Neutron Spectrum Effect and Damage Analysis on Pressure Vessel Steel Irradiation Behaviour

A comprehensive program on pressure vessel irradiation behaviour has been carried out by the Commissariat à l'Energie Atomique (CEA) and Electricité de France (EDF) from 1989 until 1995. This program is a key approach to EDF plant life assessment strategy. It is based on experimental correlations between pressure vessel steel irradiation behaviour with regard to fluences obtained in two very different irradiation spectra specially designed in the OSIRIS and SILOE CEA test reactors. The aims of this program were to assess a potential neutron spectrum effect in the range met in a Pressurised Water Reactor (PWR) between the vessel (internal surface, 1/4 thickness) and the surveillance capsules and correlatively to define the most appropriate exposure parameter for low alloy steel embrittlement.

In order to fulfill this objective, all experimental parameters had to be accurately controlled. New high-capacity irradiation rigs have been therefore developed by the CEA to test two materials in parallel with a large number of Charpy-V specimens in well-defined conditions (temperature, neutronic environment). Two weld materials were specifically manufactured from low alloy Mn Ni Mo steel. In terms of embrittling elements (Cu, Ni, P), these materials are representative of average and upper bound chemical compositions found in French nuclear power reactor vessel welds. Target fluences were 0.1 dpa in both reactors, corresponding to 7.10¹⁹ n.cm^-2 (Φ_E>1MeV) in the OSIRIS standard spectrum and to 4.10¹⁹ n.cm^-2 (Φ_E>1MeV) in the SILOE “distorted” spectrum. The spectra characteristics were derived both by damage monitor dosimetry and neutron transport calculations. For each material in each state, Charpy-V transition curves were defined.

The results in terms of Charpy-V transition shifts and fluences (Φ_E>1MeV, Φ_E>0.1MeV and Φ_dpa) are presented then discussed. In the range considered, no spectrum effect and a very good correlation using fluence Φ_E>1MeV were found.