Optimised transmutation of plutonium and americium in pressurised water reactors / Optimierte Umwandlung von Plutonium und Americium in Druckwasserreaktoren

The world energy requirement will dramatically increase during this century, because of an increasing world population on the one hand and an increasing per capita energy consumption on the other hand. Future prospects of primary energy requirement, basing on different assumptions of the future technological, economical and ecological developments, predict a multiple of todays nuclear energy production capabilities worldwide. A worldwide phasing out of the nuclear energy option is not foreseeable. To the greatest possible extend today's nuclear fuel cycle is based on the usage of uranium-based fuels in light water reactors. At this accumulating residues contain typically per tonne heavy metal beside fission products and surpluses of uranium also 11 kg transuranium elements (TRU). The TRU will dominate the hazardous potential for a very long time span in a future repository because of their long half-life and radiotoxicity whereas the fission products dominate only at the very beginning. Some of the transuranium isotopes are very good fissionable and open due to this a proliferation risk additionally. This paper analyses a possible evolution of todays fuel cycle, considering on the one hand the requirements of a sustainable waste disposal strategy and on the other hand the highest safety properties of today's western nuclear power plants. The aim is to deposit only the nuclear residuals which hazardous potential will fall below the natural hazardous potential of fresh nuclear fuel during a time period in which engineered barriers guarantee a safe encapsulation. By pin-cell, assembly and full-core simulations of the neutronics of a thoriumbased pressurised water reactor with different calculational tools the possibility to operate the reactor with (Th,Pu)O_2-fuel in a four-batch-mode up to a burn-up of 60 MWd/kg was shown. The nuclear stability of the system for all burn-up conditions was proven for the full core and especially for the void coefficients the pin-cell and assembly calculations were shown to be conservative. Under the term of minimal modifications of today's established systems further optimisation approaches with the aim of increasing the actinide consumption were performed. Beside the adjustment of moderation ratio the possibility to use spectral shifts within the pellet by a two layer approach and enlargement of the pellet diameter were analysed for different fuels of the second recycling step. Furthermore effects of hardening the neutron spectra by a cadmium filter between the inner and outer fuel zone were studied. Repercussions of those special designed fuel pins within the whole fuel assembly, the maximal fuel temperatures, heat surface stress and helium production were also part of the analysis. Very favourable is the heterogenous (Th,Am,U)O_2 fuel, because without blending with fresh fuel an advantageous americium isotopic composition was found even up to the third recycling step. (orig.) / SIGLE / Available from TIB Hannover: RA 831(4131) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische Informationsbibliothek / DE / Germany