UK government's Fukushima crisis plan based on bigger leak than Chernobyl

The British government made contingency plans at the height of the Fukushima nuclear crisis which anticipated a "reasonable worst case scenario" of the plant releasing more radiation than Chernobyl, new documents released to the Guardian show.

The grim assessment was used to underpin plans by the British embassy in Tokyo to issue protective iodine pills to expats and visitors. It also prompted detailed plans by Cobra, the government's emergency committee, to scramble specialist teams to screen passengers returning from Japan at UK airports for radioactive contamination.

The UK government's response to the unfolding crisis is revealed in documents prepared for Sir John Beddington, the chief scientist and chair of the Scientific Advisory Group for Emergencies (Sage), and released to the Guardian under the Freedom of Information Act. The 30 documents include advice from the National Nuclear Laboratory on damage to the plant, public safety assessments from the Health Protection Agency (HPA), computer models of the radioactive plume from Defra's Radioactive Incident Monitoring Network (Rimnet), and the worst case scenario that might unfold at the plant.

A substantial number of documents were withheld on grounds that they contained "information which, if disclosed, would adversely affect international relations," the government's civil contingencies team said.

The earthquake and tsunami, which hit Japan in March, knocked out critical cooling systems at the Fukushima plant, prompting immediate fears that nuclear fuel in the reactors could melt through their pressure vessels and drop onto the concrete floor beneath, causing a "corium explosion" and a major release of radiation. There were similar concerns over hundreds of spent fuel rods packed into storage pools above the reactors.

To avert nuclear meltdown, Japanese emergency teams worked frantically to rig up mobile power supplies and pump water into the reactors, while the storage ponds were doused with water from helicopters, military fire trucks and an enormous remote-controlled concrete pump.

The documents reveal how the British government scrambled to build up a clear picture of the crisis and the danger it posed in the days and weeks after the earthquake struck.

In the first week, advisers wrestled with conflicting information on the amount of spent fuel at the Fukushima site, but settled on a figure of 4,500 fuel rods in six ponds, or enough to power eight reactor cores. A separate fuel pond, known as the common pool, was "full to the gunnels," one unnamed adviser wrote.

A fear raised in one document was that spent fuel rods might overheat, melt and slump to the bottom of their storage pools where enough could gather to "go critical", that is, restart nuclear reactions. The likely result of that, the paper states, would be releases of fresh radioactive material, "which could continue for some time".

In further statements, government advisers admitted insufficient expertise to work out how high an explosion might blast radioactive material, but warned that an explosion at one reactor or fuel pond could trigger a domino effect as other reactors and the spent fuel ponds became too dangerous to deal with. The "reasonable worst case scenario" envisaged ruptures at all three reactors in operation before the earthquake, and radiation leaks from six spent fuel ponds at the Fukushima site.

The Nuclear Installations Inspectorate estimated this would release 10% of the radioactive caesium-137 and iodine-131 in the cores and one third of the caesium-137 in the spent fuel ponds. Under that scenario, wind currents carried the plume directly towards Tokyo at a speed of five metres per second. Had the event happened, it could have released the equivalent of 9.92 million terabecquerels of radiation from iodine-131 into the open air, nearly double the 5.2 million terabecquerels released by the fire at the Chernobyl nuclear reactor in Ukraine in 1986.

Even in this worst case scenario though, the accident was expected to cause less harm than Chernobyl, where the reactor's burning graphite core threw radioactive material high into the atmosphere, and local populations were not evacuated quickly or barred from consuming contaminated milk and other products.

The documents show how seriously the risk to Britons in the Japanese capital Tokyo and Sendai, a major city 70 miles from the stricken plant, was taken. A week into the crisis, the British embassy distributed iodine pills as a contingency measure.

Meanwhile, the HPA prepared advice to be published in case a major leak occurred. It explained what time the radioactive plume would reach either Tokyo or Sendai, depending on the wind direction and stated: "When the Japanese authorities instruct you to shelter you should take the first dose, or if the UK government gives you additional information."

On 25 March, two weeks after the tsunami struck, the government's emergency Cobra committee asked the HPA to draw up detailed plans to monitor air passengers for radioactive contamination as they arrived from Japan. The agency proposed monitoring all passengers if a major leak arose that could cause "some health concern", and limited monitoring to reassure the public at the government's request.

HPA documents show it had a team ready to monitor passengers landing at Heathrow within 24 to 36 hours of Cobra's request. The agency expected to start passenger screening if a major radiation leak from the plant led to predicted doses of more than six milliseverts in people beyond the 80km exclusion zone. But the agency warned it would struggle to monitor all passengers arriving from Japan at more than two airports, even if it borrowed detectors from the Atomic Weapons Establishment in Berkshire.

In April, the Fukushima incident was upgraded to a level seven, the maximum, on the International Nuclear and Radiological Event Scale. According to Japan's Nuclear Safety Commission, the accident led to a release of radiation equivalent to 630,000 terabecquerels of iodine-131. That month, a joint paper to Sage from the Office of Nuclear Regulation, the Met Office, the HPA and Rimnet stated: "The mechanism that could lead to a serious release remains unchanged, ie molten fuel coming into contact with the concrete base of a pond or reactor causing an explosion."

Other papers prepared for Sage outline the possible long term impact of the accident, which could see restrictions on food supplies in place for years and some regions around the plant ruled permanently out of bounds.

Workers at the site are still facing high radiation levels and contaminated waste water to keep three reactors and several ponds of spent fuel under control.

Report lists safety flaws

Protection added to Japan's nuclear plants after a 2002 review of tsunami risks fatally underestimated the hazards that emerged in the Fukushima disaster, the first independent assessment of the has found.

A report published on Monday by the International Atomic Energy Agency also found that:

Regulators had failed to review or approve the improved defences.

Accident plans could not cope with multiple plant failures.

Sites have no seismically robust buildings to shelter emergency teams.

Instruments essential to monitoring reactors were not sufficiently "hardened" against accidents

The risks of hydrogen explosions was underestimated.

There were "dedicated and devoted" workers available but in the crisis "complicated structures and organisations" resulted "in delays in urgent decision making".

Launching a four-day ministerial summit on nuclear safety in Vienna, the IAEA's head, Yukiya Amano, urged a worldwide review of measures to prevent future disasters.

But he added: "Even the best safety standards are useless unless they are actually implemented."