18 years of experience with water chemistry at the Dodewaard nuclear power plant

The Dodewaard nuclear power plant is of the thermal boiling water reactor type with natural circulation. This type uses water for the moderation of neutrons and for cooling the core. In the primary circuit various types of stainless steel is used as construction material. A special operation for the condensate polishing system (CPS) was created in 1982. Once a month the stand-by filter of the CPS was put into service and the other one switched off and thoroughly backwashed. This created a large decrease in the suspended iron and cobalt amounts which were transported toward the reactor. The decrease of the iron and cobalt input had a direct influence on the crude layers in the auxiliary system. Also, the amount of crud on the control rod drives has been lowered. Oxygen can cause inter granual attack followed by inter granual stress corrosion cracking (IGSCC). The conductivity of the reactor-water is very low, and this allows the e.c.p. value to be as low as -25 mV (SHE) before IGSCC will occur. Thus HWC is not implemented. To separate solids resulting from errosion, etc., a plate-filter with precoat powder has been installed. A sedimentation and flocculation tank is also used. A great number of modifications permitted water transport to and from the various (waste) water systems, but this caused serious cross-contamination. Efforts are under way to solve this problem. (Nogami, K.)