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Sci Tech
Safe transport, disposal of nuclear waste
IN RECENT experiments by Sandia National Laboratories researcher Gary Harms and his team are using a new lab-built reactor to provide benchmarks showing that spent nuclear fuel — uranium that has been used as fuel at a nuclear power plant — is considerably less reactive than the original fresh fuel. This could mean significant savings in the eventual safe transport, storage, and disposal of nuclear waste.
This results in the numbers of canisters required in the handling of spent nuclear fuel to be conservatively high, driving up shipping and storage costs.
The more realistic view is that as nuclear fuel is burned, the reactivity of the fuel decreases due to the consumption of some of the uranium and to the accumulation of fission products, the `ash' left from burning the nuclear fuel.
Accounting for this reactivity decrease, called the burn up credit, would allow for the spent nuclear fuel to be safely packed in more dense arrays for transportation, storage, and disposal than would be possible if the composition changes were ignored.
Allowing such burn up credit would result in significant cost savings in the handling of spent nuclear fuel according to the researchers, this seems obvious on the surface, but in the ultraconservative world of nuclear criticality safety, an effect must be proven before it is accepted.
Thus, prior to the Nuclear Regulatory Commission ever agreeing to the more realistic view, it would have to be proven in actual experiments and compared to computer models showing the same effects.
To do this the team first designed and built a small reactor, technically called a critical assembly, which uses low-enriched fuel. The reactor, which operated during the experiments at a lower power than that of a household light bulb, was subjected to several layers of safety reviews by the researchers. During the experiments, it performed safely exactly as predicted.
The core of the BUCCX consists of a few hundred rods full of pellets of clean uranium that originally came from the nuclear powered ship NS Savannah.
Thirty-six of the rods can be opened to insert experiment materials between the fuel pellets.
Prior to conducting experiments with the rhodium, the researchers loaded the reactor to critical with only the uranium fuel. This provided a baseline point of where uranium goes critical — information that could be compared to later experiments.
Then, the team added about 1,200 circular rhodium foils between the uranium pellets in the 36 rods. The intent was to measure the extent to which the rhodium reduced the reactivity of the uranium.
Months before running the physical experiments on the reactor, the team was modelling on Sandia's sophisticated computers to determine where the uranium doped with rhodium would go critical.
Of course, the computer codes weren't perfect, and had a small bias when compared to other criticality safety benchmarks.
The team, says two other fission products absorb neutrons better than rhodium.
However, they selected rhodium to run the experiments because it is one of the few byproducts of fission that has a single stable isotope, which means the experiment would not be contaminated by the effects of other isotopes.
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