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.

Tech Review - 2

Alas, another Sunday with coffee in tow.

In my reactor analysis course last semester, one group project focused on the effect the presence of U-236 would have on reusing uranium from spent nuclear fuel. It has long been known that U-236 introduces a long-lasting negative reactivity to the fuel. So-called penalty factors have been defined to quantify how many extra units of U-235 are needed to overcome the negative reactivity of one unit of U-236. These factors have been found to be roughly 0.30, or in other words, for every extra gram of U-236 produced, one must have present an additional 0.30 grams of U-235 to maintain the reactivity desired.

J. P. Renier et al. looked at this problem in depth, especially with respect to GNEP and waste disposal. The reuse scheme was to mix a sufficient amount of recovered uranium from an initial cycle with natural uranium to create fresh fuel for consecutive cycles. The first cycle was assigned 33 GWd/t burnup, typical of U.S. operations, and 55 GWd/t for subsequent cycles--why they focused on higher burnup for later cycles remains unclear to me. Perhaps it was merely assumed that future operations would employ such higher burnups.

Their conclusion states an asymptotic weight percentage of U-236 was found (~0.85%). The penalty factors for the cycles were roughly 0.25, which agrees pretty well with historical values.

Perhaps important to state is why U-236 would even be present. Uranium purification usually employs some form of centrifugal separation based on weight. For natural uranium, comprised only of U-235 and U-238, this works well, as the relative mass difference between the isotopes is large (enough). For U-235 and U-236 (produced during the cycle), this is not the case, and over half the U-236 remains with the re-enriched product.

The article is merely a transaction, so much detail is left out. What I'd like to know is how the data uncertainties look after propagation through 7 cycles. To me it seems perturbations in just about any factor could introduce significant changes in the U-236 concentrations (and hence the requisite U-235). Perhaps I'll solicit an opinion from the group who reviewed this work in class.

A Statement of Professional Interests

At some point it becomes relevant to write down one's interests; for me, I choose now.

My professional interests, as a student in nuclear engineering, are wide-ranging. I enjoy the computational aspects of nuclear engineering; simulations or analytical studies of nuclear phenomena are engaging and show how much we know (and do not know) about the physical world.

Specifically, I (think I) like methods of (neutral particle) transport; in other words, I like to know how radiation works in large-scale, real-world problems. I note my skepticism only because I have not myself actually dug deeply into the 'guts' of such methods; rather, I've simply 'used' them a bit.

As for applications of transport, I find reactor physics, shield analysis, and radiotherapy all to be stimulating.

To support this interest, I find also I like numerical methods in mathematics. Pure math has always seemed so abstract to me, and while beautiful, I cannot say it is 'relevant.' Additionally, high performance computing is a necessary companion to applied math and engineering.

During my research experiences, a number of relatively specific sub-genres within nuclear engineering have struck me as particularly engaging. Sensitivity analysis, especially with respect to nuclear data analysis is an important application of applied math and transport theory. Additionally, optimization, a rather broad topic within many science (or even all of life!) has come into play more than once.

Having stated these interests, which are certainly not exhaustive, I feel it is my duty as a good and soon-to-be graduate to keep abreast of the 'latest-and-greatest' within the fields. I will try using this blog as a technical log of those things which seem especially valuable. My goal will be every Sunday morning to browse a recent volume of a related journal for relevant material; upon finding an article of interest, I shall read it over coffee and then recount the very basics of it via this blog. I shall also try finding ways such work could be applied in my own work or expanded to be of further utility.