Japanese Earthquake Update (18 March 10:15 UTC)OK, so what does this mean?
Japanese authorities have informed the IAEA that new INES ratings have been issued for some of the events relating to the nuclear emergency at the Fukushima Daiichi and Daini nuclear power plants.
Japanese authorities have assessed that the core damage at the Fukushima Daiichi 2 and 3 reactor units caused by loss of all cooling function has been rated as 5 on the INES scale.
Japanese authorities have assessed that the loss of cooling and water supplying functions in the spent fuel pool of the unit 4 reactor has been rated as 3.
Japanese authorities have assessed that the loss of cooling functions in the reactor units 1, 2 and 4 of the Fukushima Daini nuclear power plant has also been rated as 3. All reactor units at Fukushima Daini nuclear power plant are now in a cold shut down condition.[UPDATE - that's Dai-no, Number-2, not Dai-ichi, Number-1 as I first thought]
Contrary to several news reports, the IAEA to date has NOT received any notification from the Japanese authorities of people sickened by radiation contamination.
In the report of 17 March 01:15 UTC, the cases described were of people who were reported to have had radioactive contamination detected on them when they were monitored.
Category 5 means that places far away have had their background count increase.
It means there's serious damage, which might or might not include some melting, in #2 and #3 reactors.
OK, let's look at the long-term contamination. As far as I'm aware, the kinds of isotopes in the Japanese fuel rods will be broadly similar to the ones in Chronobyl. There are some differences in reactor #3 as it uses MOX fuel (ie it burns plutonium from scrapped bombs), but it will do as a first approximation.
Note that this is the relative contribution of each: at 10,000 days, the amount of Cesium-137 is only half as much as it was on day 1, and the effects of the rest are negligible in comparison. So if the Cesium-137 contributes 2% of the total now, then in 30 years time, the actual increase in background from all causes will be ~1% of what it is today.
The three nasties are Iodine-131, Cesium-137, and Strontium-90. All are easily absorbed by the body, so let's look at those:
Iodine-137 is concentrated in the thyroid - and after 100 days, it's negligible in effect. Within 30 days though, especially in the first few, it's nasty, and can cause thyroid cancer. Taking iodine tablets provides a large degree of protection, so if you do that on the first day, and continue for 3 months, you largely negate the problem.'
Cesium-137 is chemically similar to potassium, and only half is gone in 30 years. That means that although it's easily gotten rid of by sweating, vegetables and fruits grown in the contaminated area should not be eaten, as it's too easily absorbed by them.
Strontium-90 is chemically similar to Calcium, so is deposited in bones. Although its contribution is small, it isn't easily gotten rid of by the body, once it's in, it stays in. This makes it far more dangerous than its minor contribution would be otherwise. It too has a half-life of 30 years, meaning half of it is still around 30 years later. Drinking lots of uncontaminated milk and eating lots of uncontaminated cheese is wise. Conversely, drinking contaminated milk is a Bad Idea(tm). However, most of the stuff will remain on-site, and it's only the venting of steam containing it that's a worry. Unfortunately, we've had to do exactly that at reactors #2 and #3.
OK, to summarise: it looks like the containment on #2 and #3 is not 100%. At least one has been generating spikes, and release of steam at this point or later will contain long-term nasties that earlier releases did not. Same with the spent fuel rods, these need covering with water ASAP.
Although there's always a possibility of accidents during the clean-up, providing no more venting is needed, we've had all the long-term consequences we're going to. The increase in continuous background, as opposed to readings during the transient spikes from short-lived isotopes, is many tens greater than what it will be long-term.
In places like Tokyo, the increase in continuous background is too small to measure. In the long-term, it might be 1/20 of that minute amount. We don't have enough data to say about places closer, but the "trough" readings at the gate indicate a background count that's still safe. I wouldn't eat vegetables grown next to the reactor though. Not even 1 km away. 10km? Not enough information, there's too much uncertainty to say that would be 100% safe. "Very probably completely safe" isn't the same as "100% guaranteed", and there's still plenty of scope for accident.
And #3 is still emitting 3.3 Rem per hour (down from 3.5 before the firehoses were used) , far too high for anyone to work there for more than an hour a year. Cleaning it up will be tricky, and #2 is damaged too.
[UPDATE 1100 JST] In order to stop confusion about microSieverts, milliSieverts, and Rem, I'll adopt a rule: non-dangerous dose rates will be in microSieverts/hr. Dangerous dose rates will be in Rem/hr.
Inside #1 reactor, it's 1.0 Rem/Hr. Inside #2 reactor, it's 1.5 Rem/Hr. Several workers have now exceeded a dose of 10 Rem. The upper limit for powerplant workers has now been raised to 15 Rem, and for SDF soldiers and firefighters, 25 Rem. Anyone who takes 10 Rem or more is evacuated from the site and not allowed to return.
The usual annual dose considered safe over a year (with some margin of safety) is 5 Rem. The average dose everyone receives in a year no matter where they live is 3.5 Rem. Where I live, at 200 metres altitude near granite mountains, and in a brick house, it's estimated as 4.1 Rem.
No biological effects can be measured from 20 Rem. There is no statistical difference in cancer rates at 10 Rem. There are no symptoms of Radiation Sickness at below 100 Rem. There is no danger of death below 250 Rem. But at a dose rate of 3.3 Rem/hr, that last recorded at #3 reactor, you hit your maximum allowable dose for the year in 2 hours. 3.5 Rem from the last year of living, and 2 x 3.3 Rem for the 2 hours spent at #3.