Experimental information on the nature of matrix defects necessarily splits into evidence for clusters formed by vacancies and interstitial point defects. Significant insight into potential vacancy clusters has come from positron studies of irradiated and model steels. Positrons are well established as a tool for probing vacancy-type defects in solids, where the defect concentration is typically 1018cm~3. (Posi­trons are attracted to regions of the lattice which are more ‘open’ than average. The most obvious such regions are vacancies and vacancy clusters (the larger clusters being stronger positron traps). Less obvious open regions include the tensile parts of a dislocation strain field (even around an interstitial loop) or incoherent particle-matrix interfaces. The positron annihilation techniques have included both positron lifetime (PA-t) and lineshape (PALA) ana­lyses or, more recently, CDB. The latter is an impor­tant recent development. CDB (also known as positron annihilation spectroscopy-orbital angular momentum distribution, PAS-OEMS) measures the energy spectrum of the gamma rays produced at the positron annihilation sites, and thus the momenta of the electrons at those sites. The energy (or momentum) spectra characterize the elements sur­rounding the positron traps.72

Overall, the experimental data provide strong evi­dence for open-volume clusters that are sensitive to positrons, and for PA being a useful technique for studying MD (see, e. g., Buswell and Highton,73 Dai et a/.,74 Carter et a/.54). In model alloys, a number of authors report evidence for large vacancy clusters or microvoids after irradiation. Most interestingly, although the positron lifetime studies have indicated the presence of vacancy clusters in model alloys, such studies have not identified vacancy clusters in com­mercial steels (e. g., Dai et a/.74). The study by Valo eta/.75 is most convincing in this aspect, as the authors investigated both model alloys and RPV steels.

Evidence for interstitial clusters has come primar­ily from studies of model alloys or steels irradiated to very high doses in excess of that of interest for most power reactor applications. Such studies have nor­mally employed TEM techniques that are sensitive to the strain field associated with small dislocation loops. The imaging of such features is difficult in ferritic materials because of the need to correct for the image distortion caused by the ferromagnetic behavior of the samples, and because of the contrast arising from surface oxide on the thinned specimens. Critically, the resolution for small dislocation loops is ^2-3 nm in even well-prepared samples imaged in higher voltage microscopes. Krishnamoorthy and Ebrahimi76 and Hoelzer and Ebrahimi77 reported the formation of visible interstitial loops in Fe irra­diated to 4.63 x 1023 nm~2; E> 1MeV at ^280 °C; the loops increased in size and decreased in number density after annealing at 500 °C.

There have been fewer studies on irradiated steels, but in MnNiMo steels little evidence for dislocation loops has been reported. Soneda and coworkers have undertaken weak-beam TEM obser­vations of RPV surveillance steels containing 0.06 and 0.12 wt% Cu irradiated to 4x 1019 ncm~2.65

Soneda reported the formation of interstitial dislo­cation loops, whose diameter and number density are 2-3 nm and of the order of 1021m — , respec­tively. At very high doses (and dose rates) a uniform density of loops has been observed. Fuji and Fukura78 undertook a weak-beam TEM study for MD in A533B RPV steel produced by 3 MeV Ni2+ ion irradiation to a dose of 1 dpa at 290 °C. The MD was found to consist of small dislocation loops. The observed and analyzed dislocation loops have Burgers vectors b = a <100> (Figure 13). The dis­location loops have a mean image size d = 2.5 nm and the number density is about 1 x 1022m-3. Most of the loops are stable after thermal annealing at 400 °C for 30 min. This indirect evidence suggests that their nature is interstitial.

Kocik et al..79 examined the radiation damage microstructures in Cr-Mo-V surveillance base metal and weld containing (^0.06-0.07 wt% Cu and 0.012-0.014 wt% P) irradiated up to 6 x 1024nm-2 (E> 1 MeV) for times up to 5 years at 265 °C. TEM examination of the irradiated materials revealed, in both the base metal and the weld metal, black dots, small (resolvable) dislocation loops, and small precipitated particles. Clouds of defects are formed along dislocations at higher neutron fluences, and it was only at the higher fluence that loops that may not be associated with dislocations could be seen. Interactions were observed between defects and (as-grown) dislocations that result in a rebuild of dislocation substructure. Miller et a/.80 examined the radiation damage microstructures in similar Cr-Mo-V surveillance base metal and weld. They reported manganese-, silicon-, copper-, phosphorus-, and carbon-decorated dislocations and other features in the matrix of the neutron-irradiated base and weld materials.

4.05.4.5.2 Development with flux and fluence and irradiation temperature

The most important inference from the mechanical test data is that hardening and embrittlement are proportional to the square root of fluence in low copper steels. Early theoretical and experimental work by Makin and coworkers81,82 demonstrated that a square root dependency on dose was consistent with the hardening arising from the cutting, by glide dislocations, of irradiation-produced obstacles, and that in the early stages of irradiation the number density of clusters is proportional to the irradiation exposure. Thus, in irradiated low Cu RPV steels, there is continuous production of hardening centers during irradiation. Further, the linear dependence of hardening on irradiation temperature from 150 to ^300 °C in CMn steels and low Ni A533B weld­ments implies that thermal stability of MD clusters is

important.83

There are relatively few studies that generate insight into the effect of flux and fluence on MD itself. Unsurprisingly, studies of model alloys tend to emphasize the increase in number density (and size) of the vacancy-rich clusters with increasing dose. Kampmann et a/.84 found void-like features 1-2 nm diameter in Cu-free ternary Fe-Ni-P/Mn alloys irradiated 2-25 x 1018ncm-2. The authors considered that the microvoid numbers increase with dose up to ^5 x 1018ncm-2, and then either remain constant or decrease. Analyzing positron annihilation data from annealing studies of neutron — irradiated A533B plate, A508-3 forging, and welds, Carter et al.54 considered that increasing the dose from 1 x 1018to20 x 1018ncm-2 at 290 °C increased the volume fraction of vacancy clusters, probably via increasing both the number density and average size of the clusters. Increasing the flux from 6 x 1011 to 5 x 1012 n cm-2s-1 increased either the number den­sity or the mean radius, probably the radius.

Postirradiation annealing has been shown to be a powerful means ofinvestigating the nature ofthe MD further. A major development has been characterizing the matrix defect term as being due to two compo­nents; first, stable matrix defects (SMD) and second, at high fluxes, unstable matrix defects (UMD) (see, e. g., Mader et al.85). UMD are matrix defects that, although thermally unstable at the irradiation temperature, are frozen into the microstructure during the cooldown after irradiation. Such studies have also established that MD and hardening of low Cu steels will be dose rate dependent at high dose rates (>1-5 x 1012 n cm-2 s-1, E > 1 MeV).85

Soneda65 modeled the effects of dose, dose rate, and irradiation temperature on the defect accumula­tion in bcc-Fe using the kinetic Monte Carlo (KMC) method.65,86 Jones and Williams83 proposed a model that describes the irradiation temperature depen­dence of the embrittlement of low Cu materials, AT = a x FT x (‘t)1/2, where AT, a, and ‘t are the transition temperature shift (TTS), constant coefficient, and dose, respectively, and FT = 1.869­4.57 x 10~3T (°C). This model was studied using a KMC simulation. The number densities of both vacancies and self-interstitial atom (SIA) clusters exhibited a linear temperature dependence with a slope equivalent to that of FT, and Soneda considered that the origin of the form of the FT term can be understood from the temperature dependence of point defect cluster formation.