The effect of radiation damage on ferritic RPV steels46-50 has for some time been considered in terms of

• The formation of matrix damage (MD), that is, defect clusters and dislocation loops. It is well established that in low copper steels the shift in impact or yield strength properties depends on Vdose.

• The irradiation enhanced formation of copper- enriched clusters (CECs). (CEC are also referred to as CRPs (Cu-rich precipitates) as they were originally assumed to be strictly Cu-rich rather than simply Cu-enriched.) It has been demon­strated that, in many low-to-medium Ni steels and alloys, the yield strength change due to copper precipitation rises to a plateau value that is then unchanged by subsequent irradiation.

• The irradiation induced/enhanced grain boundary segregation of embrittling elements such as P.

The first two mechanisms contribute to embrittle­ment by increasing the steels’ hardness as illustrated in Figure 6(a). The third mechanism induces embrit­tlement without hardening. The latter mechanism is not necessarily found in all RPV steels under operating conditions. Indeed, for MnMoNi steels irradiated in surveillance schemes in Western LWRs, the observed embrittlement is associated with the first two mechanisms; that is, the total shift in the ductile to brittle transition, as measured at the Charpy 41J level, is

A T41J = A T41J(CRP) + A T41J(MD) [2]

where CRP = Cu-rich precipitate and MD = matrix damage, or equivalently the increases in yield strength, Affj, is given by

Asy = Asy(CRP) + Asy(MD) [3]

The first two mechanisms serve to harden the mate­rial and increase the yield strength sy, while the third mechanism causes a drop in the fracture strength, sF. The effects of these changes on the fracture behavior are illustrated in Figure 6(b), where the temperature dependence of the yield stress, sy, and the fracture stress, sF, are plotted. It can be seen that the effect of

irradiation in causing hardening or a change in sF is to cause a change in the transition temperature. In the figure, ATT is caused by an increase in sy, while segregation of P to grain boundaries can lower the fracture stress and result in a shift ATT2. If both mechanisms are operative, then a combined shift of ATT3 occurs.

It is important to note that Ni and Mn are known to strongly influence hardening in steels containing low levels of Cu and also CEC hardening in Cu-containing steels. In Cu-containing steels, satura­tion of the cluster hardening has been demonstrated in steels containing up to ^1wt% Ni. At steel Ni levels above ~1.5 wt% (and with Mn 1.2-1.7 wt%>), cluster hardening has not been observed to saturate. The precise Cu, Ni, and Mn levels at which the plateau is suppressed have not been fully char­acterized, and are the subject of current research. Similarly, the exact influence of Ni and Mn on the embrittlement of low Cu steels has not been fully established and is again a subject of ongoing research. (The term standard MnMoNi steels is used to refer to steels with typical Mn levels (<1.5wt%) and Ni levels < ~ 1-1.2 wt%. The limit on the level of Ni is the subject of ongoing debate
with some workers preferring a limit of 1.0 wt%, with others promoting a higher limit.) Indeed, as will be described in Section 4.05.6, there is concern about whether at high doses (typical of those achieved in plant with extended lives) there may be deviations from the simple framework established above.