Last Thursday, TEPCO mistakenly reported that radiation levels found in a puddle of water (that two plant workers had stepped in) were ten million times normal. They later corrected this and said the levels were ten thousand times normal. The radiation was coming from an isotope called iodine-134, which has a half-life of 53 minutes; which is good news, because it means that within a short time the radiation would no longer be dangerous. It’s also bad news, because at that stage the reactors had been shut down for almost two weeks, not 53 minutes. The presence of highly radioactive isotopes with short half-life, and TEPCO’s statement last week that neutron radiation had been detected at the plant (see my post last Friday, 25th March), seems to prove almost conclusively that nuclear fission is still occuring in one or more reactors.
Nuclear fission = lots of heat and lots of radiation. If the radiation prevents workers from being able to cool the reactors, there’s a chance that one of them could go into full meltdown. The melted fuel rods then produce a lava-like substance called corium, which burns through the reactor vessel and flows down to a concrete chamber below. The concrete chamber beneath the reactor is a safety feature, designed to stop the corium from going straight down into the earth, where it would eventually reach the water table.
At Fukushima they’ve been pouring thousands of tons of sea water into the cooling pools and over the reactors, and it seems likely that the chambers beneath the reactors are flooded. If a reactor does go into full meltdown, and the corium hits that water, there could be a massive thermal explosion.
They had exactly the same problem at Chernobyl. During the early days of the crisis they poured thousands of tons of water in to the No.4 reactor. This water had absolutely no effect on the fire, but it did flood the chambers beneath the reactor. Valeri Legasov, a leading Soviet scientist and one of the team brought in to deal with the disaster, realised what would happen if the molten core of the reactor reached the water. Legasov estimated that the resulting thermal explosion would be in the order of 2 to 4 megatons, and would flatten everything within a 20 mile (30km) radius of the plant. Furthermore, the radiation thrown high into the atmosphere by the explosion would render a large part of Europe and the western Soviet Union uninhabitable for decades, if not hundreds of years.
Grim stuff. At Chernobyl, three divers volunteered to go into the flooded chambers to manually turn valves that would release the water. It was a successful mission. However, two of the divers died shortly afterwards from radiation sickness. It’s not known if the third diver is still alive. Incidentally, on the second anniversary of Chernobyl, Valeri Legasov committed suicide, out of remorse for his part in the cover-up by the Soviet authorities.
At Chernobyl, corium did eventually reach the (dry) chambers beneath the reactor. It’s now solidified, yet 25 years later it is still very hot and radioactive. The Chernobyl corium is of great interest to scientists, because it contains never-before-seen minerals and types of uranium…
One would hope that TEPCO are aware of the dangers of a thermal explosion, and are ensuring that the chambers beneath the Fukushima reactors are kept clear of water.