No abstract

The warm pre-stress (WPS) of a flawed structure occurs when it is pre-loaded at high temperature in the ductile domain then cooled and loaded up to fracture in the brittle to ductile transition temperature domain. This load history is a feature of RPV accidental transients of LOCA type. Numerous tests on non irradiated specimens and structures have shown the favourable effect of WPS on fracture behaviour. Theorical knowledge let expect that the WPS effect occurs by the same way on irradiated material, but experimental approach had to be completed in such conditions. The experimental program presented in the present article consists in fracture toughness tests under WPS loading conditions performed on two RPV steels irradiated up to a fluence of 6,5.1019 n/cm2. The CT12.5 specimens used for these tests had been irradiated in the capsules of the pressure vessel surveillance program of two french reactors. Different types of WPS load history have been applied to cover typical accidental transients. All the results obtained confirmed for an irradiated steel the two assumptions generally made about the WPS effect: no fracture occurred during the cooling step of the loading even at high load level and the mean fracture toughness value is higher than that measured with conventional mono-temperature tests.

Base nickel alloys like Inconel 600 or 182 are particularly sensitive to stress corrosion cracking. This fact is well known since Corriou’works at the beginning of the sixties and its applications to the steam generator tubes in the seventies. For the RP vessel heads, the major fact of the nineties was the leak that occurred on one penetration in 1991 in the French NPP unit of Bugey. Several important decisions were taken after discover of this leak. First of them was to understand why it appeared so quickly, then test repairs for the Bugey case, then decide to replace all vessel heads considering that the repair solutions was to high cost. In parallel many developments were launched to establish laws for PWSCC and develop non-destructive methods to inspect the head penetrations. The conclusions obtained show the decision was good and no new leak happened on the VH penetrations.

The structural integrity of the RPV is an essential issue for the plant safety. At the design stage, the demonstration is required with material properties at end of life, to ensure the adequacy of the design with the expected operating transients in all conditions. During operation, the integrity assessment is updated every ten years with new existing knowledge and feedback of operating experience, in particular in service aged material data coming from Irradiation Surveillance Program ISP, fluence evaluations taking into account the effective in service core arrangements in each vessel, in service detected flaws plus a postulated subclad crack whose detection cannot be guaranteed by the qualified ISI program. The final assessment showed that the regulatory criteria are met until the fourth decennial outage for 900 MW RPV. The analysis is performed in accordance with French regulations (use of safety coefficients) and follows a deterministic approach in which the input parameters and uncertainties are taken into account conservatively. For the future demonstration beyond 40 years, a multidisciplinary effort is committed to improving knowledge in order to reduce uncertainties in data and in methods. This extensive program involves in particular: - Thermohydraulic analysis and description of transients: temperatures and heat exchange coefficients; - Mechanical analysis: warm pre-stress effect and crack arrest. In addition, a complementary study using a probabilistic approach to rationalize the level of conservatism of input data is launched. In this report, the French deterministic approach and the main results for 40 years duration are presented and the new developments for the future.

No abstract