MIT: Chain Reactions Reignited At Fukushima After Tsunami, Says New Study

In that paper, which can be accessed from arxiv here, the author examines the ratio of I-131 to Cs-137 activities for several different measurements at the plant. Since I-131 and Cs-137 are made in known proportions during fission, but they have different half-lives, the relative amounts could (in principle) tell you something about how much fission a given piece of nuclear fuel has undergone. Once a reactor has been scrammed (shut off) this ratio should decay away with the half-life of I-131 (8 days).

The paper uses the theoretical values for the ratio between the two isotopes and compares the ratio to actual measurements at the plant. The finding is that several measurements — particularly those in the sub-drain of Unit 2 — are anomalously high and don't match the theoretical values. In fact, the values at Unit 2 are high and then rise, peaking about 40 days after the earthquake. The interpretation presented is that this is evidence that Unit 2 has undergone fission again — thus producing more I-131 to make the ratio of I-131 to Cs-137 higher than it would otherwise be.

The theoretical model used in the paper greatly oversimplifies the situation. The basic issue is that the concentrations of iodine and cesium in the water are going to depend a lot on chemical properties, temperature, and how they are transported. As a very simple example to illustrate the point, let's look at the boiling points of iodine and cesium:

In very gross terms, as a material gets close to its boiling point, more and more of it is able to evaporate. So say the fuel rods are at 1500°F. This is above both boiling points, and so we might expect iodine and cesium to be released in similar quantities. But say the temperature is lower, at around 400°F. Then a lot of iodine can evaporate from the fuel rods, while not as much cesium can. As the fuel cools, this disparity between iodine and cesium should get larger and larger. Notice that at cooler temperatures, iodine would evaporate more and so the ratio of iodine to cesium would get larger. This very simplistic model could explain why iodine becomes overabundant as Unit 2 cools down.

Again, I am just making a point and so I am also ignoring chemistry and other "messy details" that could come into play. But in my opinion, before recriticality is invoked to explain the data, these messy details need to be examined since the explanation might be in those details.

As a final note, a much better way to analyze the fission history of nuclear fuel would be to compare the ratios of different isotopes of the same element — then there are no chemical differences and the comparison truly would be "apples to apples." Cs-134 has a half-life of 2 years, which won't tell us much about the very recent history of fission. But the isotope Cs-136 has a 13 day half-life, so the ratio of it to Cs-137 could yield a great deal of information without the drawbacks of I-131. It is far less abundant than Cs-134, however, but it might still be in the TEPCO data.

Mark [BRAWM Team Member]