Water chemistry and corrosion issues in PWR/WWER SGs and current knowledge on water chemistries in SMRs

A literature survey was conducted with the aim on charting the past and current chemistry and corrosion issues in PWR and WWER steam generators and with the aim on charting publicly available information on water chemistry and material choices in small modular water cooled reactors (SMRs).

In PWRs and WWERs the corrosion related issues have been in principle similar: wall thinning (general corrosion), pits, pittings, cracks, and their combinations. In western PWRs corrosion problems encountered in Alloy 600 (both mill-annealed, MA, and thermally treated, TT, versions) have been largely overcome by replacing it with alloys 690 (TT) and 800 and by better water chemistry control. Some stress corrosion cracking (SCC), intergranular attack and pitting cases have been observed in Alloy 800 that have been attributed mainly to the presence of sulphur and thermal ageing. In the case of Alloy 690, very little corrosion related problems have been reported (only a small number of wear cases). In WWERs, the main development in corrosion mitigation methods has been better water chemistry control. A possible solution to the pitting corrosion and SCC problems is to replace the material of the tubes (austenitic stainless steel) with martensitic ferritic steel or martensitic steels. In both, PWRs and WWERs, the risks of corrosion related issues have been reduced in recent years also by structural changes.

Very little specific information is available (as of 2019) concerning the construction materials of SMRs. However, where available, the materials used in the main components in present day designs of SMRs seem to be mainly the same materials that are used in generation 3 large reactors: low carbon or stabilized austenitic stainless steels (e.g. 304L, 316L, 321, 347), low alloy steels (e.g. RPV: SA-508 Grade 3 Class 1 type pressure vessel steel), and better SCC resistant nickel base or high nickel alloys than Alloy 600 (e.g. 690 and 800). In many cases reactivity control of the reactor core takes place most often apparently without soluble boron (boric acid). In most studied SMR concepts no mention of primary side corrosion potential control by dissolved H2 nor pH control was found in the open literature. The applied secondary side water chemistries have neither been revealed. However, it can be assumed that the latest chemistry and corrosion control methods used in large nuclear power plants are applied in the secondary side.