Structural behavior during a PTS transient taking into account the WPS effect

The Reactor Pressure Vessel (RPV) is an essential component liable to limit the life duration of PWR's. Its behavior in service is limited in time due to the embrittling effects of irradiation. The structural integrity of the RPV is assessed by conventional fracture mechanical studies, where it is a ssumed that the failure of a flawed structure occurs when the stress intens ity factor at the crack tip reaches the toughness value of the structure ma terial. Toughness curves of materials are obtained from monotonously increa sing and isothermal loading. On the other hand, RPV integrity assessment in volves loading conditions with coupled cooling, heating, increasing and dec reasing load. The safety analyses made at the European level study, the behavior of defec ts in the vessel subjected to loading resulting from thermal transients. Th ese analyses usually do not take into account the effect of load history/wa rm pre-stressing (WPS) of the defects, which is observed in a wide range of experimental studies. The non-consideration of the beneficial effect of th is physical phenomenon has two major consequences: . a poor knowledge of the real margins associated with the transients to wh ich the vessel is subjected, . an economical penalty due to large under-estimation of the life duration of the vessel. This paper presents the results of two independent programs. The first dealing with four WPS tests performed at CEA in France on CT spec imens manufactured of ferritic 18MND5 steel undergoing different types of l oading during the cooling phase as follow: . Load Cool Fracture (LCF) . Loading Maintained CMOD Cooling Fracture (LM2CF) WPS effect is observed in the first case, while curve in the second case th e failure occurs during the crossing of the transition because of the monot onous increase of the force due to constant CMOD (Crack Mouth Opening Displ acement). The second program deals with tests performed at MPA in Germany in collabor ation with EDF on CT25 and CT50 specimens using the same material with five types of WPS cycles: . LCF . Load Unload Cool Fracture (LUCF) . Load Transient Fracture (LTF) . Load Oscillation Cool Fracture (LOCF) . Load Oscillation Transient Fracture (LOTF) In all these cases, WPS effect is demonstrated. For both the programs, numerical analyses were performed at MPA using Weibu ll [Metall Trans A, 14A (1987) 2277] and Chell [Fourth Int Conf Pressure Ve ssel Technol, Inst Mech Engng, 1980, Paper C22/80, London, U.K., 117] model s to predict the WPS effect, details of which are presented here. (C) 2001 Published by Elsevier Science Ltd.