Sandia experiments with new reactor prove that spent nuclear fuel is less reactive than fresh fuel
Recent experiments by Gary Harms (6423) and his team using a new Sandia-built reactor in Tech Area 5 are providing 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.
"The conservative view has always been to treat spent fuel like it just came out of the factory with its full reactivity," Gary, project lead, says. "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 product poisons. Accounting for this reactivity decrease, called burnup 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 burnup credit would result in significant cost savings in the handling of spent nuclear fuel," Gary adds.
This seems obvious on the surface, but in the ultraconservative world of nuclear critical 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.
In 1999 Gary obtained a three-year grant from the DOE Nuclear Energy Research Initiative to make benchmark measurements of the reactivity effects that fission products have on a nuclear reactor. The project was called the Burnup Credit Critical Experiment (BUCCX). Rhodium, an important fission product absorber, was chosen for the first measurements.
To do this the BUCCX team first designed and built a small reactor, technically called a critical assembly, which uses low-enriched fuel. The control system and some of the assembly hardware for the reactor came from the 1980s-era Space Nuclear Thermal Propulsion (SNTP) Critical Experiment project, designed to simulate the behavior of a nuclear rocket reactor.
"It took us most of the three years to build the reactor and get authorization to use it. Only in the last few months have we begun actual experiments," Gary says. "Much of the time was involved in getting approvals from Sandia and DOE and to make sure it meets all ES&H concerns."
"It takes a pretty big team to bring up a new reactor, even a small one, in this day and age," he added. "The team included members from all of the Area 5 departments that do reactor work. We also got considerable support from purchasing, the shops, and several of the ES&H departments."
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 BUCCX 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.
"We then compared the critical loading of the assembly with the rhodium foils to the critical loading without rhodium," Gary says.
And, not to anyone’s surprise, it took significantly more fuel to reach critical with the rhodium-doped rods than without them.
Months before running the physical experiments on the reactor, Gary was modeling on Sandia’s sophisticated computers to determine where the uranium doped with rhodium would go critical.
"I was curious," Gary says, "I did calculations ahead of time so I could lay out the experiment and get a peek at what the experiments would say. In the end, I was fairly impressed with how accurate the calculations were compared to the actual physical experiments.’
Of course, the computer codes weren’t perfect, and had a small bias when compared to other critical safety benchmarks. And in analyzing the actual experiments in the reactor, Gary took that bias into account.
Gary says two other fission products absorb neutrons better than rhodium. However, he 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. Also, no one else has done any experiments with rhodium in a critical assembly. Subsequent experiments could address the dozen or so other fission products that are important to burnup credit.
Also, to his knowledge, no other lab in the US is doing actual burnup credit experiments. Oak Ridge National Laboratory is running codes to determine how much the reactivity of spent fuel is reduced by fission products, but not doing actual experiments.
At the end of the three-year funding period, Gary says the Sandia program has come a long way in proving that the reactivity of spent fuel is considerably less than that of fresh fuel.
"In essence Sandia is helping pave the way for the Nuclear Regulatory Commission to address the safe and cost-efficient transport and storage of nuclear waste," Gary says.