Application of Local Approach Concept of Cleavage Fracture to VVER Materials

Abstract

The Prometey local approach of cleavage fracture has been applied within the TACIS R2.06/96 project: “Surveillance Program for VVER 1000 Reactors”, sponsored by the European Commission. The main tasks are: • perform special experiments on smooth cylindrical and pre-cracked Charpy (PCC) specimens for VVER 1000 RPV material in initial, embrittled and irradiated state; • perform fracture toughness tests on 2T-CT specimens for RPV steel in initial and embrittled state; • predict the KJC(T) curves by this model; • compare the calculated and experimental results with the Master Curve results. The local approach of cleavage fracture is applied to predict KJC(T) curves in the transition regime of RPV materials in the initial state, embrittled by thermal heat treatment and irradiated, samples in the latter cases taken from surveillance capsules of a VVER 1000 NPP. The test data of large fracture mechanics specimens (2T-CT) could be well described over a wide temperature range for the initial state and the embrittled material, when the test results of PCC specimens at one temperature are used for the calibration of the model parameters. It is recommended for future application cases to use PCC specimens for the calibration of the parameters. A comparison of the Prometey local approach with the Master Curve approach lead to a good agreement for all investigated materials apart from the thermally embrittled material which has a very high embrittlement level (DBTT shift). The KJC(T) curves of VVER1000 RPV steels with low and moderate embrittlement level could be well predicted by both methods. Because the Master Curve method is already accepted as an international standard, it might be easier to apply in more routine cases. The Prometey probabilistic model may be also used for the prediction of KJC(T) curves of RPV steels with a high embrittlement level.

Brittle fracture local criterion and radiation embrittlement of reactor pressure vessel steels

“…Using this model, some investigations were carried out to assess the effect of short cracks on the function K T Jc ( ) [31,32]. The Prometey model permits predicting the K T Jc ( ) curve for RPV steels with any degree of embrittlement from the test results for a limited number of small-sized specimens [22,[30][31][32][33]. We will consider below the development of this model from the standpoint of physical-mechanical models of the effect of radiation defects such as lattice defects, precipitation of elements and segregation of impurities on the cleavage microcracking processes, and will discuss its application for predicting the influence of plastic strain on the fracture toughness.…”

“…The Prometey model has been verified for RPV steels in various states (initial, irradiated, highly embrittled) [26,22,30,33]. Based on the Prometey model, an engineering method called the Unified Curve method has been elaborated [34,35] which, unlike the Master Curve method [16], allows for the change of shape of K T Jc ( ) curves due to embrittlement.…”

Probabilistic definition of local criterion for brittle fracture under complex thermomechanical loading

“…Coefficients C 1 * , C 2 * , and A d in (4), parameters σ Y , A 0 , and n in (11), constants a b c , , , and h in (20) are determined from the results of tensile testing of smooth cylindrical specimens at various temperatures.…”

“…Test Materials. Investigations to assess the TPL effect on the fracture tioughness were performed on Grade 15Kh2NMFÀ-À reactor pressure vessel steel in initial and thermally embrittled states; its mechanical properties had been detailed in [20,22]. A shift of critical brittle temperature ΔT 41J for the embrittled steel was 180°C.…”

“…Now it is known as the Prometey model [13,20,21]. This model has been demonstrated to allow an adequate prediction of the K T c I ( ) function for variously embrittled RPV materials [20,22].…”