Why is iodine-134 a problem?


Question:


TEPCO announced that it measured a large amount of iodine-134 in water from the basement of the turbine building attached to the unit 2 reactor. The news has caused an outcry, but what is so special about iodine-134 compared to other isotopes of iodine? I would like to know how to read the table. (30s, Tokyo)

Answer:

Iodine-134 is one of the radioactive isotopes of the ordinary stable iodine-127. Another of its radioactive iodine isotopes, iodine-131, is now found in the environment and some vegetables. If finding iodine-134 is scarier than finding iodine-131, what is that makes iodine-134 so special?

Let us first look at the table in question. The below lists the measurement of radioactive substances found in water in the basement of the turbine building attached to the unit 1, 2, 3, and 4 reactors.


The left-most column lists the radioactive substances tested for the measurement. The result confirmed that the reactor containment has failed to some degree, releasing portions of radioactive substances to the outside.

The third column shows the radiation density for each type of radioactive substance. The table says that iodine-134 (“I-134”) was detected 2.9 billions times every second per cubic-centimeter. The figure is large, even when compared to other substances, but the problem isn’t the amount.

The problem is the number in parenthesis to the right in the first column. The number is what is called half-life, the amount of time a radioactive substance takes to decrease by half. This means that Iodine-134 should decrease by half in 53 minutes. Most of iodine-134 is produced by fission reaction, so the initial amount should be halved in about one hour after the reactor was shutdown. After a day, the amount should be a 17 millionth of the initial amount, and after two weeks iodine should safely be undetectable.

But the figure showed otherwise. If the measurement was correct, experts would have to conclude that the once successfully terminated fission reaction had started again, which is the worst case scenario one can imagine at this point.

TEPCO did retract the announcement later in the evening, however, admitting that they did the analysis wrong:


The second and the third columns show the radiation density before and after the reexamination of water sample taken from the unit 2 basement. The large figure of the iodine-134 was corrected to be “undetectable”.

At the end of the day, everyone agreed that fission reaction did not occur. We could generally deduce what is going on inside the reactors by looking into what is leaking out from them, but that is only when the data is reliable.


Science Communicator: Misato Hayashida