The NESC-1 large scale pressurised thermal shock experiment was designed partly as a test of the performance of structural analysis methodologies. One of the activities performed by the NESC network was a “codes and standards” round robin, which applied national failure avoidance codes to the defects in the NESC-1 cylinder. Six procedures were applied: ASME Section XI, R6 and BS PD6493:1991 (UK), KTA (Germany), SKIFS 1994:1 (Sweden), and RCCM/RSEM (France). The results were uniformly pessimistic. All the assessments predicted acceptable defect sizes in the range 1–10mm, nearly two orders of magnitude smaller than the large defects in the NESC-1 cylinder which produced only very limited crack growth during the test. The reasons for this excessive pessimism were reviewed using the R6 assessment as a representative example. Three major areas of pessimism were identified: the defect model used to represent real defects in the test cylinder, the methods used to estimate crack driving force in the presence of thermal shock stresses that cause plastic flow, and the transition fracture toughness model used. Excess pessimism in predictions of crack driving force can be minimised by use of an appropriate defect model (through-clad for surface defects, sub-clad for buried defects), and by taking account of the effects of plasticity on secondary stresses in estimates of crack driving force. Fracture toughness curves based upon the traditional ASME approach with safety factors are very pessimistic for the NESC-1 material. A statistical lower bound based upon the Master Curve provides a much better description of the material behaviour.

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