It was pointed out in Section 4.05.4.1 that the segre­gation of certain impurities to grain boundaries could cause nonhardening embrittlement. This phenome­non has received less attention than the hardening from the production of small clusters. Several reliable techniques (AES, FEGSTEM, and atom probe) exist with which grain boundary segregation may be not only observed, but also quantified,41 and there have been a number of critical studies that have both measured and modeled the segregation ofimpu — rity elements under irradiation.39,101-105 (Extensive experimental programs on long-term aging have per­mitted the accumulation of segregation data on a variety of model alloys and steels. It has been possible to interpret these data in terms of the simple McLean theory of equilibrium segregation (McLean, D. Grain Boundaries in Metals; Clarendon Press: Oxford, 1957). The success of the McLean model in describing segregation in these alloys and steels indicates that segregation is generally thermodynamically con­trolled, and defect gradients have no effect.)

The segregation of P and C to grain boundaries in irradiated materials has received greatest atten — tion.41,100 Overall P segregation increases with irradi­ation dose in all of the model alloys and steel types examined. The rate of P segregation under irradia­tion appears quite variable, both in different classes of steel and within a given class. It is possible that P segregation under irradiation is slower in welds than in the CGHAZ microstructure, because of the presence of additional traps for P in the welds. Other causes of variability are less consistently observed. The behavior of C is less consistent. In the model alloys and the CMn steels, grain boundary C gener­ally decreases with fluence, but in the MnMoNi steels C segregation may either increase or decrease. Desegregation of C appears more likely to be related to carbide precipitation in these materials with rela­tively high free C than merely to trapping of C at matrix defects.

Quantifying the data has been attempted in sev­eral cases.9 , 0 The majority of models indicate that P is dragged to grain boundaries during radiation by the flux of irradiation-induced defects to sinks. Consis­tency between the models and data need not necessarily confirm the validity of the model, as all have adjustable parameters, and no data set is large enough or coherent enough to test the models with much stringency.

Importantly, a conclusion from the European Commission 5th Framework PISA programme was ‘‘On the basis of the observations made here and else­where, it appears unlikely that nonhardening embrit­tlement will influence RPV condition during normal operation for homogeneous MnMoNi steels (i. e., A508 Class 3, A533B, 22NiMoCr37) of <0.02 wt% P.’’101

4.05.4.2 Summary

The formation of matrix defects has been established as an important contribution to RPV embrittlement. The studies of low Cu steels have established that they are produced continuously during irradiation but the nature of these defects is still uncertain. The experimental data provide strong evidence for clus­ters that are sensitive to positrons; and for PA being a useful technique for studying MD. Although micro­voids have been identified in model alloys from posi­tron lifetime studies, none has been identified in commercial steels from such studies. It is important to note that there is increasing evidence that vacancy clusters are not responsible for the observed harden­ing; it has been inferred that interstitial clusters are responsible for the observed hardening in RPV steels. Postirradiation annealing has been shown to be a pow­erful means of investigating the nature of the matrix defects further. A major development has been char­acterizing MD as being due to two components; first, SMD and second, at high fluxes, UMD. UMD are matrix defects which, although thermally unstable at the irradiation temperature, are frozen into the micro­structure during the cooldown after irradiation.