Ukraine's WWER-1000 primary coolant chemistry optimization

One of the main requirements for improving the WWER NPP coolant chemistry is to enhance the reliability of elements and systems through reduced corrosion impact of the chemically loaded coolant on structural materials. Not least important and ever more significant are the issues related to accumulation of corrosion products in the primary circuit, and, as a consequence, reduced radiation fields from the primary circuit equipment, high consumption of chemical agents, and regeneration of significant amounts of liquid radioactive waste. There is a global tendency towards imposing additional restrictions on the fuel performance consisting in higher burnup and longer cycles. The established primary coolant chemistry quality norms in principle are justified and meet the requirements of the first task, but accomplishment of the other objectives calls for new approaches; therefore, the need to optimize Ukraine's NPP primary coolant chemistry is becoming ever more desirable. The paper systematizes the overseas primary coolant maintenance and optimization experience. It considers and analyzes the following possible ways of primary coolant optimization at NPPs with WWERs: transfer to injection of gaseous hydrogen instead of ammonia, replacement of the KOH corrective additive with LiOH, use of boric acid enriched with ¹⁰B isotope, and zinc injection. The paper provides the results of research into effects of zinc injection on corrosion behavior of stainless steels and E110 zirconium alloy in the simulated WWER-1000 primary coolant chemistry environment. The autoclave tests conducted demonstrate an inhibitive action of zinc on the uniform corrosion rate of stainless steels and absence of adverse impacts on the corrosion resistance of zirconium alloys. We studied the possibility of replacing the KOH corrective additive with LiOH as an option for optimizing the primary coolant chemistry of Ukraine's NPPs. The corrosion tests demonstrated higher E110 corrosion rate in the environment with the LiOH additive compared to standard composition and parameters of the WWER-1000 primary coolant. Based on the experimental results, however, the life-time of E110 fuel rod cladding will not be limited by any of the researched corrosion parameters, and the fuel rod claddings are expected to perform in longer cycles. Optimization of Ukraine's WWER primary coolant chemistry should take into account the differences between the WWER and PWR designs and be implemented into pilot and commercial operation only upon completion of all the material studies and validation. (author)