Tuesday, April 12, 2011

Even More Radiation Stuff From Japan

The IAEA posted that the accident is now provisionally rated a l International Nuclear and Radiological Event Scale (INES) 7, the same rating a Chernobyl. Before it had rated each of the three reactors as 5 (TMI) and the reactor four spent fuel pool at a level 3 incidents. It make perfectly good sense that the total event be rated a level 7 "provisionally". INES level 6 may apply, Serious Accident, that really depends on the total radiation released. Right now, the radiation released is listed at "An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of terabequerels of I-131." 10,000 terabequerels is 100,000 times 10^12 becquerel or 100 time 10^15 becquerels, one terabecquerel is 27 Curies, 100,000 terabecquerels is 27,000,000 Curies. A Becquerel is a very small amount, 27,000,000 Curries is not a small amount. How much radioactive material was released is difficult to tell. The IAEA report mentions that the release of radioactivity is about 10% of the total released by Chernobyl. So that is a lot. The big question is how much fallout of which type of isotope is on the ground around the Fukushima power plant and in what concentrations. Most of the radiation fell harmlessly in the sea, but some areas do appear to be highly contaminated.

From the IAEA Facebook release:
"On 11 April, deposition of both iodine-131 and cesium-137 was detected in 6 and 8 prefectures respectively. The values reported for iodine-131 ranged from 2.1 to 35 Bq/m2 and for cesium-137 from 5.2 to 41 Bq/m2.

Gamma dose rates are measured daily in all 47 prefectures, the values tend to decrease. For Fukushima, on 11 April a dose rate of 2.1 µSv/h, for the Ibaraki prefecture a gamma dose rate of 0.15 µSv/h was reported. The gamma dose rates in all other prefectures were below 0.1 µSv/h.

Dose rates are also reported specifically for the Eastern part of the Fukushima prefecture, for distances of more than 30 km to Fukushima-Daiichi. On 11 April, the values in this area ranged from 0.2 to 25 µSv/h.

In an additional MEXT monitoring programme, on 11 April measurements were reported for 25 cities in 13 prefectures. In Fukushima City, a value of 0.42 µSv/h was observed. In all other cities, gamma dose rates ranged from 0.04 to 0.13 µSv/h. Typical normal background levels are in the range of 0.05 to 0.10 µSv/h.

On 11 April, the IAEA Team made measurements at 9 different locations in the Fukushima area at distances of 30 to 58 km, West to Northwest from the Fukushima nuclear power plant. At these locations, the dose rates ranged from 0.1 to 2.2 µSv/h. At the same locations, results of beta-gamma contamination measurements ranged from 0.01 to 0.28 Megabecquerel/m2.

Analytical results related to food contamination were reported by the Japanese Ministry of Health, Labour and Welfare on 11 April, and covered a total of 21 samples taken on 8 April and 10 to 11 April. Analytical results for all of the samples of various vegetables, spinach and other leafy vegetables, fruit (strawberries), various meats (chicken, beef and pork), seafood and unprocessed raw milk in eight prefectures (Fukushima, Gunma, Hyogo, Ibaraki, Miyagi, Niigata, Saitama and Yamagata) indicated that I-131, Cs-134 and/or Cs-137 were either not detected or were below the regulation values set by the Japanese authorities."

In the Bold, the hottest area measured for both microsieverts and Mega(10^6) Becquerels was 2.2 microSV/h with 0.28 MegaBq/m2. So there is significant fallout is this area, probably more in the area closer to the power plants. The source in this case was not listed. If the main source is Iodine 131, its half life is 8 days, so it is less of an issue long term. Cs 137 has a half life of 30 years, so that is a long term problem.

Cs 137 melts or turns liquid at a low temperature. So summer heat should disperse the Cs 137 into the soil deeper, reducing its impact. Its concentration is much stronger on the surface of the soil. If the level is low enough to allow farming, tilling the soil will further mix the Cs 137 greatly decreasing its radiation hazard. While it will take many years to degrade, it is not typically taken into plants growing in radiated soil, in any significant amount.

There are a few odd hot spots found to the Northeast of the power plants. While it is likely that the radiation is due to the reactor accident, it is not uncommon to find a mix of natural deposits, previous spills and fallout when doing in depth radiation testing. Depending on the type and quality of the meter used in testing, what is attributed to I 131 and Cs137 may be caused by other isotopes interfering with the readings. That is something that will be determined once the reactors are back under full control. Note that some of the more accurate tests require a day or two of laboratory time to process.

The amount of radiation noted in the IAEA report is considerably higher than what would be expected give the design of the reactors. That may be due to radiation from the spent fuel pools, the radioactive water leakage, or increased venting while trying to temporarily cool the reactors with available water supplies early in the event.

While the INES event level news is not unexpected or comforting, the tests of the food products in the Fukushima region is encouraging. That would tend to indicate that most of the radiation was either from I131 which has degraded or Cs137 that was washed into the soil by precipitation. Most of the radiation test points have shown reductions in radiation since fresh water cooling of the reactors was re-established. That is very encouraging.

The biggest concern is of course controlling the reactors to spot further release of radiation. That process is painfully slow, but progress is being made. The second concern, since the evacuations appear to have been effective, is the degree of soil contamination.

On April 1st, the MEXT listed one of the highest test points as point 83 west and a little north of the power plant with a high reading of 70.9 MicroSV/h. That same point on 12 April had readings of 53.5 at 14:44 and a second reading of 5.3 at 16:45 while it was raining. So radical drops of radiation should happen as more rain washes the radiation into the soil or there is runoff.

Different organizations are taking readings with different equipment. So there will continue to be confusion. If you live in the area or are concerned you can check the MEXT site for radiation updates posted daily.

It will take some time for the situation to stabilize enough for consistent readings to be made. Until then, the best source I have found is the MEXT site which includes maps of the area showing test points and readings. Despite the frustration and uncertainties, the radiation levels appear to be dropping fairly well indicated less long term impact.

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