Review of a procedure for performing constraint and attenuation-corrected fracture mechanics safety case calculations for Magnox reactor steel pressure vessels

An assessment methodology put forward by BNFL Magnox Generation to take account of effects of constraint and dose attenuation in determining the margin between onset of upper shelf temperature (OUST) and normal steady state service temperature has been examined in detail by the UK Technical Advisory Group on Structural Integrity of Nuclear Plant (TAGSI). The standard methodology assumes that the constraint at a service flaw corresponds to that in a deeply notched three point bend or compact tension fracture toughness specimen. It also assumes that the maximum level of neutron and thermal ageing embrittlement at the vessel inner surface applies throughout the thickness. The revised procedure uses the local approach with detailed finite element analysis of specific defect geometries to determine constraint effects and allows for attenuation of embrittlement effects through the thickness. Demonstration examples of the analysis procedures were provided by BNFL Magnox Generation and AEA Technology Ltd. TAGSI endorses the proposed methodology, subject to validation in two specific areas, and to sensitivity studies being undertaken on the effects of other related factors. The key areas requiring validation are: (a) the use of start-of-life yield strength and work hardening properties for estimating end-of-life constraint benefits, and (b) the transferability of local approach model parameters to situations of differing constraint. TAGSI has given consideration to circumstances in which the methodology could be used. In carrying out this review, it has been recognised that OUST is sensitive to constraint and attenuation adjustments and it can no longer be regarded as a material property. The value of OUST determined from highly constrained fracture toughness specimens should be regarded as a 'reference' value for a given material but will not necessarily represent the value of OUST for a service application