Preliminary Analysis of Reactivity Feedbacks for Inner-Side Chromium Coating

Abstract

Protective coatings were proposed as near-term concepts to enhance the accident tolerance of nuclear fuel claddings; it is expected to improve corrosion resistance in severe events, and enhance cladding performance during normal operation, without introducing major design changes. Various aspects such as corrosion resistance, thermal and neutronic performances are being evaluated. However, the fact that inner uncoated side of fuel cladding may expose to the oxidizing environment under some circumstances is still a concern. Thus, a complementary inner-side coating was proposed. Metallic chromium is extensively studied as a potential coating material, and showed promising performance. Neutronic penalties are expected if chromium used as protective coating. In this study, reactor's inherent safety feedback, when chromium coatings used for fuel claddings inner-side protection, is evaluated and compared with the case of using external coatings. These parameters have not been studied before. Five different coating thicknesses were used in this evaluation. Results showed that inner side coatings will induce less negative moderator temperature feed backs, while feedback to changes in fuel temperature will be more negative than the reference uncoated case. Boron coefficient was found to be less negative compared to the reference. It was found that each coating thickness will induce unique changes in neutrons flux; generally, there will be reduction in thermal portion of neutron flux, which will be less for the case of inner-side coatings, when compared to external coatings. The magnitude of feedback varies from thickness to another. Reactivity feedbacks also calculated for some cases where inner and external coatings used simultaneously, results showed similar behavior, with the magnitude varying from a combination to another. The presence of chromium coatings will introduce some changes in reactor's neutron flux; nevertheless, the reactor will still keep its inherent negative feedback to changes in different operation parameters studied.