There are currently many advanced nuclear reactor designs under consideration around the world. These next generation reactors promise clean energy with enhanced safety and efficiency and at a potentially lower cost to build. Xiaodong Tang, a M.S. student working with Center for Nuclear Security Science and Policy Initiatives (NSSPI) Faculty Fellow Dr. Shikha Prasad, conducted research to evaluate safeguards instrumentation for one such advanced reactor, the Pebble Bed Modular Reactor (PBMR). The Xe-100/Xe-mobile by X-energy in the US and the HTR-PM in China are examples of two PBMR reactors currently under development.
According to Tang, “Pebble Bed Modular Reactors could allow nuclear plants to support the goal of reducing global climate change in an energy hungry world. They are small, modular, inherently safe, use a demonstrated nuclear technology, and can be competitive with fossil fuels.”
PBMRs use a specialized “meltdown-proof” spherical TRISO pebble fuel that can be passed through the reactor multiple times to extract energy. Tang is performing research on how best to measure the burnup of these pebbles for safeguards purposes. He is developing a design for an extremely fast (sub-nanosecond) detection system to perform high efficiency gamma-spectroscopy for very high burnup pebbles.
“The PBMR is interesting from a Material Control and Accounting point of view because it obtains very high enrichments of U-235,” explains Tang.
Traditional Light Water Reactors found in the majority of nuclear power plants in the US use fuel with an enrichment of only 3-5%, whereas the Xe-100, for example, uses fuel with around 15% enrichment. In addition, Tang points out, PBMRs have double the burnup of traditional reactors, which leads to the production of plutonium. Accounting for this material and ensuring it is not misused or diverted to non-peaceful uses is thus a concern.
Tang was able to employ a barium fluoride (BaF2) scintillation detector to perform these extremely fast gamma measurements. Previous attempts at gamma detection in the PBMR using a high purity germanium (HPGe) detector suffered from slow response times and thus large dead time losses incompatible with the requirements for a fast and efficient measurement system.
In his study, Tang found that the HPGe detector offered better energy resolution and was a better instrument for unambiguously identifying nuclides than the BaF2 detector; however, the increased speed of the BaF2 detector makes it a viable option for use in an extremely fast detection system capable of performing on-the-fly pulse analysis of pebbles in the PBMR.
After successfully defending his thesis, Tang took a position in nuclear criticality safety engineering for Savannah River Nuclear Solutions. He will be earning his master’s degree in nuclear engineering from Texas A&M University in December. Before becoming a NSSPI student, Tang received his bachelor’s degree in nuclear engineering from Texas A&M with a minor in radiological health engineering. During his undergraduate studies, he was enrolled in the Nuclear Criticality Safety program in the Texas A&M Department of Nuclear Engineering, which is a collaboration between Texas A&M, Los Alamos National Laboratory, and the Y-12 National Security Complex. The program focuses on introducing various aspects of the nuclear criticality profession to build a pipeline from the university to the national laboratory complex.