1994:14 Evaluation of acceptance criteriafor data on environmentally assistedcracking in light water reactors

The assessment of possible sub-critical crack growth through environmentally assisted cracking (EAC) is a commonly encountered problem in safety evaluations of light water reactor components. The numerous factors influencing EAC fall into three main groups (material, medium and loading), but their interactions are highly complex. Field assessments usually rely on the evaluation of experimental results, with or without parallel attempts to model the cracking process, but they are hampered by the extreme scatter often exhibited by laboratory data under nominally identical testing situations. Furthermore, doubts often exist as to the extent to which the field conditions have been adequately simulated in autoclave tests.

The approach presented in the present report to resolve this problem involves a systematic assessment of the quality of the experiments performed by reference to acceptance criteria for specific aspects of testing, followed by downgrading, on a weighted scale, of those data which are associated with inadequate experimental techniques. Subsequently, cut-off points can be set so as to rationalize the overall data assessment.

Generalised criteria are developed and justified for the individual test parameters within the three groups of factors influencing EAC. However, it has to be recognized that the weighting will vary for different classes of material and operating environments. The viability of the approach is demonstrated by means of a full-scale analysis of data on the stress corrosion cracking behaviour of low-alloy steels under simulated BWR conditions.