Estimation of Master Curve Based RT_To Reference Temperature From CVN Data

The ASME code cases N-629 and N-631 permits the use of a Master Curve-based index temperature (RT_To ≡ T₀ + 19.4°C) as an alternative to traditional RT_NDT-based methods of positioning the ASME K_Ic, and K_IR curves. This approach was adopted to enable use of Master Curve technology without requiring the wholesale changes to the structure of the ASME Code that would be needed to use all aspects of Master Curve technology. For the brittle failure analysis considering irradiation embrittlement additionally a procedure to predict the adjustment of fracture toughness for EOL from irradiation surveillance results must be available as by NRC R.G. 1.99 Rev. 2 e.g.: ART = Initial RT_NDT + ΔRT_NDT + Margin. The conservatism of this procedure when RT_NDT is replaced by RT_To is investigated for western nuclear grade pressure vessel steels and their welds. Based on a systematic evaluation of nearly 100 different irradiated material data sets, a simple relation between RT_Toirr, RT_Toref and ΔT_41JRG is proposed. The relation makes use of the R.G. 1.99 Rev. 2 and enables the minimizing of margins, necessary for conventional correlations based on temperature shifts. As an example, the method is used to assess the RT_To as a function of fluence for several German pressure vessel steels and corresponding welds. It is shown that the method is robust and well suited for codification.