Saturday, March 19, 2011

An Update on the Fukushima Situation

Some of the following will make more sense if one has a basic understanding of the composition and construction of nuclear fuel. I provided a basic introduction to nuclear power in my last blog post and have created a standing page on the subject.

Fallout Over California
Previously I discussed the possibility of fallout from the Fukushima incident reaching the West Coast. Although I erred slightly when I said that the fallout "would probably be so widely dispersed that the radioactivity would be effectively undetectable" (I neglected the fact that radioactivity detection systems used to monitor nuclear weapons tests have become extremely effective), I was right when I also said that it "would have a negligible impact on public health". Recent reports indicate that the levels of contamination reaching the United States are "about a billion times beneath levels that would be health threatening".

Damage Control and Radiation Exposure
A damaged reactor building at
the Fukushima nuclear plant
In the meantime, nuclear plant workers (the heroic "Fukushima 50", although there are actually about 180 of them) have been attempting to stabilize the reactors. Working in shifts, these plant employees are being limited to 50 rem (0.5 Sv) total radiation exposure. This is ten times the annual Federal limit for radiation workers in the US but is about half the amount of exposure at which point the effects of acute radiation sickness can be felt; i.e., 100-200 rem (1-2 Sv). Although these workers will undoubtedly face an increased cancer risk in the future (perhaps four times greater than normal if they end up like Chernobyl's "Liquidators"), as long as they observe the 50 rem limit it is very unlikely that any of the Fukushima 50 will die from radiation sickness. In other words, the plant workers have been given a dangerous task, but it isn't a "suicide mission" as some in the media have called it.

Dry Cooling Pools
The latest reports also seem to indicate that some of the greatest problems being faced are not coming from the reactors themselves but from spent fuel cooling pools that may be going dry. If the fuel assemblies aren't being properly cooled they can become damaged due to decay heat. The zirconium cladding can bubble and burst open, releasing fission products into the air. Even worse, it is feared that the overheated zirconium alloys will begin to oxidize rapidly, which can result in a fire. In the presence of steam, the oxidation process can produce hydrogen gas. This process occurred earlier this week when hydrogen was created by the oxidation of the cladding of the overheated fuel assemblies inside the reactors. To reduce the likelihood of damage to the reactor vessels, operators vented the hydrogen out of the reactors and into the reactor buildings, which caused several explosions. The decision to vent the gas was definitely the right one since an explosion within the reactors would have been even more disastrous.

Given the potential consequences of allowing the cooling pools to dry out, plant workers have been desperately trying to refill them. However, in addition to cooling the fuel the water also provides radiation shielding. Without shielding, it is very difficult to approach the pools due to extremely high radiation levels.

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