The facts that have emerged show that workers at the complex made a simple but terrible mistake.

They were preparing uranium fuel for use in a nuclear reactor. Normally only 2.3 kgs (5 pounds) of liquid uranium is placed into a container containing nitric acid. By error nearly eight times that amount was used.

The indications are that the uranium was of a higher grade than had been used before. This meant that the consequences of using too much was more serious. By bringing together 16 kilograms (35 pounds) of enriched uranium they produced a nuclear reaction in which the uranium became very hot.

In this so-called criticality, a self-sustaining chain reaction occurred. The intense heat caused a build-up of pressure inside the container and an explosion. Radioactive gas gushed into the atmosphere.

The building in which the accident occurred was not designed to contain radiation, and three workers in the near vicinity received massive doses of radiation.

Safety protocols

The management at JCO, the private firm that runs the Tokaimura plant, has blamed its workers for ignoring safety protocols. However, why the company was using a system in which the creation of a "critical mass" of material was even possible has yet to be explained.

Neutron readings taken in the vicinity of the plant show that the radiation has declined to previous background levels.

Officials believe that the self-sustaining reaction may be fading but entering the stricken building will still be a very hazardous procedure requiring full-scale radiation protection. It is unlikely to be used again.

Since 1945 there have been 60 known accidents at nuclear reactors and processing plants. Thirty-three have occurred in the United States and 19 in the former Soviet Union.

If the major accidents in nuclear history have told us anything, it is that the potential for operator error must, as far as possible, be designed out of the system.

Checks and double-checks must be used and backed-up by a fail-safe design. The behaviour of nuclear material is well understood. Accidents usually occur when the rules are broken.

Chernobyl

The 1986 Chernobyl accident was the supreme example of human incompetence on many levels.

The power station's operators wanted to test the efficiency of its electricity generators after the supply of steam coming off their RBMK nuclear reactor had been switched off. They knew this was dangerous, but pressed ahead even when a problem started to develop in the reactor's core.

Left to itself, the Chernobyl reactor should have shut itself down automatically. But the operators overrode the safety systems.

By the time they realised their mistakes, they could not insert the control rods into the reactor fast enough to shut it down. The reactor blew its top.

Had the RBMK reactor been better designed, the accident would have had less disastrous consequences. It had no containment and the resulting explosion scattered radioactive debris all over Europe. Such a design would not have been allowed in Western countries.

Three Mile Island

Human error also played its part at Three Mile Island in Harrisburg, Pennsylvania, in 1979.

Valve failure led to a loss of water used to cool the reactor. This would have triggered an emergency core cooling system to come into operation but the workers at the plant misread the situation and turned it off.

Fortunately, very little radioactivity was released into the environment - mainly the noble gas isotope of krypton-85.

Three Mile Island was a turning point in nuclear safety. It showed that a potentially serious incident could happen in mainland USA and forced a re-evaluation of safety systems. It was realised that with the system they were working with the operators did not stand a chance to gain control of the situation.