POSSIBLE ALTERNATIVES TO SODIUM PHOSPHATE FOR NON-VOLATILE pH CONTROL IN STEAM GENERATORS

摘要

Since the first civil PWRs were commissioned, a variety of strategies for steam generator chemistry have been applied. For pH control this has included the use of coordinated sodium phosphate or an all volatile treatment (AVT), using ammonia, morpholine or monoethanolamine, together with hydrazine to ensure reducing conditions. When the early approaches proved not to be entirely satisfactory, restrictions on impurity levels were tightened. The use of amines has become more widespread and other additives, such as boric acid, are used on some stations. All such approaches have not been without their problems for steam generators with mill annealed Alloy 600 steam generator tubes. This led to the development of an alternative material, thermally treated (TT) Alloy 690, which has improved properties and has proved entirely satisfactory to date. Two alternative systems for non-volatile pH control have been suggested: ? The replacement of sodium phosphate with potassium phosphate. ? The use of the boric acid/borate buffer system. This paper describes an investigation into the possible use of potassium phosphate or the boric acid/borate system. In particular the results of both isothermal and heat transfer stress corrosion cracking and corrosion tests are described and the implications discussed.