Tech Review - 4

As I begin reviewing the literature for my summer internship (and potential master's work), I think it is helpful to explain the background of the topic--burnup credit--and what implications it could have. A recent paper out of ORNL* summarizes quite well the value implementation of full burnup would have.

Burnup credit is the process of taking into account the reduction in reactivity due to irradiation of nuclear fuel (i.e. burnup). The reactivity is decreased via a net reduction of fissile isotopes and the introduction of parasitic neutron absorbers (i.e. non-fissile actinides and fission products).

The paper opens with a discussion of burnup credit and the varying degrees to which it has been implemented. For years, regulations called for the "fresh fuel" assumption. That is to say, when doing safety calculations for used fuel, the fuel was assumed to be unirradiated--which is a grossly conservative assumption! More recently, regulatory guidelines state the major actinides (i.e. changes in U and Pu) can be considered.

However, even these new guidelines neglect the roughly one-third of the possible decrease in reactivity produced by fission products (e.g. Sm-151, Rh-103). If a high-capacity, 32-assembly cask were to be used for shipping and containment, neglecting these fission products allows for only about 30% of current used fuel to be put in such containers; however, and the key point, up to 90% of such fuel could be shipped if we could account for all relevant fission products. The financial ramifications are huge: $156 million is a target minimum savings if we could actually move that much fuel around in these casks.

Thus, the goal is to strive toward guidelines allowing full burnup credit (i.e. accounting for ALL the reduction in reactivity).

Two problems exist: 1) we need better data that says exactly how much negative reactivity a given isotope introduces and 2) we need to know better exactly how much of said isotope is present in used fuel.

The first problem deals largely with our nuclear data and the nuclear codes we use for analysis. Essentially, we need to analyze our database of critical experiments that analyze the specific isotopes of interest with respect to their effect on reactivity. However, we can't just use any old critical experiments--they must be similar to our application, namely the cask and used fuel of interest. ORNL currently is evaluating some foreign experiments for this "similarity" or "applicability"; Sandia has performed one such experimental suite to probe Rh-103 specifically.

While this data all looks pretty good right now, we need more! Last summer, I looked at theoretical modifications to a given critical experiment to see how useful it could be in analyzing all the relevant isotopes. I found several setups that could be easy and useful to implement; I won't go in depth, because I hope to have it presented or published in the near term. Moreover, I expect this summer and for my M.S. thesis to expand on this work and design robust experiments that will satisfy the "applicability" requirements for the isotopes needed.

Furthermore, in regard to the second problem, chemical assays are needed of current used fuel. One might think this is easy enough to do, but suffice it to say, it is not easy for anyone--even the Labs--to get their hands on much of this stuff.

* C. V. Parks, J. C. Wagner, and D. E. Mueller, "Full Burnup Credit in Transport and Storage Casks: Benefits and Implementation," Proc. of the International High-Level Radioactive Waste Management Conference, Las Vegas, NV, April 30-May 4, 2006, pp. 1299-1308.