ORIGINALLY PUBLISHED: 5.22pm GMT
UPDATE 1: Addition of data on Japanese average dose
UPDATE 2: 8.48pm, correction to Tokai radiation values
Radiation levels in the wider area around the Fukushima Daiichi nuclear power plant trended downwards for several days, but have recently increased on rainfall. Contamination of food appears limited at present.
Conditions for workers on the plant site have been alleviated somewhat in the last few days thanks to successful efforts to refill fuel ponds. But safety considerations there are on a wholly different scale from those in nearby areas already evacuated or further afield in cities such as Tokyo.
According to the Research Organisation for Information Science and Technology, a typical person in Japan receives an average radiation dose of about 3750 microsieverts per year. Some 60% of this is voluntarily received from medical procedures - such as X-rays and CT scans. A CT scan of the chest can contribute some 7000 microsieverts, according to Japan's Ministry of Education, Culture, Sports, Science, and Technology (MEXT).
A nuclear worker by contrast is allowed to receive a dose of up to 20,000 microsieverts per year, although in practice they often receive very much less. In emergency situations it is acceptable for workers to receive up to 100,000 microsieverts. Below this it is statistically difficult to connect radiation dose to cancer rates, but above this the relationship starts to become apparent. An exception has been made in the current crisis to allow emergency workers to receive exposures up to 250,000 microsieverts. Only one worker involved in a steam venting operation some days ago has been confirmed to have received more than the 100,000 microsievert level.
Radiation detectors in central Japan have shown elevated readings since the Fukushima accident worsened on 15 and 16 March, with venting from reactor containment, the apparent damage to unit 2's torus suppression chamber and fires and explosions at units 1, 3 and 4.
Detectors in several regions, but especially to the south in neighbouring Ibaraki prefecture, have shown clear spikes in radiation. At around 7.30am on 15 March one meauring point showed 5 microsieverts per hour in Tokai village as a short-term peak, but this decreased (with some oscillation) to less than 1.0 microsieverts per hour on 20 March. This rose again to just under 3 microsieverts per hour as of 6:00pm on 21 March, most likely due to the widespread rainfall in the area. Measurement of background radiation for Ibaraki provided by MEXT show a range of 0.036 microsieverts per hour to 0.056 microsieverts per hour.
These readings are caused by the passage overhead of a mixture of gases and possibly fine particles from the nuclear power plant, which disperse with distance. In addition the radionuclides involved are also decaying, and thus reducing in potency. The readings represent a value for a single location at a single time and so give only a rough idea of what any member of the public in that area may have actually been exposed to. At the rates described above the passage of the materials will have had no measurable effect on the general public's health.
The possibility of higher exposure to the public nearer the power plant has been precluded by the early evacuation of the people closest to the plant. An evacuation of the first two kilometres was ordered within hours of the earthquake and tsunami when it was clear that the Fukushima Daiichi plant was in trouble. Among the evacuees would have been most power plant workers as well as their families, along with other residents.
This zone was subsequently widened to three kilometres, then ten, then to 20 kilometres as people were cleared from the highest priority areas and the situation developed. Currently the zone stands at 20 kilometres with people in a further ten kilometres recommended to stay indoors. The Japanese government has given advice to populations in the wider area to help avoid contamination during rain showers.
Despite unhelpful contradictory comments from some overseas nuclear regulators, the World Health Organisation backed the Japanese authorities, saying "These recommendations are in line with those based on accepted public health expertise."
Monitoring of food
The presence of certain radioisotopes in food has been confirmed by government checks for iodine-131 and caesium-137. Some have been noted to be above the Nuclear Safety Commission of Japan's conservative indices, which are for the time being enforced as regulation in Japan.
The indices are based on a person's consumption of the foods at elevated levels for a whole year, not simply a single meal. Iodine-131 has a half-life of eight days, meaning its risk passes in a brief period after a release. Caesium-137, however, has a half life of almost 31 years and so could potentially present a longer-term risk to agriculture. Both of these would be subject to the action of water over time, which may wash contaminants to sea depending on the kind of ground upon which they were deposited.
In Ibaraki prefecture some samples of leaf vegetables, such as spring onions and spinach, have been measured above index levels of 2000 becquerels per kilogram for iodine-131, or of 500 becquerels per kilogram of caesium-137.
In the town of Kawamata, three milk samples showed above 300 becquerels per kilogram in iodine. Caesium-137 was detected in one sample but in concentrations below the indices.
As yet, 40 of 46 tests of drinking water have shown no detection of iodine-131 or caesium-137. Six have exhibited signs of iodine-131, said the International Atomic Energy Agency, though the concentration was below emergency monitoring criteria.
It is possible that current rainfall will bring contaminants to earth more quickly than would happen in dry conditions. This could of course result in more finds of contamination above the index levels in coming days.
Potassium-iodide pills have been distributed to evacuation centres to protect against the possibility that iodine-131 could be absorbed by a person's thyroid gland, which is the clearest risk of cancer following this kind of radioactive release. People leaving the 20 kilometre zone to travel through the further ten kilometre zone have been recommended to take the appropriate dose of potassium iodide since 16 March.
Researched and written
by World Nuclear News