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.
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).
(report) [ISRN] Evaluation Au 66eme 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 t1/2 of 2.07 years, and 137Cs has a t1/2 = 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.)
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:
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.