Warm Pre-Stressing Tests for WWER 440 Reactor Pressure Vessel Material

During years 2006 – 2008, warm pre-stressing tests on small (Charpy size) and 1T CT specimens were performed at NRI Rez. The specimens were made from WWER 440 reactor pressure vessel material in as-received, thermally treated (artificially aged) and irradiated conditions, the last two conditions simulating the end of life state of the RPV. In this paper, only results of tests performed for this material in as-received and irradiated conditions are presented. Evaluation of WPS tests was performed with using Chell and Wallin predictive models. The attention was paid to 5% probability level fracture predictions, since this level of probability is important for WPS application in pressurized thermal shock evaluation performed within the RPV integrity assessment. From point of view of this 5% probability fracture prediction, both Chell and Wallin models appeared not to be sufficiently conservative for LCF regime (prediction of “Case 2”); for other regimes (LUCF, LPUCF, LTUF and LPTUF) they appeared to be sufficiently conservative (in almost all cases). Based on the results of the tests, Wallin model was selected for implementation into the RPV integrity evaluation procedure, but simultaneously a decision was adopted to decrease its predictions when the “Case 2” is predicted: instead of predicting some surplus (15% of virgin K_IC) above the value of K_WPS, only value of K_WPS (without any surplus) is predicted. This measure enhances conservativeness of the Wallin model to a sufficient level: the performed WPS experiments then well confirm the Wallin model predictions decreased in this manner. Taking 90% of the value of K_WPS represents an additional margin implemented currently in the WPS methodology.