The effects of high hydrazine chemistry on corrosion product transport and feedwater electrochemical potential at Pickering NGS-B, a mixed feedtrain station

Pickering NGS-A (Units 1-4) and -B (Units 5-8) is a 8 x 540 MWe CANDU station on Lake Ontario. The in-service date for Unit 7, the subject of this paper, is 1984 October. This unit has been running continuously for 643 days (as of 94.02.01) which extends the previous world record of 533 days for a water power reactor. The materials of construction of the major secondary system components are Admiralty Brass (B111 alloy 443) (Condenser, with a stainless-steel air-removal zone and LP Heaters) and 90/10 Cu/Ni alloy (SB111 alloy C70600) HP Heaters. Each Pickering-B unit has 12 recirculating B and W steam generators, with Monel 400 tubes, carbon steel trefoil broach tube support plates and integral preheaters. Morpholine is injected at the condensate extraction pump discharge and hydrazine is added in the downcomer between the top deaerator spray section and the bottom storage tank. In early 1992, under-deposit pitting of the Monel boiler tubing was identified in Units 5 and 6. Remedial actions taken include steam generator cleaning and a decision to replace copper alloys in the secondary system. One operational action taken was to raise the hydrazine levels on all Pickering-B units with the objective of lowering the feedwater electrochemical potential (ECP) to reduce the risk of further pit initiation and growth. A corrosion product transport (CPT) survey of Unit 7 was performed between 1992 June and 1993 March with extensive use of phase analysis to assess the net benefit of high hydrazine chemistry. Changes in the nature of the corrosion products, especially the ratios of Fe(II):Fe(III) and Cu(II):Cu(I):Cu(O) were monitored to correlate with the reducing conditions produced by the system chemistry. (authors). 4 figs., 3 tabs., 7 refs