The ROXf is a kind of inert matrix fuel U-238-free matrices, it has a high plutonium transmutation capability. The ROXf consists of chemically stable phases of fluorite ‘stabilized’ ZrO2 or ThO2, and spinel MgAl2O4. In this fuel, PuO2 is solidified in a fluorite phase. With U-238-Free matrices, a large part of the plutonium can be burned after irradiation in conventional LWRs. The spent ROXf consists of natural analogous geologically stable phases, and is disposed directly as high level wastes ‘HLWs’ after about 50 years cooling. From the high plutonium burn up rate and the high stability of the fuel, the ROXf-LWRs system has proliferation resistance and environmental safety. Characteristics of two types of ROXf, Zr-ROX and Th –ROX with weapons-Pu, in an LWR core arrangement are evaluated by cell burn up calculations and 2-D core calculations using the SRAC code system and JENDL-3 nuclear library. In an LWR of moderator to fuel volume ratio = 1.9, which corresponds to current PWRs, Pu transmutation rates with the two types of ROXf are large enough and more than 80% and 99% of Pu and Pu-239, respectively, can be burned. The calculated kinetic parameters indicate less moderate characteristics of ROX cores, especially with Zr-ROX. The fertile Th-232 in Th-ROX works like U-238 in the UO2 fuel, making kinetic parameters more moderate and reactivity drop due to burn up smaller than that Zr-ROX. The neutron capture of Th-232 to generate U-233, causes the safeguards problem. Thus, the characteristics of Zr-ROX as a typical example, were investigated in this study.

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