Microstructures of ZrC coated kernels for fuel of Pu-burner high temperature gas-cooled reactor in Japan

Abstract

Concept of Pu-burner high temperature gas-cooled reactor (HTGR) was proposed for the purpose of more safely reducing the amount of recovered Pu. In the Pu-burner HTGR concept, coated fuel particle (CFP), with ZrC coated yttria stabilized zirconia (YSZ) containing PuO2 (PuO2-YSZ) kernel and with tri-structural isotropic (TRISO) coating, is employed for very high burn-up and high nuclear proliferation resistance. Japan Atomic Energy Agency (JAEA) has carried out ZrC coatings of kernels simulating PuO2-YSZ kernels. Kernels which simulated PuO2-YSZ kernels were YSZ containing CeO2 (CeO2-YSZ) kernels fabricated by Nuclear Fuel Industry, Ltd. (NFI) or commercially available YSZ kernels. Ce was used as simulating element of Pu. In this manuscript, microstructures of ZrC coated CeO2-YSZ or YSZ kernels are reported, and future direction of development of Pu-burner HTGR is discussed. CeO2-YSZ kernel consisted of Ce-rich grains and Zr-rich grains. Observation results indicated CeO2-YSZ kernel was corroded during ZrC deposition process. Then Ce can be extracted from CeO2-YSZ kernels. If Pu can also be extracted from PuO2-YSZ kernels, that PuO2-YSZ kernel is undesirable in view of nuclear proliferation resistance. Fuel design and/or reactor core design of Pu-burner HTGR must be revised, if it is very hard to fabricate PuO2-YSZ kernels, with composition used in the present design of Pu-burner HTGR, and from which Pu is hard to be extracted. Observation results also indicated the possibility that only Ce-rich grains were corroded. If Pu can be extracted only from Pu-rich grains, PuO2-YSZ kernels containing no or only a small amount of Pu-rich grains, should be fabricated. ZrC layer was bound to YSZ kernel, but was completely debonded after heat treatment simulating sintering of fuel compacts. Remarkable grain growth occurred both in ZrC layer and YSZ kernel, after heat treatment. Voids were distributed in ZrC layer both in grain and on grain boundary.