There’s now much more data emerging on radiation levels around Fukushima and beyond, compared with the utter dearth last week. Indeed, there is now so much disparate data from many sources, in different units, and on various aspects of radiation, that there still seems much confusion about what it all means, and although a clearer picture is slowly emerging even experts sometimes don’t seem sure and are awaiting further data.
Some data released on Tuesday from a US DOE aerial monitoring survey of the immediate vicinity of the plant carried out on 17-19 March suggest that higher levels of radioactivity were only deposited in a narrow band up to 25 km northwest of the Fukushima plant. Jim Smith, an environmental physicist at the University of Portsmouth, UK, says he was “relieved” by the data, as they suggest contamination around Fukushima will be much lower than after Chernobyl. But the rates in the hot zone would nonetheless cause significant radioactive exposure to anyone staying there for long periods, and are therefore likely to be uninhabitable for some time, he says. The length of time such an ‘exclusion zone’ might be enforced would depend on the mix of radionuclides there. A large amount of cesium isotopes, with half-lives of years, would mean that evacuations and bans on farming could be long-term, he says. That may also be the case for the pockets of high radioactivity found by IAEA and Japanese teams as far away as 60 km from the plants, and outside of the zone surveyed by DOE. Such pockets aren’t surprizing, as the spread of radioactives plumes isn’t uniform and depends on weather conditions.
MEXT, the Japanese science ministry, today released new environmental data. Radioactivity readings for soil and pond samples were highest at one location 40 km northwest of the plant. On 19 March, upland soil there contained 28,100 Bq/kg of Cs-137 and 300,000 Bq/kg of I-131. One day later, these same figures were 163,000 Bq/kg of Cs-137 and 1,170,000 Bq/kg of I-131.
My colleague Geoff Brumfiel has written excellent accounts of what all these units mean, here, and here.
This evening, I asked Jim Smith for his take on these new data. Here’s what he had to say:
“These are really interesting data. The difference from one day to the next may well not be significant, because activity concentrations in soils may be very variable. The higher reading on the second day isn’t necessarily due to a fresh release, it may be because a slightly different site has been sampled. To calculate a deposition value (Bq/m^2) accurately from these data, you need to know the depth to which the soil was sampled, the soil density and whether the readings are per unit wet or dry weight (I guess the latter, as it is standard). Given that we don’t know the sampling depth it’s difficult to interpret these data. But some of the soil readings are very high and would be consistent with maximum dose rate estimates of 125 microSv/h or more, similar to those seen in the US DoE aerial survey of the area closer to Fukushima. These new measurements in soil confirm the previous reports by the IAEA of likely very high radiation levels at more than 20 km from the site. I would expect that in areas where dose rates were of order 100 uSv/h, evacuation would be necessary, if it hasn’t been done already. If there are significant areas of Cs-137 soil concentration of order 100 000 Bq/kg, evacuation of these areas could be effectively permanent. More detailed maps of Cs-137 surface contamination density (i.e. Bq/m^2) would confirm this.”
Meanwhile Kyodo news today reported that water exceeding safety levels for infants had been found at water purification plants serving several cities, including Hitachi in Ibaraki, where levels were detected of 298 Bq/kg of I-131, almost three times the safe limit for infants, and bordering the 300 Bq/kg limit for adults. Levels above safe limits for infants were also reported in Chiba prefecture, and for a brief period at the city of Kawaguchi. I-131 levels in water above recommended limits were first found at a Tokyo plant yesterday, after which officials advised against giving young infants tap water, for example in milk formula. Thatadvice was lifted today after levels fell to 79 Bq/kg. At least I-131 has a short half-life of 8 days – so far I haven’t seen any data on levels in water of longer-lived cesium isotopes.
Some other sites providing data which you may find useful are listed below:
Austria’s weather service, the Central Institute for Meteorology and Geodynamics in Vienna
Graphs of Earthquake and Radiation Data in Japan
If you know of other good data sources, do let us know using the Comments facility below. We’d also love to hear from any scientists in Japan working firsthand with emerging field data.
For full coverage of the Fukushima disaster, go to Nature’s news special.