It started with an unusual bright blue flame flickering above the tank of uranium. It meant it was already too late. Japan's worst nuclear accident was under way and could not be stopped.

Hisashi Ouchi and Masato Shinohara, two workers at the JCO Co Ltd nuclear fuel processing plant at Tokaimura, had spent the early morning placing uranium in a tank of nitric acid, part of the nuclear fuel production process. It was fairly routine work, although it carried the added edge of involving radioactive materials and hazardous chemicals, so care was always required.

But with that little blue flame their shift ended dramatically, for it was the first indication that something was going seriously wrong. Within minutes they were overcome with nausea, the first signs of radiation poisoning, and were whisked away by white-gowned and masked emergency medical staff.

Some time later the inter-action between the uranium and acid became critical and the tank exploded, ripping a hole in the roof of the building. A spontaneous nuclear chain reaction had begun spewing radiation skywards.

It has yet to emerge who caused the accident at the JCO plant at Tokaimura, a town about 120 kilometres north-east of Tokyo. What is certain, however, is that Hisashi Ouchi (35) and Masato Shinohara (39) are unlikely to survive the massive burst of radiation signalled by that little blue flame.

Another 47 people, including plant workers and rescuers, are being treated for the effects of radiation exposure caused by the incident.

Tokaimura is a medium-sized town in the Ibaraki prefecture (county), but sits on a crossroads of the Japanese nuclear energy industry. The region is dotted with nuclear facilities, including power plants, fuel manufacturing facilities and service companies, all committed to the maintenance of the nuclear ethic.

Tokaimura was the site of Japan's first nuclear plant, completed in 1966, and is home to no fewer than 17 nuclear facilities. It has also experienced a firestorm of recrimination following another nuclear accident in March 1997 when a plutonium reprocessing plant caught fire, exposing 37 people to radiation.

So perhaps it was no surprise to the people of Tokaimura when the alarm bells started ringing and the local police came to clear people closest to the JCO plant from their homes last Thursday.

The work day for Hisashi Ouchi and Masato Shinohara began probably much the same as it had the day before and the day before that. They worked in a "conversion experiment building", one of the dozens of large buildings on the JCO campus.

The company's main activity is the production of uranium fuel pellets for the nuclear power industry. The process involves relatively conventional industrial chemistry, but the risks were, of course, higher because the men were working with radioactive materials.

Nuclear power stations rely on fission, a process that involves splitting large heavy uranium atoms into smaller atoms. When they split, the atoms release large amounts of energy which is captured as heat, used to power boilers and steam turbines to generate electricity.

JCO's role was to provide the uranium fuel used by these stations. This is not just a matter of shaping uranium ore into fuel pellets, however. Ordinary uranium does not have enough uranium 235 (U235), a particular isotope that splits easily under the right conditions.

The company, and others like it, take ore and enrich it, raising the level of U235 to between 2 and 4 per cent of the total fuel weight. This is "rich" enough to make the power stations work and drive the turbines.

Large amounts of fuel are then loaded into a nuclear reactor and the fission reaction is allowed to get under way. The U235 is split by a particle called a neutron, and each time a U235 atom splits it releases energy and another neutron. This particle is then free to split another atom and there follows a growing cascade of splitting atoms called a chain reaction.

The reaction is held in check in the reactor to help keep the energy release under control. Unimpeded, the chain reaction occurs nearly instantaneously in an atomic explosion similar to the A-bombs exploded over Japan at the end of the second World War. Hisashi Ouchi and Masato Shinohara weren't involved in ordinary fuel production at the JCO plant. They were formulating super-enriched fuel, which experts now believe was destined for an experimental new nuclear reactor. They were enriching the uranium to 19-20 per cent.

Tragic error had crept into the system, however. For reasons yet unknown, the men continued to add additional highly enriched uranium to their nitric acid tank. They added 16 kg of uranium to the tank, which should have held no more than 2.4 kg.

Either the level of enrichment or the large amount of uranium in the tank brought too much U235 together and it is thought that the acid might have acted to help the chain reaction to get under way.

Whatever the cause, the little blue flame - ignited by escaping radiation as it began breaking down air molecules - was the first indication of trouble. A powerful burst of radiation was released at 10.35 a.m. local time, (1.35 a.m. Irish time), delivering a likely fatal dose to Hisashi Ouchi and Masato Shinohara. They were whisked away to hospital, where their condition remains grave.

It took another 40 minutes for the company to contact Japan's Science and Technology Agency, 18 minutes more for the news to reach the Ibaraki prefecture and nearly two hours had passed before the Prime Minister, Keizo Obuchi, learned of the accident.

It was not until 12.41 p.m. local time (3.41 a.m. Irish) that JCO's unsuspecting near neighbours got any indication of the drama unfolding only dozens of metres from their homes. Police moved in and blocked roads and banned entry within a radius of 200 metres of the plant. It was not until 3 p.m. however, four hours and 25 minutes after the explosion, that the authorities issued an evacuation advisory to 150 people living within a 350 metre radius of the JCO campus.

Back at the plant, chaos reigned. The hole in the roof underlined the unsuitability of the building for dangerous nuclear work. Concrete and steel shielding was considered unnecessary because this type of accident was not supposed to be possible.

The building was cleared, but this left the company's experts unsure of what was happening to the uranium in the tank. Clearly it had reached criticality, the point at which spontaneous fission occurs. But was the fission reaction continuing, accompanied by the release of radioactive gases, particles and neutrons? They didn't know. They couldn't re-enter the building for fear of fresh explosions.

As word of the accident spread and police action continued, fear stalked the streets of Tokaimura. The 310,000 residents in a 10 kilometre danger zone were told to stay in their homes and not to go out. A light rain was falling and people were told to wipe their feet well and not track the rainwater in. Farmers were told to stop any harvesting and leave their crops in the fields.

An invisible but more dangerous presence also passed over Tokaimura.

Radiation levels outside the plant sky-rocketed. Initial measurements recorded levels 10 times higher near the plant. This was soon upgraded to 4,000 times normal. These levels eventually peaked at 20,000 times the normal. These were so high that a person standing outdoors two kilometres from the plant would receive in an hour more radiation than they might have expected to receive in a typical year.

These high levels provided the proof that radioactive materials were escaping from the plant. Many criticality incidents simply blow themselves out with a short burst of radiation and a small explosion, separating the uranium and stopping the chain reaction. This clearly was not the scenario at Tokaimura.

The criticality persisted for 17 hours and all during that time radioactive materials were released. It was not until 6.15 a.m. local time yesterday (9.15 p.m. Thursday, Irish time) that Japan's Nuclear Safety Commission said the chain reaction had stopped after chemicals were injected into the plant to absorb the neutrons.

Radiation levels fell rapidly and by 3 p.m. yesterday Tokaimura's residents were told they could leave their homes. The crisis was over.

The plant and its neighbours still face an uncertain future, however. There were 49 people being treated for radiation exposure and there may be more to follow, given the clean-up at the plant and further afield.

It is already known that long-lived radioactive materials including radio-caesium have been discovered in the soil away from the plant, according to early reports from the Radiological Protection Institute of Ireland. The Japanese authorities are still trying to assess how much and what substances got out of the plant during the emergency.

More is perhaps known about the future for two if not three of the most highly irradiated workers at the plant, including Hisashi Ouchi and Masato Shinohara. They began showing the typical signs of severe radiation poisoning even before they reached the hospital. While they continue to receive treatment, there is really nothing that can be done to reverse the effects of a large radiation dose.

Their families will take little comfort in the apology given yesterday by Moriki Aoyagi, president of Sumitomo Metal Mining Co Ltd, which owns JCO Co. Nor will they find reassurance in the Japanese government's promise to review safety procedures in the country's nuclear plants.