New UNSCEAR Report on Fukushima: Collective Doses

On April 2, UNSCEAR published its long-awaited Report on Fukushima. Of prime importance are its estimates  of collective doses to the Japanese population.

Page 60 of Annex A of the UNSCEAR report contains the following table 8 on estimated collective effective doses and collective absorbed doses to the thyroid for the population of Japan (approximately 128 million in 2010).

Exposure duration Over first year Over ten years Up to age 80 years
Collective effective dose 18,000 man-Sv 36,000 man-Sv 48,000 man-Sv
Collective absorbed dose to thyroid 82,000 man-Gy 100,000 man-Gy 112,000 man-Gy

These estimates are slightly higher than in the draft UNSCEAR report in November 2013. For example, the 80 yr whole body dose was 41,000 man Sv and thyroid dose was 110,000 man Sv in last year’s draft.

In an early preliminary view, these are realistic collective doses, as they are relatively consistent with some independent estimates in Europe. For example, the most detailed model used by the Report published by IPPNW Germany in late March 2013 estimated 95,000 man Sv: ie the UNSCEAR 48,000 man Sv estimate is within a factor of 2 of this, which is good agreement given the uncertainties in the IPPNW’s methodology and in this area generally.

My own estimate in early March 2013 for Fukushima Prefecture (the most contaminated region) alone was 34,000 man Sv.  If I were to add an estimate for the rest of Japan of ~13,000[1] man Sv, this would total 47,000 man Sv – very close to UNSCEAR’s estimate of 48,000 man Sv. In fact, the agreement is slightly unnerving! :-)

In terms of the fatal cancers these doses would cause, the new  UNSCEAR estimates imply (via the LNT theory) that in future ~5,000 people in Japan will die from Fukushima’s fallout, if we applied a fatal cancer risk of 10% per Sv. (This is because the UNSCEAR report like the previous WHO reports no longer applies a DREFF of 2 to risk estimates.)

However a more detailed scrutiny will be required of the methodologies and assumptions used in the new UNSCEAR report before a final view can be given.

PS. The UNSCEAR report on page 60 adds that “The collective effective dose to the population of Japan due to a lifetime exposure following the FDNPS accident is approximately 10-15% of the corresponding value for European populations exposed to radiation following the Chernobyl accident. Correspondingly, the collective absorbed dose to the thyroid was approximately 5% of that due to the Chernobyl accident.”

From this, one can work out what UNSCEAR now thinks the whole body collective dose to Europe was from Chernobyl: ie 320,000 to 480,000 man Sv, leading to ~32,000 to ~48,000 fatal cancers. This has never been stated before by UNSCEAR. These estimates are close to the 2006 independent TORCH report’s estimates of 30,000 to 60,000 fatal cancers.

[1] UNSCEAR estimates the average whole body dose to the rest of Japan outside Fukushima Prefecture was ~0.1 mSv. Multiplied by 126,000,000 people outside Fukushima gives a collective dose of 12,600 or 13,000 to 2 sig figs.

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