Question

Quick question for the nuclear engineers/physicists out there

Where does I-134 come from? I cant find it in any of the charts of standard decay products of Uranium fission, but there is tons of the stuff in Fukushima reactor 2 building right now (2900 MBq/ml of water! Nasty!)

half life is 52 minutes, so either it is being made by fission (which would be bad news) or something from weeks ago is still decaying into it in large quantities.

Answer 1

First, I need to point out that the news about the "10 million times higher radiation" that spread across the world media today were just an error. TEPCO revealed that the error came from a misinterpretation of the concentration of cobalt-56 as iodine-134. Because of this fixed mistake, it's completely plausible that iodine-134 was never detected over there, and there was just cobalt-56. However, let me continue with the bulk of the answer that was written before the mistakes in the reporting began to be appreciated.

Chernobyl and similar accidents have been usually associated with iodine-131 (half-life of 8 days) as the primary source of thyroid cancer. However, iodine-134 (and other isotopes) is usually produced in greater quantities. Its immensely short lifetime makes it quickly irrelevant, however.

On the other hand, when one studies "acute" (and therefore also short-lived) problems with high radiation, such as the "10 million times increased radiation in the water" today, the quickly decaying isotopes such as iodine-134 are just dominant.

Where do nuclei such as iodine-134 come from? Well, there is fission going on in the reactor, so fissiles such as uranium-235 are broken into pretty much arbitrarily smaller pieces and iodine-134 is often among them. See

http://en.wikipedia.org/wiki/Nuclear_fission

for the typical products of fission - the mass is often divided in the 3:2 ratio etc. so isotopes such as iodine-134 are common. Yes, iodine-134 is a direct product of fission but most likely not ordinary neutron-driven fission but probably only photofission (fission initiated by gamma rays hitting the large nucleus), see these papers for details:

http://adsabs.harvard.edu/abs/1982PhRvC..25.1546T

http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4816218%2F4823592%2F04823601.pdf%3Ftp%3D%26arnumber%3D4823601%26punumber%3D4816218&authDecision=-203

If the uranium-235 splits into a Z=53 iodine isotope and one more, this "one more" has to be yttrium with Z=39. The half-life of yttrium-99 and nearby isotopes you may get (after emitting a few neutrons) is comparable to a second.

Why is exactly iodine-134 so much more represented among the fission products than other iodine isotopes? Well, because it has the rigth proton-neutron ratio. The fissile, uranium-235, has Z/A equal to 92/235. And because iodine's Z=53, the same ratio of protons and neutrons - a democratic fission - is obtained for the total nuclear mass
235/92