One theory says that the steam reacted with the zirconium tubes forming zirconia (an inert oxide of zirconium) and hydrogen gas which subsequently ignited resulting in an enormous, but chemical, explosion. Another says that part of the core went super-critical, vaporising at 7000 Celsius in a nuclear explosion.
Whichever, the subsequent melt-down of the remaining radioactive fuel (the China syndrome) released enormous plumes of the volatile but highly radioactive nuclides iodine-131 and caesium-137 into the atmosphere, as well as dust from over 100 different radioactive isotopes, a massive 14 exabecquerals of radioactivity! [14×1014Bq]. which were carried by the winds onto England, and deposited where it rained: in Wales and in the Lake District Fells. Altogether, 190 tonnes of toxic materials were released into the atmosphere. This contaminated great tracts of land, where grazing by animals for the foodchain is still forbidden (AD 2003). Seven million people were evacuated from the immediate vicinity of the reactor and up to 30 mile radius around. It remains vacant today except for the operators of the other three nuclear reactors at the site which were not damaged. The preventable accident has left a trail of cancer, mutation and death in its wake. Almost all the soldiers sent to put the nuclear fire out have either died or else are still receiving treatment for ghastly radiation burns, and they only spent up to two minutes on the roof of the burning reactor. 5 million people now live on contaminated land around Chernobyl, and the land will remain contaminated for centuries before the radioactively reduces sufficiently. In order to try to quench the runaway nuclear reaction, tons of monazite sand were thrown from helicopter into the white-hot molten mass, presumably because of the small amount of gadolinium it contains which has a very high thermal neutron absorption. Most of the volunteer soldiers in the helicopters died from radiation poisoning in the first few days. The total death toll through cancer for the Chernoble accident is estimated to be 30,000 to 60,000 people; about ten times higher than was forecast by the IAEA in 2005!
The molten fuel fortuitously mixed with tons of surrounding sand which diluted and stabilised it turning it into a lava which then flowed into the many rooms below the reactor before finally solidifying and producing many entirely new and novel forms of (highly radioactive) minerals. Metamict is the term given to minerals which have been damaged and changed from a crystalline state to an amorphous state by their own irradiation. This lava is slowly disintegrating due to its intense radioactivity, becoming dust which may one day be disturbed sending clouds of highly dangerous plutonium dust into the atmosphere when the encasing sarcophagus hastily built around the reactor finally collapses, perhaps before the next century unless anything is done. For 20 years nothing has been.
Plutonium-241 is one of the very many radioisotopes the explosion strew over Belarus. Plutonium-241 has a halflife of 14.3 years decaying by beta decay into americium-241, an alpha emitter of halflife 432 years. But although the halflife of americium-241 is more than 30 times longer than is plutonium-241, the energy released per decay at 5.63 MeV is 28 times higher than the electrons of up to 0.2MeV released by plutonium-241. Also, the alpha rays are far more damaging, and this transmutation has now, 2005, more than doubled the amount of alpha radiation being released than was initially, and are expected to remain this high until the year 2276. This transmutation has increased the area seriously contaminated by alpha radiation from the initial 950 square kilometres to 3500 square kilometres. The radiation ranges from 740 to 3700 becquerals per square metre, being between 14 and 70 times higher than the average of the whole country.
The iodine-131 from the accident caused a 90-fold increase in childhood thyroid cancers. The caesium-137 which was also released persists in the soil being re-cycled by plants, and is the main reason why sheep in hundreds of farms in the rainy districts of the UK (those regions raining at the time when the volatiles in the radioactive cloud passed overhead) are still, in 2005, subject to slaughter and movement restrictions.