3-D Core Design of the TRU-Incinerating Thorium RBWR Using Accident Tolerant Cladding

This study investigates the safety of the full core optimal design for the RBWR-TR – a reduced moderation BWR with a high transuranic (TRU) consumption rate. This design is a variant of the Hitachi RBWR-TB2, which arranges its fuel in a hexagonal lattice, axially segregates seed and blanket regions, and fits within an ABWR pressure vessel and is capable of unlimited TRU recycling as do fast reactors. The RBWR-TR eliminates the internal axial blanket, eliminates absorbers from the upper reflector, and uses thorium for the fertile fuel. Both designs are initially presented with Zircaloy-2 cladding. The neutron spectrum in the RBWR-TR is softer than in the RBWR-TB2, which results in a lower cladding fast neutron fluence; however, the peak fluence of fast neutrons (E > 0.1 MeV) the cladding is exposed to exceeds the bounds of Zircaloy-2 at accident scenarios, limiting its material feasibility and affecting the reactor safety. The constraining phenomena which are enhanced by the high fast neutron fluence include accelerated corrosion, accelerated embrittlement rates, and hydrogen pickup. Alternative cladding materials to Zr-based alloys are being investigated for accident-tolerant scenarios. These include stainless steel based materials, which are not limited by hydrogen pickup phenomena. Since these alternative claddings have larger absorption cross sections than Zr-based alloys, the impact on the achievable discharge burnup and other key neutronics parameters is assessed. The design variables used in the parametric studies include: the cladding material, cladding size, gap between cladding and fuel, and fuel-to-moderator volume ratio. The changes of the void feedback, cycle length, burnup, shutdown margin, and critical power ratio to variation in each of the design variables are calculated to determine their impact on the design. The design presented in this paper does not exceed material bounds at same burnup value of the original design. By increasing the gap and reducing the cladding dimension we were able to meet all design constraints. However, due to significant changes in gap and cladding dimensions, the RBWR cores require further intensive studies related to swelling accommodation and pellet-cladding interaction effects. (author)