Consider this news story (and accompanying video) from the BBC about radioactivity in the sea within 300 m of the Fukushima nuclear plant:
Levels of radioactive iodine in the sea near the tsunami-stricken Fukushima nuclear plant are 1,250 times higher than the safety limit, officials say.
The readings were taken about 300m (984ft) offshore. It is feared the radiation could be seeping into groundwater from one of the reactors.
But the radiation will no longer be a risk after eight days, officials say. [Emphasis added]
There is no explicit by-line on this article, but the video contains an interview with BBC reporter Chris Hogg in Tokyo that repeats that a half life of 8 days means "that after 8 days the risk will have dissipated".
The reporter is WRONG. Twice, because that is also not what the officials said. His ignorance of basic physics, in this case a topic I always teach in a college general education class, led him to misinterpret what was actually said by a government spokesman and hence mislead the public.
The risk will not dissipate after 8 days.
First, what did the official say? The article reports that
"Generally speaking," spokesman Hidehiko Nishiyama told a news conference, "radioactive material released into the sea will spread due to tides, so you need much more for seaweed and sea life to absorb it." He continued: "And, since [the iodine] has a half-life of eight days, by the time people eat the sea products its amount is likely to have diminished significantly."
This (emphasis added) is correct.
The most significant effect is dilution. Levels will be very high near the source, but get reduced significantly as the source gets mixed into a larger volume of water. (Like smoke when you are a long way from a fire.) It also matters what the ratio is of radioactive iodine to the iodine that is naturally in the water, since the seaweed can't tell the difference.
It is also correct because in 8 days the radiation level in the seawater will be half what it is today. Half. Not "dissipated", half. The radiation level of the iodine in that bay will only fall to 625 times the safe level, not zero, in those 8 days. The risk is reduced, but not gone.
But there are other factors. For example, I have no idea how long it takes from the time seaweed is harvested and when it shows up on shelves all nicely dried and packaged, but it is unlikely to be a few days. Only fresh items like milk and vegetables appear "just in time" in supermarkets. Radiation drops every day it sits in a warehouse. It falls to half after 8 days, one quarter after 16 days, one eighth after 24 days, and one sixteenth after about a month.
The obvious fact (pull out your calculator) is that what remains after a month is still 1250/16 = 78 times the safety limit if all of the iodine stayed in the ocean near the plant. This shows why dilution is so important.
Side comment 1:
The other contaminant in seafood, mercury, does not go away with time and is not as easy to detect and monitor as radiation from I-131. I-131 emits a gamma ray which can be detected through the usual plastic packaging used for seaweed, right on the shelf. Mercury requires a careful (and destructive) chemical test.
Side comment 2:
The reporter quite correctly puts attention on cesium, which has a 30 year half life. Isotopes with very short half lives are "hotter" but go away quickly, so you just have to keep your distance for a month or two. Gram for gram, cesium isn't as hot but you have to avoid it for a longer time. That can be hard to do.
However, this ignores the other significant factor, which is biological activity. Our body needs a regular supply of iodine, so it will go looking for it in anything you eat. (Naturally iodine deficient diets in the Ukraine contributed to the uptake of I-131 after Chernobyl.) Further, it gets concentrated in one place, the thyroid.
Cesium (Cs) is in the same chemical family as sodium (Na), in table salt, and potassium (K), in sports drinks and bananas, which are both essential to the operation of our body. However, since it is much heavier, I doubt if it can substitute for the many ionic processes the body uses Na and K for. Any biologist or chemist know if Cs is concentrated by the body?
By the way, one reason I knew this was a major incident was that Cs-137 could be detected above background in California. You see, it takes a significant release to see it above the Cs-137 that still remains from atmospheric nuclear testing. As big as Chernobyl was, its Cs-137 was barely detectable over the stuff left from weapons tests done decades earlier once it got diluted by one trip around the globe.
Side comment 3:
There has been no new I-131 made since fission was stopped on March 11, fifteen days ago. That means only one quarter of the original I-131 remains in the fuel rods inside the three reactors that had been operating at the time of the quake.
The most important thing in this article might be that the levels in sea water had increased by a factor of 8 in the past week. That means I-131 from inside fuel rods inside the reactor vessel is not only finding its way into the water, but a larger fraction of it has been released from the fuel rods. (There is less I-131 available to leak out, but more of what remains is getting out. Did I say that clearly enough?) This is further indication that the fuel rods have been damaged significantly, which we already knew, but might just result from the iodine -- already vented from the reactor vessel -- being washed out of the containment building as they can now pour more water onto and into the containment building.
Side comment 4:
That observation in the quotation at the top of this article, that radiation "might" be seeping into ground water, struck me as strange. There is I-131 in Tokyo drinking water. This is because Tokyo's water supply comes from surface water (mostly behind dams based on a city water department document I found), which will be contaminated by radioactive rain carrying I-131. But everyone should know that rain also soaks into the ground. Apart from geochemical processes that would capture iodine, it will go into the ground water.