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

The Transuranus Burn-up (TUBRNP) model for (Th,Pu) O2 and (Th,U) O2 fuels is described and validated. To this end, the depletion equations of the nuclides 232Th, 233U and 234U were included as well as the breeding of 235U through a neutron capture in 234U. 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 232Th and 240Pu. Finally, TUBRNP is completed by extending the fission yields of 233U for the most representative isotopes of Nd, Xe, Kr and Cs. The validation of TUBRNP for (Th,Pu) O2 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) O2 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.