Reply #113: Technetium: Fission yield then the bad news about Technetium.

[NNadir]

There are in my mind only three really seriously problematic fission products (as distinguished from actinides) found in so called "nuclear waste." In this series of a detailed examination of the properties of the fission products that many people, most people probably, define as "nuclear wastes," I have already examined the first of these, the isotope of the element cesium that has a mass number of 135. I have discussed this isotope at length above.

The second of these nuclei is iodine-129 which I will discuss later in the series.

(Some people might include Zr-93, but I don't.)

For the present discussion of technetium, the element I introduced above, I will follow very much the same format as I used with cesium. I will discuss the problems of the chemistry and physics of technetium and the difficulties dealing with it, the "bad news" so to speak. This post will serve as an introduction to the bad news.

Technetium is rather routinely produced in all of the worlds nuclear fission reactors. Most of the world's nuclear reactors operate on a thermal neutron spectrum and for the time being, the majority of the fission events take place in the isotope U-235. Some fissions, generally around 20%, take place in Pu-239, the fissionable plutonium isotope that is produced in all fission reactors from neutron capture in the uranium isotope U-238.

The fission yield (the percentage of fission events resulting in the production of a technetium) for Tc-99 is for thermal neutron fission (0.0253 eV) U-235 and Pu-239 respectively, 6.11047% and 6.14029%.

It is significant that the yield of technetium is somewhat higher in Pu-239 than it is in U-235. The distributions of fission products for the fission for almost all actinides (excepting some of the exotic superheavies) is not symmetric, there is a heavy group with a mass number distribution centered around the mass number 139 (which yields all fissioning actinides. Fission products that are exactly most probable in the heavy group, those having a mass number of 139, quickly yield the stable non-radioactive isotope of lanthanum, La-139, and no isotopes with a mass number of 139 can be considered problematic as the radioactive ones are all extremely short lived. There is also a light group in the distribution of fission products. The most probable mass number in the light group distribution varies with the mass of the fissioning actinide.

As an element that is a constituent of the "light" group, the yield of technetium therefore varies significantly depending on the size of the fissioning nucleus. The mass numbers of maximum probability in the light group (for thermal neutrons) are as follows: For U-233, not yet widely used in nuclear reactors, in spite of its many advantages, the most probable isotope the mass number 90, which yields the radioactive isotope of Strontium, Sr-90, which decays with about a 30 year half-life to the stable isotope of zirconium, Zr-90, with Yttrium-90 as a short-lived intermediate in the decay chain. For U-235, again the most probable mass number in the light group is 92, which yields (rapidly) the stable nonradioactive zirconium isotope, Zr-92. No isotopes with a mass number of 93 are problematic. For Pu-239, the maximum probability in the light group belongs to the mass number 95. This yields over a period of about 1 year's decay, the stable isotope of Molybdenum, Mo-95. For Pu-241, the maximally probable mass number in the light fragment is 99, the mass number that yields technetium (and ultimately over a period of hundreds of thousands of years, the valuable precious metal ruthenium, the isotope Ru-99.

It is worth noting that an actinide recycling program will certainly involve the fissioning of significant quantities of Pu-241. Thus in a sensible world sensibly using nuclear resources, the management of technetium is a serious matter involving serious attention.

(For the discussion of fission yields from which most of this post is paraphrased, please see Seaborg and Loveland, "The Elements beyond Uranium", Wiley and Sons, 1990, pp 176-184.)