Embrittlement correlation methods USA

The Revision 1 result was a simple expression for embrittlement as a function of Cu and P content (wt%) and neutron fluence (f, lO n/cm E IMeV)  

The trend curve was pieced together by P.N. Randall of the NRC using evaluations and analyses performed by others. The Revision 2 trend equation uses CF values that are tabulated chemistry factors (in UK units) for welds and base metals based on Cu and Ni content (wt%), and FF is a more complex fluence function (f is again expressed as lO n/cm, E lMeV)  

This embrittlement equation was also incorporated into an ASTM Standard Guide E 900-87. The residuals of predicted minus measured values were used to derive a standard error for both welds and base metals the computed standard error (cta) for welds was 15.6°C (28.0°F) and 9.4°C (17.0 °F) for base metals. These standard error values are used to derive a bound to the embrittlement prediction using essentially 2oa. 

Another significant change was the attenuation model for fluence through the vessel wall. In Revision 1, the attenuation model used a fast neutron exposure (n/cm, E 1 MeV) as the attenuated fluence, whereas Revision 2 used a displacements per atom (dpa) model for attenuation through the vessel wall. The attenuation using dpa is less than for fast neutron, which means that the projected embrittlement at 14-thickness and %-thickness is higher using the dpa model. 

The US NRC was developing the Pressurized Thermal Shock (PTS) Rule at the same time that Revision 2 of Regulatory Guide 1.99 was being revised. The original PTS Rule was based on an intermediate embrittlement correlation on a slightly smaller database before Revision 2 of the 

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