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.

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