4.05.5.1 Introduction

DDRs are equations that describe the changes in mechanical properties (yield stress, Charpy transition temperature, upper shelf toughness, hardness, etc.) as a function of neutron dose. The purpose of this section is to set out the mechanistic DDRs that have been developed to describe the embrittlement of RPV steels incorporated in the different reactor clas­ses worldwide. However, it is important to note that the earliest DDRs were purely statistical in nature. The formulas were derived in different countries from the analysis of (different) databases. The first prediction formula in the United States that included the effect of residuals (‘residuals’ refers to levels of solutes such as Cu) was published in 1975. It was then followed by the Regulatory Guide 1.99 revision 1.106 This Guide has been particularly influential and presents some important features, such as the exis­tence of thresholds in chemical composition, and an explicit dependence on Cu and Ni. It was followed in 1988 by USNRC Regulatory Guide 1.99 Revision 2.107 Petroquin108 has reviewed and compared the formulas employed for the prediction of irradiation embrit­tlement of reactor vessel materials, including the empirical DDRs developed in France (FIS and FIM formulae109), Germany (KTA110,111), and Japan (Japan Electric Association Code (JEAC) 4201 1991112,113).

We focus here on DDRs (or embrittlement corre­lations) that have been developed with the form of the equations reflecting mechanistic understanding of the development of radiation damage in RPV steels, while the exact parameterization has been undertaken through fitting the models to large mechanical property databases arising from the test­ing of irradiated surveillance samples. The major driving force for such developments has been the

need to take advantage of the greatly improved understanding of embrittlement mechanisms in DDRs that enable interpolation or extrapolation with improved confidence to a parameter space poorly covered by a given (national) surveillance database. A common feature of such DDRs is that they follow the same mechanistic framework (described in Section 4.05.4), but give different weights to the parametric dependencies of radiation damage that have been described in the previous sections. Properly describing the effect of flux on the embrittlement of both low Cu and Cu-containing steels has been subject to extensive debate (see Section 4.05.6). A further common fea­ture is that the DDRs have been refined as new surveillance data have become available, frequently with changes in the form of the equations in order to accommodate more sophisticated mechanistic under­standing. It is important to note that they have been developed to describe embrittlement under relatively low dose rate conditions that apply to specific steel types, that is, CMn or MnMoNi steels.

Two classes of steels have been described by such DDRs, namely, CMn RPV steels employed in Mag — nox reactors and MnMoNi steels used in Western LWRs (primarily in the United States and Japan). The mechanistically based DDRs for CMn steels were developed in the 1980s while it was not before 1998 that the first such DDR was published describing embrittlement in US RPV steels.