In reviewing radiation damage effects in ferritic steels, it is important to recognize that a range of ferritic steels have been employed in commercial reactors. Such steels were necessarily employed in thick sec­tions and the fabrication of the vessels involved weld­ing of preformed plates or forgings (for a description of typical vessels, see Steele and Sterne1). Frequently, in early designs, the welds were located opposite the center of the reactor core and received the highest neutron dose. Commercially available ferritic steels were employed in the construction ofthe first reactors (both for Magnox and LWR designs).

For LWRs, which have to contain higher pressure than the pressure vessels of Magnox reactors, the desirability of using steels of high toughness, ade­quate strength, and weldability in thick sections, combined with good service experience, has nar­rowed the choice to a number of low alloy steels; for example, those containing manganese, nickel, and
molybdenum. Indeed, the ASME specification for quenched and tempered vacuum-treated carbon and alloy steel forgings for pressure vessels permits only SA-508 Class 2 and Class 3 compositions. Early ves­sels were constructed from A302 and A302B plates. Vessels constructed in Russia (so-called VVER reac­tors) are fabricated from high-strength CrMoV steels.

The steels used in vessel construction of Magnox pressure vessels generally consisted of simple C-Mn ferritic plates, either Si-killed or Al-grain refined, in the normalized and stress-relieved condition. Table 12 shows the average, or range of, chemical compositions for the plates, manual metal-arc (MMA) welds, submerged-arc (SMA) welds, and forgings, respec­tively, as derived from the populations of vessel cut-outs ex-construction, surveillance samples, and contemporary reproduction vessel materials.

Compositions of commercial MnMoNi steels used for modern LWR pressure vessels are given in Table 2 9-12 A detailed description of the develop­ment of modern pressure vessel steels can be found in

Druce and Edwards.13 It is to be noted that commer­cial steels are complex with many elements present. Although the concentrations of additional elements are low, all have the potential to influence the radia­tion damage response of the steel.

The deleterious effects of residual impurities such as Cu and P on the in-service degradation in properties (see Section 4.05.3) have been increas­ingly recognized. Consequently, over time, the levels of Cu and P in ferritic pressure vessel steels have been reduced and in modern steels, such as the for­gings and welds employed in fabricating the vessel for the PWR at Sizewell in the United Kingdom, the levels of such impurities are well-controlled (see Table 3).

Finally, it is important to recognize that these steels have a complex microstructure at start of life (SOL), and that there may be significant spatial vari­ation in the local microstructure.14 For example, the local dislocation density or size and number of second-phase precipitates may vary significantly both within a given material and also between plates, welds, or forgings produced to the same nominal specification.