Influence of Cladding Microstructure on the Low Enthalpy Failures in RIA Simulation Tests

Abstract

Welding used during the preparation of specimens for Reactivity Initiated Accident (RIA) simulation testing from fuel rods previously irradiated in reactors introduces the following four microstructural changes in the specimen: (a) annealing of irradiation damage, (b) a change from an α-phase structure of Zircaloy to a transformed β structure in the cladding, (c) dissolution of the hydride rim formed under the oxide on the cladding tube outer surface during normal irradiation and possible radial-oriented hydride reprecipitation at the transformed β platelet boundaries, and (d) reprecipitation of second-phase particles previously dissolved due to radiation damage. A fifth factor, the change in the texture of Zircaloy, is also introduced due to the welding operation. The possible effect of these five changes on the specimen fracture toughness and failure enthalpy is evaluated in this paper. The published data on the mechanical properties of irradiated and unirradiated transformed beta structure of Zircaloy charged with hydrogen are reviewed to evaluate the impact of the five anticipated microstructural changes on the failure enthalpy. The available RIA simulation test fracture data with low-failure enthalpy are reviewed. The limited failure path information available appears to indicate that microstructural factors have contributed to the low-enthalpy failures. The applicability of the results from the low-enthalpy RIA test failures to Light Water Reactor (LWR) nuclear fuel should be based on the representativeness of the RIA specimen microstructure to that of the LWR fuel cladding.