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

摘要

The Reactor Pressure Vessel (RPV) is an essential component 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 is assumed that the failure of a flawed structure occurs when the stress intensity factor at the crack tip reaches the toughness value of the structure material. Toughness curves of materials are obtained from monotonously increasing and isothermal loading. On the other hand, RPV integrity assessment involves loading conditions with coupled cooling, hearing, increasing and decreasing load.The safety analyses made at the European level study, the behavior of defects in the vessel subjected to loading resulting from thermal transients. These analyses usually do not take into account the effect of load history/warm pre-stressing (WPS) of the defects, which is observed in a wide range of experimental studies. The non-consideration of the beneficial effect of this physical phenomenon has two major consequences:· a poor knowledge of the real margins associated with the transients to which 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 specimens manufactured of ferritic 18MND5 steel undergoing different types of loading during the cooling phase as follow:· Load Cool Fracture (LCF)·Loading Maintained CMOD Cooling Fracture (LM2CF)WPS effect observed in the first case, while curve in the second case the failure occurs during the crossing of the transition because of the monotonous increase of the force due to constant CMOD (Crack Mouth Opening Displacement).The second program deals with tests performed at MPA in Germany in collaboration 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 Weibull (Metall Trans A, 14A (1987) 2277) and Chell (Fourth Int Conf Pressure Vessel Technol, Inst Mech Engng, 1980, Paper C22/80, London, U.K. 117) models to predict the WPS effect, details of which are presented here.