The TRANSURANUS burn-up model for thorium fuels under LWR conditions

The Transuranus Burn-up (TUBRNP) model for (Th, Pu) O-2 and (Th, U) O-2 fuels is described and validated. To this end, the depletion equations of the nuclides Th-232, U-233 and U-234 were included as well as the breeding of U-235 through a neutron capture in U-234. The Monte Carlo code Serpent was utilized to derive the one-group effective cross sections and the radial power-shape functions that account for the resonance absorption in the epithermal range of neutron energies in Th-232 and Pu-240. Finally, TUBRNP is completed by extending the fission yields of U-233 for the most representative isotopes of Nd, Xe, Kr and Cs. The validation of TUBRNP for (Th, Pu) O-2 type fuels was carried out in two steps. A comparison of the normalised radial distributed concentrations of Th, U, Pu, Nd, Xe and Cs between TUBRNP, Serpent and Electron probe microanalysis (EPMA) data points measured on a sample from a rodlet irradiated at Kernkraftwerk Obrigheim (KWO) allows checking the correctness in the derivation of the radial functions. In a second step, a comparison of the radially averaged values of the same elements between TUBRNP, Serpent, EPMA and benchmark codes under same conditions indicates the agreement in the computation of the one-group cross sections. For (Th, U) O-2 fuels isotopic compositions measured in a rod segment from the Light Water Breeder Reactor (LWBR) Shippingport irradiation programme were employed to validate the calculation of the 1-group cross sections in TUBRNP for this type of fuel.