ISRN on Fukushima radiation effects; I-131 thyroid exposure by inhalation

French public radiation-protection agency Institut de radioprotection et de sûreté nucléaire released a report Monday (in French only). Based on the French standard of 10 mSv/year to the public after a nuclear accident, they are calling for further evacuations of an additional 70,000 Fukushima residents.

[AFP] 70,000 more should evacuate after Fukushima: Watchdog

Staying in this area means the inhabitants would be exposed to radiation of more than 10 millisieverts (mSv)in the year following the disaster, according to the IRSN.

This level is used in French safety guidelines for protecting civilian populations after a nuclear accident. In France, 10 mSv is three times the normal background radiation from natural sources.

"Ten mSV is not a dangerous dose in and of itself, it's more a precautionary dose," said Champion, noting however that this figure that does not include any additional doses from contaminated food or water.

(If I remember right, Japan is using a 20 mSv/year standard).

(summary) [IRSN] Gestion radiologique des territoires contaminés à la suite de l’accident de Fukushima : l’IRSN rend public son rapport d’analyse de la situation

(report) [ISRN] Evaluation Au 66^eme Jour Des Doses Externes Projetees Pour Les Populations Vivant Dans La Zone De Retombee Nord-Ouest De L’Accident Nucleaire De Fukushima - Impact Des Mesures D’Evacuation Des Populations

Since the report is in French only for now, my comprehension is limited. Here is the map they are using, similar to a dozen other maps I've linked on this blog (except that this one depicts city population sizes). Their recommended evacuation is on the green (10 mSv in 1st year) line, following French standards. (Note that if the standard were more conservative by just a factor of two, several major cities would be encompassed, including over one million people. Or: about one million people will be exposed to ~5 mSv external dose this year, the equivalent of a CT scan.)

By their accounting, this corresponds to a combined ¹³⁴Cs/¹³⁷Cs deposition level of 600,000 Bq/km², or an outdoors dose rate of 2 μSv/hr. (There's no inconsistency there; people do not spend all their time outdoors, and buildings shield gamma radiation somewhat.) I made a graph of how this dose rate will decay over time:

(¹³⁴Cs has a t_1/2 of 2.07 years, and ¹³⁷Cs has a t_1/2 = 30.08 years. Based on surveys (two are below), the ¹³⁷Cs/¹³⁴Cs activity ratio is currently about 1.2. Comparing the decay radiation (¹³⁴Cs, ¹³⁷Cs), ¹³⁴Cs gammas are about 2.7 times more energetic (there's also a small contribution from beta rays, if they're not shielded). So the initial dose ratio is around 2.5 in favor of ¹³⁴Cs; this goes away fairly quickly.)

[MEXT] Readings of dust sampling

Interim Report on Radiation Survey in Iitate Village area conducted on March 28th and 29th

The report also mentions potential ¹³¹I thyroid doses to children in excess of 1 Sievert. (Note that Sieverts are intensive quantities, they have units of J/kg. A dose to an individual organ is not the same as a whole-body dose.) This is the main risk factor for thyroid cancer, one of the most destructive effects of the Chernobyl disaster. (I don't know off hand what doses were involved there, or what thyroid doses are considered dangerous).

¹³¹I ingestion (e.g. through contaminated milk) was probably limited by food bans. But, in Nuclear Safety Commission data I found (in Japanese), they're talking about internal doses through inhalation of iodine from air:

[NSC] 文部科学省　緊急時迅速放射能影響予測ネットワークシステム（SPEEDI）による計算結果

This map is made on the (conservative) assumption of an infant staying outdoors 24 hours a day (it's cumulative over the first month or so):

Not sure what to make of this.