PWR Cores with Silicon Carbide Cladding

Abstract

The use in present generation PWRs of fuel clad with silicon carbide rather than Zircaloy has been evaluated as an aid to reaching higher discharge burnups and to operation at higher reactor power levels. A preliminary fuel design using fuel rods with the same dimensions as Westinghouse RFA fuel assemblies but with fuel pellets having 10 vol% central holes has been adopted. The central holes mitigate the higher fuel temperatures that occur due to the lower thermal conductivity of the silicon carbide, and the open gap between the fuel and cladding that persists over most of the irradiation. With this fuel design, it has been found possible to achieve 18 month cycles that meet present-day targets for peaking, boron concentration and shutdown margin while allowing average discharge burnups up to 80 MWD/KgU, as well as power uprates of 10% and possibly 20%. For non-uprated cores, the silicon carbide clad fuel has a clear economic advantage that increases with increasing discharge burnup. Even for comparable discharge burnups, there is a fuel cost savings of several million dollars per cycle as long as it does not increase the cost of fabrication by more than 50%, which seems highly unlikely. With 10-20% power uprates, the economics of the fuel cycle will improve, but the total value of such an uprate depends on the cost of needed plant modifications. Modifications to the control rod configuration or absorbing material may also be required to meet the shutdown margin criterion, particularly for the 20% uprate. Silicon carbide’s ability to sustain higher burnups and higher duty than Zircaloy also allows the design of a licensable two year cycle that has a fuel cost comparable to that of the reference 18 month Zircaloy core, and will furthermore reduce the average annual outage time.