Plutonia-Thoria Fuel Cycle as Starting Solution for a wider Thorium Use

This paper is focused on a description of thoria fuel option. Our opinion is that this option, beyond being a valuable way to exploit the energy content of plutonium without further breeding it, may be a starting point for introducing an Uranium-Thorium fuel cycle, based on a different strategic context with respect to past proposals. The option is based on the adoption of current or advanced PWRs, the latter characterised by a reduced fuel power density, always adopting conventional fuel rods and assemblies. A three-batches full core loading scheme is assumed. The thoria-plutonia composition is determined by the constraints to obtain at Beginning Of Life (BOL) a non positive void coefficient, and to reach a burnup as high as possible. Different fuel compositions and pellet radius are considered. The plutonium content is in the range of 4.5÷15%, mainly depending on the plutonium quality, namely Weapon Grade (WG) or Reactor Grade (RG). The results are in terms of dynamic coefficients, life duration, plutonium consumption and final isotopic compositions. These fuels show the capability to destroy about 40÷60% of total plutonium for RG, while this figure rises to 65÷70% for WG. These values are well above those obtained by MOX option. A variant to eliminate any proliferation concern foresees the addition of small quantities of 238U at the expenses of a reduction of the fuel burnup and its capacity in burning RG plutonium, while the opposite occurs for WG. The low boron worth is not different from the MOX one, being related mainly to the plutonium content, and much less to the chosen fertile isotope. Therefore, modifications of control devices for a full core strategy is not expected to be different in the two cases. The results confirm the viability of this proposal, apt to future variants, including those connected to accelerator actinide burning solution. An irradiation experiment, expe-cted to take place at Halden HWBR in the context of the ENEA participation to the Halden Project, is the main part of the Inert Matrix – Thoria fuel R&D activity presently underway as a Polytechnic of Milan–ENEA co-operation.