Behavior of underclad cracks in reactor pressure vessels: Evaluation of mechanical analyses used in French RPV integrity assessment by cleavage fracture tests on large scale plates

Innocuity of underclad flaws in the reactor pressure vessels must be demonstrated in the French safety analyses, particularly in the case of a severe transient at the end of the pressure vessel lifetime, because of the radiation embrittlement of the vessel material. Safety analyses are usually performed with elastic and elasto-plastic analyses taking into account the effect of the stainless steel cladding. EDF has started a program including experiments on large size cladded specimens and their interpretations. The purpose of this program is to evaluate the different methods of fracture analysis used in safety studies. Several specimens made of ferritic steel A508 Cl 3 with stainless steel cladding, containing small artificial defects, are loaded in four-point bending. Experiments are performed at very low temperature to simulate radiation embrittlement and to obtain crack instability by cleavage fracture. Three tests have been performed on mock-ups containing a small underclad crack (with depth about 5 mm) and a fourth test has been performed on one mock-up with a larger crack (depth about 13 mm). In each case, crack instability occurred by cleavage fracture in the base metal, without crack arrest, at a temperature of about -170 C. Each test is interpreted using linear elastic analysis and elastic-plastic analysis by two-dimensional finite element computations. The fractures are conservatively predicted the stress intensity factors deduced from the computations (Kcp or KJ) are always greater than the base metal toughness. The comparison between the elastic analyses (including two plasticity corrections) and the elastic-plastic analyses shows that the elastic analyses are often conservative. The beneficial effect of the cladding in the analyses is also shown; the analyses are too conservative if the cladding effect is not taken into account