Advanced LWRs aiming at a burnup higher than 100 GWd/t and/or a conversion ratio higher than 1.0 have been developed for reducing the cost of electricity and the quantity of radioactive wastes. As fuel cladding alloys, Fe-Cr-Ni alloys with sufficient irradiation resistance and mechanical strength are preferred over Zircaloys with their high thermal neutron economy. The candidate stainless steels are modified to overcome problems such as IASCC and ductility loss experienced in the current LWRs (Fig. 1-1). From basic analyses concerning irradiation assisted corrosion and degradation, the effect of chemically active oxygen on metal surfaces and the austenite phase stability at irradiation temperatures were critical reliability issues of candidate steels (Fig. 1-2). The candidate steel with 25Cr-35Ni-0.2Ti was selected and optimized for the cladding tube fabrication process using electron beam melting and a thermomechanical treatment called SAR ("strained, aged and recrystallized") (Fig. 1-2).
On the candidate steel, the susceptibility to intergranular corrosion (IGC) and stress corrosion cracking (SCC) was markedly suppressed by decreasing harmful impurities to less than 100 ppm through the new steel fabrication process. Nb-Mo alloy was selected as a liner material to inhibit PCI and tritium (T) permeation. Diffusion bonding was selected as the appropriate lining method for the candidate steel. The irradiation properties of this steel were evaluated by ion irradiation tests simulating the neutron spectrum of LWRs using the triple ion beam irradiation facility of TIARA. The post-irradiation experiments showed the growth of secondary irradiation defects and irradiation hardening were markedly suppressed in the candidate steel (Fig. 1-3). They also showed Cr depletion at the grain boundaries due to radiation assisted segregation was less than 10% over 10 dpa. With the candidate steel having 25% Cr, it is possible to maintain sufficient Cr content to inhibit IASCC after heavy neutron irradiation (Fig. 1-4).
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