Use of on-line ion chromatography in controlling water quality in nuclear power plants

Abstract

Electric utilities that operate nuclear power plants must adhere to a tighter range of water coolant specifications that are commonly required in steam systems powered by other means such as coal, oil or natural gas. The reason for this situation is the higher negative impact of a failure in the coolant and steam piping relative to non-nuclear heat sources. This negative impact arises from the unique need of a nuclear reactor core to require cooling after shutdown of the chain reaction —due to the heat from undecayed radioactive fission products in the uranium fuel rods. Also, the cost of down time for repairs of corrosion damage in such stations is greater than for fossil fired plants because these nuclear plants are base loaded (running at ca. 100% power for several hundreds of days) and the replacement power required during their shutdown time must be provided by sources more costly by factors up to ca. 2. Since corrosion damage in nuclear plants usually occurs under crevice conditions in components such as steam generators where boiling occurs, even very small (parts per billion, ppb) initial feedwater concentrations of corrosive impurity salts such as sodium, chloride and sulfate may concentrate to corrosive brines —leading to breaches of the pressure boundaries from localized cracking or intergranular attack. This attack penetrates even the more corrosion-resistant alloys such as Inconel 600, Incoloy 800 and the stainless steels. Ion chromatography is the only technology able to readily measure ppb concentrations of such corrosive ions on a near real time basis, and to differentiate among different oxidation states of ions such as nitrate, nitrite, sulfate, thiosulfate, etc. These oxidation states respond to the general oxidizing tendency of the coolant —another variable that controls localized corrosion. A number of examples of the use of on-line ion chromatography in troubleshooting corrosion problems in light-water reactors will be given.