Sieverts, the Fukushima evacuation zone, and today's government announcement

22 Aug 2011 by Jim Fickett.

At many locations within the current evacuation zone around Fukushima, radiation levels remain at levels above the government's safety limit (the threshold is a level that may raise lifetime cancer risk from 42% to 42.2%). Consequently, the government has announced that some areas may remain off limits for years. This will have a strong psychological effect on evacuees and others living near nuclear plants.

Much of the Japanese government's policy thinking around Fukushima, for example the decision to evacuate those within 20 km, is based on the idea that exposure should be kept below 20 milliSieverts (mSv) per year.

A Sievert is a medical risk unit

A Sievert is a medical unit, not a physical unit:

Because the term “radiation” applies to three different processes, it is not straightforward to figure out how it will affect the body. Gamma radiation is the one that most people probably think about when they think of radiation. This is the powerful stuff that can go through your body, and is used in a lot of medical imaging. It will hurt you for the time you're exposed to the source, but once you leave the area, there is no further risk.

By contrast, alpha and beta radiation cannot penetrate far beyond skin or clothing. These latter types are completely harmless if you're properly protected, but if you ingest or inhale some of the material … It's a bit like carrying an x-ray machine in your chest. …

To try and simplify radiation measurements, the international community has introduced the Sievert (Sv). It is actually a complicated quantity to compute, but it attempts to weight the various risks and provide a net value for the biological effects of the radiation detected, no matter the type.

20 mSv in a year might raise lifetime cancer risk from 42% to 42.2%

The web comic and scientific commentary site XKCD published a helpful radiation chart giving some idea of different exposure levels. One way to keep in mind what a Sievert represents is to remember that medical scans are typically between 1/2 and 7 mSv, and the average yearly dose for a person is about 4 mSv (about 85% natural background and about 15% medical). The yearly dose allowed for US nuclear workers is 50 mSv.

So how did someone figure out that 20 mSv is a reasonable threshold for safety? The best discussion of this point that I've seen can be found in a recent report from MIT:

“Since the cancers induced by radiation are the same types of cancers observed ‘naturally’, determining the number of additional cancers caused by a small dose of radiation when baseline cancer rates are already high has not been possible for doses in the 20 mSv range (or even higher).

Although no data have ever demonstrated that 20 mSv over 1 year results in measureable harm, this dose range has long been relevant to the occupational radiation protection field and thus there has been a need to generate radiation risk estimates, even in the absence of actual data. These estimates come primarily from the long-term evaluation of the A-bomb survivor population and are a result of adopting a hypothetical model of extrapolating the risk per unit dose at high dose levels down to the low dose range. … scientific bodies evaluating risk often specifically caution against … predicting the long term effects of small doses to a large population. Unfortunately, more applicable risk estimates do not exist and so this caution is routinely ignored when the potential impact of low doses is of interest.

The linear extrapolation model has long been viewed as a conservative approach to estimating radiation risk at low doses and, in particular, for low doses accumulated over long periods of time. It can be used, however, to generate an upper estimate of the risk posed by the radiation doses encountered from a contaminated environment. Using the linear extrapolation model, the U.S. National Academies of Sciences’ BEIR VII committee estimates that 1 cancer could result if 100 people received a single dose of 0.1 Sv (a risk of 0.01/0.1 Sv), with lower doses resulting in proportionally lower risk. Thus, a dose of 20 mSv (if delivered acutely) x 0.1 per 0.1 Sv = 0.002. In other words, the 20 mSv dose ceiling pursued by the Japanese authorities represents a 0.2 % chance of being diagnosed with cancer later in life, in addition to a 42 % risk an individual already faces from ‘natural’ causes. This estimate is expected to be high by a factor of 2-10 and possibly more, according to NCRP 64, to account for the reduced impact of protracted radiation delivery, relative to the same dose received all at once.

So in other words, the risk is not measurable, but an educated guess is that one's lifetime risk of cancer is raised at most from 42% to 42.2%, if exposed to 20 mSv.

Recent measurements of radioactivity in the Fukushima evacuation zone

Real data has been frustratingly lacking in many areas around the Fukushima disaster. There seems finally to be some fairly systematic data on radiation levels at many points within the evacuation zone.

The Nature News Blog points to survey data from 13 August, published by the Ministry of Education, Culture, Sports, Science and Technology. One document contains a map with readings in microSieverts/hour (multiply by 8.8 to get mSv/year). Taking 2.3 microsieverts/hour = 20 millisieverts/year as a threshold, a short summary is that most locations within 10 km show readings above the government threshold, while about half between 10 and 20 km do (other reports show that some locations outside the evacuation zone altogether also have readings above the limit).

(Click for larger image.)

Government says some will not be returning home any time soon

Up until now the government has encouraged people to think the evacuation order would be lifted once cold shutdown was achieved at the reactors. This has been doubted by many observers, so it came as no surprise today when the government announced that some areas might remain off limits for a long time. The Wall Street Journal reports:

“Some places may have to be kept off-limits to residents for a long period of time even after clean-up operations are undertaken,” Chief Cabinet Secretary Yukio Edano said at a news conference. His comments followed Friday's announcement that levels of radioactive contamination were higher in some areas in the 20-kilometer evacuation zone than were found in the plant compound itself. …

Other government officials noted that decontamination is possible but will take time. Goshi Hosono, minister in charge of the Fukushima crisis, stressed that “nothing has been decided on the evacuation policy, and the desires of local residents will come first in any decision.”

The total area involved is small, even in the context of a small island country, but the psychological effect on those who cannot return home, and on others living near nuclear plants, will be large.