2018 | ANNUAL REPORT 77 This project is intended to experimentally verify the irradiation-induced inverse stress behavior and determine its impact of the stress on the performance of SiC/SiC as the LWR fuel cladding. Project Description Among the unique challenges for fuel cladding made from SiC/SiC composite is internal stress arising from differential irradiation strain. SiC swells rather rapidly during the first week or two of operation as the LWR fuel cladding until the point-defect accumulation saturates.The swelling of SiC accompanies a decrease in the thermal conductivity, resulting in a steep temperature drop across the cladding-wall thickness due to high heat flux from fuel to coolant. The saturated swelling strain exhibits negative dependence on temperature. The results are the tensile stress at the inner surface and compressive stress at the outer surface of the cladding wall i.e., the opposite effect from normal thermal stress (Figure 1). The implications of the predicted “inverse thermal stress” range from significant (such as microcracking initiating from the inner surface) to severe (crack networking leading to a loss of hermeticity and a threat to structural integrity) for the fuel cladding.This R&D project is intended to experimentally verify the irradiation-induced inverse stress behavior and determine its impact of the stress on the performance of SiC/ SiC as the LWR fuel cladding . The project consists of three technical tasks: 1) design and build an irradiation vehicle that enables neutron irradiation of small tubular specimens under an LWR-relevant radial heat-flux condition, 2) experimentally verify the thermal gradient through the tube wall thickness, and 3) examine the damage in the irradiated tubes and other effects of neutron irradiation under a steep temperature gradient. Each task is first-of-a-kind development involving significant technical obstacles that have to be overcome. However, success of the project will have implications beyond its immediate objectives because this innovative approach will enable studies on irradiation effects accompanying significant dimensional instability and irradiation behavior of any material under a steep temperature gradient.