New Burnable Absorber and Zirconium Reflector for Advanced PWRs

Abstract

This thesis presents a new high performance burnable absorber (BA) design and a Zircaloy reflector design for advanced Pressurized Water Reactors (PWRs) with the aim of enabling long-cycle operation with a low soluble boron concentration. The new BA consists of a UO2-157Gd2O3 rod covered with a thin layer of Zr167Er2. A key feature of this new BA is that enriched isotopes, 157Gd and 167Er, are used as absorber materials. Since the high absorption cross section of 157Gd can reduce the mass fraction of Gd2O3 in UO2-Gd2O3, the thermal margin of fuel rods will increase with higher heat conductivity. Also, the 157Gd transmutes into 158Gd by neutron absorption and therefore the residual penalty at the end of cycle (EOC) will decrease. Since 167Er has a resonance near the thermal neutron energy region, the moderator temperature coefficient (MTC) will become more negative, and the control rod worth will increase. The Zircaloy reflector reflects fast neutrons due to the cross section of zirconium. Recoiled neutrons are moderated to thermal neutrons which affect fuel assemblies (FAs) of the peripheral core. There are three cases for the verification of Zircaloy reflectors: water, stainless steel (SS) and Zircaloy. The above mentioned advantages of the new BA and the reflector are demonstrated with three verification cases: a 17x17 Westinghouse (WH) type fuel assembly, a 16x16 Combustion Engineering (CE) type fuel assembly, and an OPR-1000 equilibrium core.