Material Surveillance Concepts and Acceptance Criteria for Future High Temperature Reactors

Abstract Structural materials in service in future high temperature reactors will experience thermomechanical creep-fatigue damage, which could be potentially accelerated by the component environment. These environmental effects could include high neutron radiation fluxes and, for some reactor concepts, exposure to corrosive coolants. There is almost no combined effects test data available to quantify the influence of the environment on the mechanical strength and durability of likely reactor materials. An alternative strategy would be to conduct a materials surveillance program as the reactor operates, with the goal of monitoring the degradation of the reactor materials in the actual component environment. Such a surveillance program would provide valuable data on degradation rates for remaining life assessment and more generally to help plan reactor repair and maintenance schedules and to support future life extensions. Operating light water reactors maintain similar surveillance programs, for example monitoring the radiation embrittlement of the pressure vessel steel in pressurized water reactors. While surveillance programs are well-established for light water reactors, there is currently very little guidance for high temperature reactors. The techniques for material surveillance in light water reactors, and even the key material properties being monitoring, will be very different in future high temperature reactors. Past work led to the development of passively-actuated mechanical test articles that induce creep-fatigue loading in a test section driven only through fluctuations in the surveillance article temperature, for example caused by reactor transients. These test articles drive mechanical cycling in the test section using differential thermal expansion. This paper describes how to use surveillance articles of this type to run a material surveillance program in a future high temperature reactor. This includes describing three potential surveillance program concepts: (1) a canary approach where the failure of the test article provides early warning of expected failure in the corresponding component, (2) a twin approach where a second sample is maintained outside of the reactor, experiencing the same thermal history but none of the reactor environmental effects, and (3) a full remaining life assessment where detailed measurements on surveillance articles removed from the reactor provide an updated estimate of the material degradation and remaining life. This description includes detailed acceptance criteria, unique to each surveillance concept. These acceptance criteria would be used by the operating engineering team to determine if the component can continue to operate through the next inspection interval or if mitigation needs to occur immediately. These surveillance concepts could be extended and standardized to provide operating guidelines for surveillance programs in future plants.