This paper describes a procedure for failure assessment based on elastic/plastic fracture mechanics, proposed by the Central Electricity Generating Board. The basis of the assessment for failure is the Dowling-Tonwley approach. The procedure enables the user to determine the maximum tolerable flaw size, determine the acceptability of a discovered or postulated flaw, examine the sensitivity of the results to variations in input data and/or methods of calculation, and define appropriate margins of safety.