April 12, 2011

expert reaction to the raised severity rating at Fukushima

The Japanese government raised the severity level of the Fukushima incident to 7, the highest on the scale.

Prof Richard Wakeford, Visiting Professor in Epidemiology at the University of Manchester, said:

“The Japanese authorities have temporarily assessed the Fukushima events as Level 7 on the International Nuclear Event Scale (INES) on the basis of the estimated quantities of radioactive iodine and caesium released since the earthquake on 11 March – around 10% of the releases from the Chernobyl accident, the only other INES Level 7 event. However, the Chernobyl accident had a much greater radiological impact, and the Japanese authorities have acted to limit the radiation doses received by both emergency workers and members of the public. Indeed, the only INES Level 6 event – the ‘Kyshtym’ accident in the USSR in 1957*, when a radioactive waste tank exploded – had a much greater radiological impact than is predicted from the events at Fukushima.

“The Japanese authorities have extended the 20 km evacuation zone around the Fukushima site to include some additional communities that have been particularly contaminated by releases of radioactive material. It has been known since mid-March that a sector to the north-west of the site had been affected by a plume of radioactivity released a few days after the earthquake on 11 March, and the Japanese authorities had banned the consumption of contaminated foods from the area as a consequence. Radiation levels from deposited radioactive material have fallen since the deposition, but have remained sufficiently elevated to warrant the evacuation of people. Most other areas, including those within the 20 km evacuation zone, are less contaminated, but it is unlikely that people will be able to return to the evacuation zone until the Fukushima site is under control.

“The only INES 6 is the “Kyshtym” accident (at Mayak) in 1957 when an explosion in a waste tank led to 740 PBq (740 000 TBq) of fission products being released (10% beyond the Mayak perimeter), >10 000 residents being evacuated with an average effective dose of 120 mSv, >5000 workers receiving doses up to 1 Sv within a few hours, and ~30 000 clean-up workers receiving doses >250 mSv during 1957-59.”

Dr. J.T. Smith, Reader in Environmental Physics at the University of Portsmouth, said:

“The preliminary grading of the Fukushima accident as Level 7 on the International Nuclear Events Scale reflects the severity of radionuclide releases to land and sea. But there are key differences between Fukushima and Chernobyl, the only previous Level 7 accident. From the data so far, it seems that about ten times more radioactivity was released at Chernobyl. Crucially, key health protection countermeasures have been put in place at Fukushima. At Chernobyl, local people were not evacuated until about 48 hours after the accident: children were still playing outside as the reactor burned, and potassium iodide tablets were not distributed. In the weeks after Chernobyl, people continued to eat milk and leafy vegetables highly contaminated with radioactive iodine.

“By evacuating and sheltering the population, implementing food bans, and distributing potassium iodide tablets, the Japanese authorities will have prevented the most serious health effects of Fukushima. Levels of radioactive iodine are declining rapidly by radioactive decay – current levels are about a tenth of what they were when the accidents happened. But, it is clear that there has been a major contamination of land and sea. The effects of this are being seen now, with evacuation and food bans in place over a wide area. It appears from the data available so far, that there has been significant radiocaesium contamination, particularly of an area of land to the northwest of Fukushima. Radiocaesium remains in the environment for the long term and initial data suggests that this could mean that countermeasures (for example, food bans) will need to be in place in some areas for decades.”

Prof Laurence Williams FREng, Professor of Nuclear Safety at the John Tyndall Institute, University of Central Lancashire, said:

“I am a little surprised by the uprating to level 7. On the basis of the publically available information there has been no significant change in the state of the 3 affected reactors or the 4 spent fuel ponds and there has been no sudden increase in radioactivity released into the atmosphere.

“I can only assume that the Japanese authorities are taking a very conservative line given that the 3 reactors are not yet under control and the conditions in the fuel storage ponds in reactors 1 to 4 where there are significant quantities of spent nuclear fuel is also not fully under control.”