The Japanese government has announced that radioactive iodine from the damaged Fukushima Daiichi nuclear powerplant has been found in tapwater, and that infants should not drink it. However there is little reason for concern once the facts are understood.
Japan's Ministry of Health, Labour and Welfare says that tests have revealed levels of the radioactive isotope iodine-131 in tapwater samples in Tokyo that range from 100 to 210 becquerels/litre. The radio-iodine health limit in force for iodine-131 is 300 Becquerels/litre, but there is a separate limit for baby milk fed to infants less than a year old of 100 Bq/l – hence the recommendation.
As ever, we hear of "more than twice the safe level", though the BBC does add that "officials have stressed that children would have to drink a lot of it before it harmed them".
Indeed they would. As World Nuclear News makes clear, the health safety limits in question are based on a year's consumption: in other words, a baby could drink milk containing 100 Bq/l of radio-iodine for a year without ill effects. The same dose could be sustained by drinking the Tokyo tapwater (assuming continuous iodine-131 levels at the maximum so far seen) for a bit under six months.
You'd really struggle to achieve that, however, as iodine-131 has a half-life of just eight days. Most of what was in the plant when it shut down Friday before last has now turned into inoffensive Xenon, and the heat which drove initial emissions to atmosphere is now a small fraction of what it was in the first days of the incident. Levels in the water can be expected to peak soon and then drop, fast at first and then slowly, to almost nothing over a six-month timespan.
Higher levels have been seen in water supplies near the plant – one very high reading of 965 Bq/l iodine-131 has been recorded and another of 365, though that was last week following the height of the crisis and levels have since fallen to within the drink-it-for-a-year-no-bother safety limit.
Caesium radioisotopes have also been detected, but the effects of these should be minimal or nil based on experience following Chernobyl. The only health hazard presented by the Russian disaster was from iodine ingested in milk by children.
The Japanese announcement isn't totally unreasonable. It guards against a sudden worsening of the situation – for instance much more severe damage to a reactor core than anything yet seen, despite the fact that residual heating in the cores has now sunk to a low level.
On the whole though, the Japanese government seems to be facing the classic dilemma following a nuclear incident of any type. If you say there's no cause for concern, people will assume you're lying: if you then say there is cause for concern, you actually are lying. ®