Neutronic analysis for accident tolerant cladding candidates in CANDU-6 reactors

The neutronic penalty and advantages are quantified for potential accident tolerant claddings in a CANDU-6 (Canada Deuterium Uranium) reactor. Ferritic-based alloy (FeCrAl and APMT), steel-based alloy (304SS and 310SS) and silicon carbide (SiC) claddings are compared with Zircaloy-II. High thermal capture in nickel-59 and iron-56 imply a neutronic penalty in iron and steel-based bundles. A minimum enrichment of 1.0% and 1.1% is required for ferritic and steel-based claddings, respectively, to achieve the CANDU-6 burnup average criticality. An average increase of 0.35% in reactivity is introduced when SiC is considered, while keeping a natural enrichment condition. Average thermal neutron absorption rate is found to be 203, 8 and 6 times higher in UO2 pellets than in silica, iron and steel-based claddings, respectively. A spectral hardening is observed in fuel, cladding and coolant for all enriched cells. Neutron flux is 30.5% higher at 0.0253 eV in SiC and Zr bundles. For FeCrAl, APMT, 304SS and 310SS, less 239Pu is produced during all fuel residence time and a minimum of 50% more 135Xe poison is being produced at equilibrium. As well, a minimum of 60% more 235U is left at end-of-life. For bundles cladded with SiC and Zr, 7.6% higher fission rates are found on the pellet periphery. Decreasing the cladding thickness by 200 μm made it possible to satisfy the criticality requirements for all claddings with an enrichment below 1%. Moderator and coolant temperature coefficients are found higher in SiC and Zr bundles. At mid-burnup, the Doppler effect and the voiding effect are higher in FeCrAl, APMT, 304SS and 310SS cladded cells.