Cleavage fracture of large scale cladded mock-ups containing an underclad crack: Preliminary analyses by the local approach of cleavage fracture

Innocuity of underclad flaws in the reactor pressure vessels regarding the risk of fast fracture must be demonstrated in the French safety analyses, particularly in case of severe overcooling transient. The safety analyses are usually performed with elastic and elasto-plastic analyses taking into account the effect of the stainless steel cladding. A new method called the local approach of cleavage fracture is considered here. This method allows evaluating the probability of failure of a component submitted to a mechanical or thermal loading. EDF has started a structural integrity verification program including experiments on large size cladded specimens and interpretation of the test results. The purpose of this program is to evaluate the different methods of fracture analysis used in safety studies. Four tests in four point bending have been performed at low temperature on mock-ups containing an underclad crack. In each case, the crack instability is obtained by cleavage fracture in base metal, without crack arrest, at a temperature of about -170 C. The experimental results and the main mechanical analyses are presented in other papers. The authors present in this paper the preliminary interpretations of the test results using the local approach in cleavage fracture. Each test is interpreted by two-dimensional finite element elastic-plastic computations using the Weibull model with the parameters proposed by FRAMATOME (m = 22 , σu = 2,630 MPa). The probability of failure is evaluated with different meshes in each test. The results show an important effect of the size of the elements near the crack tip and of the Weibull stress σw calculation method. The effects are confirmed on a CT specimen