbled together on the floor of the storage pool. Tokyo Electric has reconfigured the storage racks in its pools in recent years so as to pack more fuel rod assemblies together in limited space.

If recriticality occurs, pouring on pure water could actually cause fission to take place even faster. The authorities would need to add water with lots of boron, as they have been trying to do, because the boron absorbs neutrons and interrupts nuclear chain reactions.

If recriticality takes place, the uranium starts to warm. If a lot of fission occurs, which may only happen in an extreme case, the uranium would melt through anything underneath it. If it encounters water as it descends, a steam explosion may then scatter the molten uranium.At Daiichi, each assembly has either 64 large fuel rods or 81 slightly smaller fuel rods, depending on the vendor who supplied it. A typical fuel rod assembly has a total of roughly 380 pounds of uranium.

One big worry for Japanese officials is that reactor No. 3, the main target of the helicopters and water cannons on Thursday, uses a new and different fuel. It uses mixed oxides, or mox, which contains a mixture of uranium and plutonium, and can produce a more dangerous radioactive plume if scattered by fire or explosions.

According to Tokyo Electric, 32 of the 514 fuel rod assemblies in the storage pond at reactor No. 3 contain mox.

Tokyo Electric has said very little about the biggest repository of spent fuel assemblies at the site: 6,291 assemblies located in a common storage pool immediately inland from reactor No. 4.

The electric utility has been making elaborate plans for years to build an intermediate-term storage facility elsewhere. The plan was to take spent fuel rod assemblies from the common storage pool and put them in more secure dry casks.

Construction on the intermediate-term storage facility was supposed to have started next year.

This article “Danger of Spent Fuel Outweighs Reactor Threat” originally appeared at The New York Times.

© 2011 The New York Times Company

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