The insidious nature of nuclear accidents is their horrible uncertainty. Nowhere is that more obvious than in Japan's latest blunder with a technology incessantly promoted as being entirely safe. The radiation leak at the Tokaimura plant is already the "worst nuclear accident" in the country's history.
It remains to be seen if the cascade of nuclear reactions triggered accidentally by plant workers will spread beyond its immediate surroundings. They are likely to be still occurring within the plant itself, whose roof appears to be breached by an explosion - while the environment for many hundreds of miles in all directions, could yet face the terrible consequences of fallout.
Uncertainty has been compounded by conflicting reports on whether what were initially dangerously high levels of radioactivity - some 4,000 times higher than is considered safe for humans - have returned to normal outside the plant. Japanese government officials and management at the privately-owned JCO processing facility, just 70 miles north of Tokyo, were unable to clarify the discrepancies.
What is clear, is that the plant has suffered "a critical mass accident" of sufficient significance to trigger a series of chain reactions, similar to what occurs in a nuclear reactor. The workers' description of seeing the ominous "blue glow" as they mixed too much uranium with nitric acid is confirmation of its critical nature. It was enough to ignite fears that a powerful neutron-induced nuclear reaction could be simmering. Continuing indications of high radioactivity outside the plant late yesterday, with one indicating a level 15,000 times above the safe human dose, suggest the accident is becoming gravely serious.
Radiation levels should fall quickly after an incident if there is to be any hope of containment. Mr Chihiro Kamisawa, a nuclear expert at the anti-nuclear group Citizen's Nuclear Information Centre, confirmed this accident's worrying significance. He said that the avoidance of a "criticality incident" is the priority in nuclear safety.
Japan relies on 51 commercial reactors to provide a third of its electricity. This accident could have a serious impact on its domestic economy. In the global sense, this is one of the world's most technologically advanced countries, supposedly a leader in nuclear power. The circumstances of Chernobyl - a badly-run plant suffering from lack of investment - have little resemblance to this case, but the world may have to live with its consequences.
Greenpeace said the accident confirmed its fears that "the entire safety culture within Japan is in crisis". It may even force postponement of a plan to restart the nuclear reprocessing plant in Tokaimura and may affect Japan's MOX fuel programme. The first shipment of MOX nuclear fuel - a mix of uranium and plutonium recycled from spent nuclear fuel - docked in Japan on Monday.
Tokaimura is a lesson in how nuclear accidents can occur anywhere, even in the best-run plants with the best safety records. Sellafield is one such facility, which proudly and regularly displays its high performance on the safety scale. This latest radiation leak should signal to the Government that the next one may be much nearer home and it must ensure that it intensifies its efforts to remove the nuclear threat on our doorstep.