EONY model

The development of embrittlement correlation methods is schematically illustrated in Rg. 11.1, where embrittlement correlation methods are very roughly classified into two groups as mentioned above statistically-based methods and mechanism-guided methods. Most of the embrittlement correlation methods authorized by the regulatory bodies are statistically-based, but the EONY model which is used in the US alternative PTS rule. 

The general form of the revised embrittlement correlation equation, widely known as the EONY model, is similar to the previous models. Instead of simplifying the existing models, some new key models were introduced into the revision of the models. The use of effective fluence, which had already been suggested by Odette and others, is one such modification. The idea is based on the experimental evidence on enhanced embrittlement, particularly in Cu-containing materials irradiated at low fluxes. This is attributed to the enhanced diffusion of solute atoms, and is modeled by the effective fluence, te, as ... 

The EONY model consists of two terms, i.e. the matrix feature (MF) term and Cu-rich precipitate (CRP) term as follows ... 

The coefficient B on the product form of the CRP term is more sensitive to the product form than the coefficient A for the MF term. A fiux-time at temperature parameter that appeared in Eqs 11.8 and 11.13 is no longer used in Eq. 11.28. Instead, the effective fiuence of Equation 11.22 is also used in the CRP term. Cu and Ni contents are also considered in the CRP term as in the EWO model and, in addition, P content is newly considered in the EONY model. 

A significant amount of new data became available in 2003 when capsules from the BWR Supplemental Surveillance Program (SSP) were tested. Because low-flux BWR shift data were significantly under-predicted by the existing shift models, additional refinement of the NRC model was performed. Furthermore, the Eason database was updated for new PWR capsule test data (about 140 shifts) and then used to derive the embrittlement correlation known as EONY (after the developers Eason, Odette, Nanstad and Yamamoto) (Eason et al., 2007) which was codified in the Alternative PTS Rule, 10 CFR 50.61a (10 CFR, 2010). 

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