Iodine isotope question

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.

Cheers

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 $$ 235 / 92 \times 53 = 135.4.$$ So it's most likely to produce iodine isotopes whose $A$ is close to 135. Iodine-135 has 6.6 hours of half-life, so it's not that important "acutely", while iodine-136 has 83 seconds and disappears too quickly. It just happens that the contamination of water, as was measured, is dominated by half-lives close to 1 hour, and iodine-134 is the key representative in this realm.

Other elements, different from iodine, have typically much longer lifetimes for the same $Z/A$ ratio. Still, it's plausible that much of iodine occurs from decay of other fission products, e.g. from beta-decay of tellurium-134 (see the other answer): fission directly prefers to produce even $Z$ elements while iodine has $Z=53$, odd. But this even-$Z$ rule is not "strict".

Otherwise, fission is a messy process and it can produce "almost anything" in the list of isotopes. Again, just to be sure, fission is not about getting incremental alpha and beta decays only; it's the process of splitting the large nucleus into two comparable pieces.

Answer 2

go to this chart

http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=235ufy

in little box on right marked "nucleus" type 134I and click GO

click at top the box 235U FY

click just below on "Zoom = 1

if you're there you'll see colored squares, one in center says 134I

that means iodine 134 . the other squares in the row are other isotopes of iodine, the row above is xenon and one below is tellurium.

the color gives a clue as to likelihood a uranium atom will split into a I134 atom. red is fairly likely.
Number at bottom IS that probability, 0.0036 means that you'll get 3.6 I134's for every thousand fissions of U235. That's what the 235U FY button meant, makes chart show fission yield.

Note the B- 100%. That means 100% of I134 decays by Beta which is electron emission. Beta decay moves the isotope diagonally up one and left one, into 134Xe's box. Note they also give the halflife 52.5 Minutes. That means : however much I134 you have right now, in 52.5 minutes half of it will have turned into Xe134, moving up one and leaft one..

Does that mean you lost half your i134?
Well sorta, half of the atoms you had indeed morphed into Xenon. BUT look one square right and one down at13Te. It is a almost 20X more likely fission product -- 62.2 atoms of Te134 per thousand fissions compared to 3.6 of I134. And it decays by beta so it morphs into I134.

So your I134 box will simultaneously empty into the Xe134 box and fill from the Te134 box. Since fission made more Te atoms than I, the inflow from Te will exceed outflow to Xe until you run low on Te.

So yes, I134 comes from fission but by two paths - directly and via Te134. The Te path looks to make most of the I134, judging by fission yields.

did i get it across?

i dont see any precursors with long halflives. You'll enjoy this site too...

http://www.periodictable.com/Isotopes/053.134/index.html

old jim