A Comparative Study of Integral Fuel Burnable Absorbers : Coupling Depletion and Fuel Performance

Sammanfattning

Integral fuel burnable absorbers (IFBA) is a type of nuclear fuel developed by Westinghouse Electric Company that coats the fuel in a standard UO2 fuel rod with ZrB2, a burnable absorber. IFBA is used for long term reactivity control, with it being especially useful for extended fuel cycles. ZrB2 has many desirable material properties that make it suitable for IFBA. However, it predominantly captures neutrons through the 10B(n,a) reaction which produces helium. This helium leads to an elevated rod internal pressure (RIP), which becomes a safety concern at high burnup where fission gas release is also occurring. This thesis compares a range of alternatives to ZrB2 with the goal of finding a material with similar performance, but lower helium production. To this end the depletion and fuel performance were compared for IFBA rods using a range of borides (ZrB2, DyB6, EuB6, YbB6, SmB6, HfB2, and ErB4) alongside cadmium oxide CdO.With regards to fuel performance, the differences were found to be negligible besides the difference in helium production and therefore RIP. It was concluded that the key metric for fuel performance in IFBA rods is the rod internal pressure. The depletion calculations demonstrated that two of the candidates ErB4 and YbB6 provided similar reactivity worths and lifetimes to ZrB2,but with the potential to reduce helium production. ErB4 demonstrated higher potential in this regard, but both compounds were deemed to merit further investigation into manufacturability, cost and chemical compatibility. Additionally, the compounds GdB6 and EuB6 were found to have distinctly different depletion behaviours to ZrB2 but could potentially be an interesting avenue for future research due to their high fraction of non-helium producing captures.