Estimation of Master Curve Based RT_(TO) Reference Temperature From CVN Data

摘要

The ASME code cases N-629 and N-631 permit the use of a master curve-based index temperature (RT_(To) ≡ T_0 + 19.4℃) 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 the 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 an additional procedure to predict the adjustment of fracture toughness for end of life (EOL) from irradiation surveillance results must be available as by NRC R.G. 1.99 Rev. 2, e.g., the adjusted reference temperature is defined as 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.