Tuesday, January 15, 2013

To reprocess or dispose? A look at fuel cycle triage

A recent study by former colleagues of mine from Oak Ridge National Laboratory raises some interesting questions about the future direction of U.S. nuclear fuel cycle. My colleagues have been presently engaged in a scientific triage study for used nuclear fuel disposition options. One of the largest parts of their work has simply been in collecting the massive amount of data on the 67,600 metric tons (1 MT = 1000 kg) of commercial used nuclear fuel in the U.S., including issues such as how long it was burned in the reactor, the fuel type, and the initial enrichment, with an objective of being able to accurately characterize the composition and location of every used nuclear fuel assembly presently in the U.S. (I also am tangentially involved in this work, funding an undergraduate for data collection and am hoping to expand my role into doing modeling work in support of this effort).

The overall goal of this work is to support a more informed decision framework to specifically look at how we deal with spent fuel inventories in the U.S. - in other words, performing a triage analysis on what fuel would be the best candidates for various fuel cycle options (including direct disposal versus recycling). Given that some fuel is inherently going to be less suitable (read: more expensive) for recovering actinides as future fuel material, the goal is to sort out what can be disposed of immediately and what might be preserved for future fuel cycles.

Their (surprising) finding was that of the present inventory, 98% of the current used fuel inventory (by mass) could be disposed of without leaving open the option of future retrieval while still allowing for the ability to facilitate a future closed fuel cycle in the U.S. This conclusion was based upon the assumption that the U.S. would eventually open a fuel reprocessing facility; even under this assumption most of the present inventory of used nuclear fuel is not needed to support such a cycle. Some of this is simply due to the large inventory of used nuclear fuel in the U.S. - at nearly 68,000 metric tons of heavy metal with the largest fuel reprocessing centers having a throughput on the order of 1,000-1,500 MTHM per year, there is simply more "legacy" fuel out there than a typical facility would ever usefully process.

Their decision analysis was based on several factors, including the value of material which would be recovered (older fuel tends to have less plutonium available for recovery, and the plutonium is of lower quality); complexity (older fuel has other complicating factors such as different types of cladding material - like stainless steel - which can complicate potential recovery and thus make it less preferable to newer fuel), and simply the amount of material needed to sustain a closed fuel cycle (given the time before such a facility would come online, it is anticipated more than sufficient inventories would be present to sustain a closed fuel cycle without drawing into older fuel). Likewise, they considered what fuel assemblies might be useful to future reprocessing research efforts by DOE (such as used, highly-enriched fuel from naval and research programs).

To many who advocate exploiting the resource potential of used nuclear fuel (myself included), this is a jarring conclusion. There has always been a tacit assumption in mind that domestic reprocessing would not only include future inventories of used nuclear fuel, but help to alleviate the pressure on current demand for geologic repository space by making use of the readily available inventories out there. Yet beyond looking at what is economically practical (i.e., prioritizing the most valuable fuel for recovery), the report brings in an eye-opening reality - given the fact that the U.S. has spent the last thirty years committed to a once-through fuel cycle track, there is simply more used fuel than a single modern reprocessing facility would have capacity to handle, especially given the stable influx of fuel coming out of future reactors which would form the foundation for a future closed fuel cycle. As a result, much of this "legacy" fuel becomes unnecessary to support such future fuel cycles.

A more important implication relates to geologic disposal itself. The plans for the (now likely former) Yucca Mountain site called for a 50-75 year "retrievability" window; in other words, the repository was to be operated for an extended period which would allow for retrieval of used fuel out of the repository for other uses. (After the retrieval period, it was generally assumed if no use case had emerged by this point, permanently closing the repository was the most reasonable option).

Designing a repository with future retrievability in mind doesn't come for free; it essentially adds another engineering constraint (read: cost) to the problem and ultimately requires further analysis of how the repository will perform in containing waste in addition to the "post-closure" period. (It also tends to bias one's choice of geology - a feature of salt-based repositories like WIPP is that they are explicitly not designed to be retrievable - the heat from nuclear waste packages generally causes salt to plastically deform around waste packages, effectively "sealing them in.")

Thus, figuring out what spent fuel has little potential prospect for future recovery represents an technical triage which can help simplify a future repository design (as well as open up options for where such repositories might be located). In essence, separating out the "wheat" (fuel more useful for recovery) from the "chaff" (fuel which has limited potential for recovery) allows for a more intelligent approach to used fuel disposition which can ultimately make constructing a future permanent geologic repository cheaper and easier.

Of course, the standard caveat applies: the hardest part of opening any geologic repository has never been technical so much as it has political. Nonetheless, the ORNL report offers a rather bracing conclusion as to what a future U.S. fuel cycle may look like, even if the decision is made to restart reprocessing in the U.S. Ultimately, the vast majority of the current inventory of used nuclear fuel may yet still be destined for direct disposal, simply due to the realities of waiting over three decades before finally deciding to reconsider our rather ill-fated national decision to abandon a closed nuclear fuel cycle.