Tuesday, October 25, 2005

DOE Directs Contractor on Repository Design

I’ve spent the vast majority of my career in the nuclear industry working in waste and used fuel management. First, I interned at DOE’s Office of Civilian Radioactive Waste Management as an undergraduate, then I spent a summer in France modeling breeder reactor cores, next I worked on waste management issues at the Defense Nuclear Facilities Safety Board, for my master’s thesis I modeled gas evolution from Hanford N-Reactor fuel in sealed canisters, and I worked for seven years in Dominion’s nuclear spent fuel group.

In that time, I’ve witnessed many DOE proposals for repository design and function: a “hot” versus a “cold” repository; wet fuel transfer versus dry fuel transfer versus no fuel transfer; standard canister design versus standard canister design criteria versus “let the utilities decide what to ship it in"; and on and on. All of these proposals are technically feasible and also have pros and cons.

So, I read with great interest today DOE’s announcement that it has instructed its contractor to

devise a plan to operate the Yucca Mountain repository as a primarily “clean” or non-contaminated facility.
What this means to the layperson is that instead of transporting fuel from reactor sites to Yucca Mountain in one container, removing it (either in a pool or inert, dry environment), and then repackaging it in another container, most of the used fuel will be shipped in standardized containers that can be placed into an overpack and installed directly in the repository.

This plan makes a lot of sense.

Moving individual fuel assemblies around whenever the mood strikes is the best way to damage them. And while there are several methods to safely handle damaged fuel, they are costly and time-consuming. This proposal means that the vast majority of fuel will be handled the minimal number of times between core discharge and placement in a repository. A standardized design for the canisters would also facilitate efficient removal of the fuel for recycling if such technologies are later available.

Most importantly, I’m hopeful that focusing on this reasonable approach will, in DOE’s words, simplify the design and license application for the repository; because ultimately, after protecting the health and the safety of the public, DOE’s next greatest responsibility is to expedite the removal of used fuel from reactor sites.

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4 comments:

Solomon2 said...

for my master’s thesis I modeled gas evolution from Hanford N-Reactor fuel in sealed canisters

Hey, you're exactly the person who I hoped would catch this DOE announcement. Sure, everything can stay in the containers, but won't outgassing from fuel assemblies eventually burst the rods and release radioactives into the surroundings?

Lisa Stiles-Shell said...

No. Fuel rods and fuel containers are designed to withstand internal pressue from gases. Furthermore, when putting used fuel into a container, certain fuel characterization calculations must be done to make sure that the fuel loading does not challenge the design of the container. Parameters like gamma and neutron doses as a function of time, decay heat, and the amount of pressure that could be generated from off-gassing are determined based on the original fuel characteristics, power history, cooling time, etc.

Solomon2 said...

Specifically, I recall learning that after several hundred years the rods would burst due to the pressure generated from gaseous radioactive decay products. Is this correct?

Lisa Stiles-Shell said...

For all scenarios I can think of, no, that is not correct.

I'm sure someone could come up with some sort of chain of extremely unlikely events (fuel for which the enrichment was incorrect, plus unknowingly operating at powers beyond what a plant is licensed for, etc. etc.) after which a fuel rod could burst. But as I mentioned, fuel containers are designed to withstand high internal pressure, so there would still be no release to the surroundings.

You see, the design criteria for used fuel containers are determined by imagining the worst possible configuration. In this case, even though operators are prohibited from loading fuel that is known to have significant defects (without canning it first, but that's a different story) the container is designed to maintain its sealing integrity even if every fuel rod fails.