Abstract
MHI has been developing an advanced light water reactor (LWR) “SRZ-1200”[1]. For continuous use of Plutonium (Pu), it is desirable for light water reactors to be capable of loading a large fraction of MOX fuel.
Conventional MOX fuel assembly for Japanese Pressurized Water Reactors (PWRs) is designed with all MOX fuel rods – unlike the ones for BWRs – with distribution of Pu content to mitigate the power peak in the assembly. Today, the MOX fuel assemblies have been loaded to several Japanese PWRs; however, a fraction of MOX fuel assembly loading is limited to the one fourth of the core (1/4MOX core) to ensure a sufficient shutdown margin. With the aim of increasing a fraction of MOX fuel in the core, it is necessary to enhance the control rod wort, which is a key parameter for shutdown margin, against MOX fuel assembly.
As one of the measures to enhance the control rod worth, the advanced MOX fuel assembly, which consists of both uranium dioxide (UO2) fuel rods and MOX fuel rods, has been developed in this study. Inside the advanced MOX fuel assembly, UO2 fuel rods are placed around the control rod guide thimbles, and a fraction of MOX fuel rods in the assembly is approximately 80%. In addition to the measure mentioned above, combination of Ag-In-Cd (AIC) and boron carbide (B4C) with enriched boron, which has larger neutron absorption cross section than AIC does, is used for neutron absorber material of control rod. By implementing both the advanced MOX fuel assembly and AIC-B4C combined control rod, this study confirmed that the core can be fully loaded with the advanced MOX fuel assemblies (i.e., over twice more of annual Pu consumption to the core compared to the 1/4MOX core can be achieved) while maintaining a sufficient shutdown margin.
