The ASME code cases N-629 and N-631 permit the use of a master curve-based index temperature $(RTTo≡T0+19.4°C)$ as an alternative to traditional $RTNDT$-based methods of positioning the ASME $KIC$ and $KIR$ 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 $ART=initialRTNDT+ΔRTNDT+margin$. The conservatism of this procedure when $RTNDT$ is replaced by $RTTo$ 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 $RTToirr$, $RTToref$, and $ΔT41JRG$ 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 $RTTo$ 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.

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