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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The effect of warm pre-stress (WPS) is well known on cleavage fracture resistance of ferritic steels and has been confirmed on numerous tests on non irradiated materials. Theoritical knowledge let expect that this effect occurs by the same way on irradiated material but has to be demonstrated with experiments in such conditions. An experimental program has been conducted by EDF in Chinon hot cell laboratory on two RPV steels irradiated up to a fluence of 6,5.10 19 n/cm 2 .The ½ TCT specimens (CT12.5) used in this programme have been irradiated in the capsules of the EDF RPV surveillance program of two PWR reactors. Different types of WPS loading path have been applied to cover typical postulated accidental transients. The results confirmed the effect on WPS on the cleavage fracture resistance of the irradiated materials. No fracture occurred during the cooling phase of the loading path and the fracture toughness values are higher than that measured with conventional isothermal tests.The analyses of the experiments have been conducted based on use of simplified engineering models and more refined fracture models based on local approach to cleavage fracture. These analyses are in agreement with the experimental results.