The assessment of reactor pressure vessel defects allowing for crack tip constraint and its effect on the calculation of the onset of the upper shelf

摘要

The paper shows how a specialised application of the R6 method could be used to calculate a pressure-temperature failure envelope for postulated defects in a reactor pressure vessel (RPV), making due allowance for the distribution of constraint around the crack front. As such, the technique provides a means of estimating a defect-specific onset of upper shelf temperature (OUST). A material's constraint-based toughness behaviour can be described using separate curves to represent the variations of ductile fracture toughness with constraint and cleavage fracture toughness with constraint and temperature. Armed with such a constraint-based material response, and the calculated variation of constraint with pressure determined at different points along the crack front of a defect, the pressure-temperature failure envelope can be estimated by solving equations inherent in the R6 method. The method is illustrated for a spherical RPV fabricated from ferritic steel, with radius 10m and wall thickness 100 mm, by application to three postulated surface-breaking defects: an extended, circumferential defect; and two semi-elliptical defects. Constraint-based fracture toughness curves can be obtained by applying micro-mechanical models to a series of modified boundary layer Finite Element analyses. For each postulated defect an estimate of the OUST is presented and compared against the value which would be obtained from an assessment of high constraint fracture toughness data. For the 5 mm deep extended, circumferential defect, allowance for loss of constraint resulted in a beneficial shift (i.e. to a lower temperature) in the OUST of about - 18.8℃. For the 5 mm deep by 30 mm long surface breaking, semi-elliptical defect, there was a shift in the OUST of - 17.8℃, a value which did not change appreciably when residual stresses were allowed for. For the 25 mm deep by 150 mm long surface-breaking, semi-elliptical defect, the OUST was shifted by - 19.7℃ without allowance for residual stresses, and by - 18.7℃ allowing for residual stresses. All these values represent significant improvements in the OUST. Because the direction of these shifts in the OUST is opposite to that which would occur due to material degradation by in-service neutron irradiation, it is evident that crack-tip constraint is an important factor that should be taken into account in assessing RPV lifetime.