Irradiation temperature monitoring with SiC for RPV steel at low fluence

Abstract

Accurate knowledge of the irradiation temperature is a key concern in irradiations of reactor pressure vessel (RPV) steel. We report results of passive temperature monitoring of RPV steel with SiC. Two un-instrumented capsules containing RPV steel blocks were irradiated in Belgian Reactor 2, at SCK CEN in Belgium. Because un-instrumented capsules were used, the irradiation conditions (gamma heating and irradiation temperature) were calculated. To have experimental verification of the irradiation conditions each capsule contained a passive silicon carbide (SiC) temperature monitor. After irradiation the resistivity and the radiation-induced swelling (lattice parameter) is measured using x-ray diffraction (XRD). These measurements were repeated after annealing treatments to determine the peak irradiation temperature. The best method to determine the peak irradiation temperature with the lowest uncertainty in this study was the resistivity technique. Although lattice parameter measurements could be improved by using material without preferential orientation. Tensile tests of the RPV blocks were compared to a large available database of RPV material with the proper sensitivity towards neutron fluence and irradiation temperature to find the irradiation temperature of the RPV steel. The data showed a discrepancy between the temperature obtained from SiC and the temperature obtained from tensile tests. The temperature differences were discussed and rationalized by finite element modelling with respect to uncertainties in the helium gap between the capsule and the RPV steel block. The aluminium holder accounted for the temperature difference and the use of a steel holder is recommended for future similar irradiations to minimize the temperature difference between SiC and tensile specimens . In-depth analysis showed that when the irradiation temperature dropped at the end of the irradiation for a considerable time, the SiC temperature monitor had a memory effect. In this case the post-irradiation resistivity analysis showed two peak irradiation temperatures with a region of constant resistivity between the two.