Spectrum Effect Mitigation and Monitoring of the EPR Irradiation Surveillance Program

Abstract

The EPR (initially European Pressurized water Reactor then Evolutionary Power Reactor and considered today as only a trademark) reactor pressure vessel (RPV) has been designed taking into account the requirement of an end-of-life (EOL) RTNDT lower than or equal to 30°C after 60 years of operation. The maximum acceptable fluence at the RPV inner wall, calculated with RCC-M formula and emerging from that requirement, is 3.3 × 1019n/cm2 (E > 1 MeV). To not exceed this fluence level, a heavy reflector has been introduced together with an increased water gap in the reactor downcomer. According to neutron calculations, the expected actual EOL fluence on EPR RPV in the presence of the heavy reflector is between 1 and 2.25 × 1019n/cm2 (E > 1 MeV) depending on the fuel management route. The irradiation surveillance program consists of positioning representative specimens of core region materials in the capsules attached to the outer face of the RPV core barrel. In former reactors equipped with a conventional core baffling, the neutron energy spectrum affecting the surveillance capsules is quite similar to the one affecting the RPV inner wall. In EPR, the heavy reflector presence distorts the neutron spectrum at the capsules’ location and, consequently, the EPR surveillance specimens used to monitor progress of the embrittlement in service would be subjected to a significantly different neutron energy spectrum from that on the RPV inner wall. The aim of this paper is to describe the approach followed by AREVA to limit this neutron spectrum effect and to design an appropriate capsule withdrawal schedule. This approach includes: (1) mitigation, with a new capsule basket design introducing an additional water gap between the reflector and the specimens, and (2) assessment of the embrittlement, not only with the conventional fast neutron fluence (E > 1 MeV), but also with the dose per atom as an additional dose damage parameter.