Primary water chemistry for NPP with VVER-TOI

Nowadays within the framework of development of the nuclear power industry in Russia the VVER-TOI reactor is under designing (Standard optimized design). The given design provides for improvement of operation safety level, of technical-economic, operational and load-follow characteristics, and for the raise of competitive capacity of reactor plant and NPP as a whole. In VVER-TOI reactor plant design the primary water chemistry has been improved considering operation experience of VVER reactor plants and a possibility of RP operation under load-follow modes from the viewpoint of meeting the following requirements: - suppression of generation of oxidizing radiolytic products under power operation; - assurance of corrosion resistance of structural materials of equipment and pipelines throughout the NPP design service life; - minimization of deposits on surfaces of the reactor core fuel rods and on heat exchange surface of steam generators; - minimization of accumulation of activated corrosion products; - minimization of the amount of radioactive processing waste. In meeting these requirements an important role is devoted to suppression of generation of oxidizing radiolytic products owing to accumulation of hydrogen in the primary coolant. At NPP with VVER-1000 reactor the ammonia-potassium water chemistry is used wherein the hydrogen accumulation is provided at the expense of ammonia proportioning. Usage of ammonia leads to generation of additional amount of radioactive processing waste and to increased irregularity of maintaining the water chemistry under the daily load-follow modes. In VVER TOI design the primary water chemistry is improved by replacing the proportioning of ammonia with the proportioning of gaseous hydrogen. Different process schemes were considered that provide for a possibility of hydrogen accumulation and maintaining owing to direct proportioning of gaseous hydrogen. The obtained results showed that transition to the potassium water chemistry with proportioning of gaseous hydrogen does not need changes in the primary water chemistry parameters. The important purpose of improving the primary water chemistry of VVER-TOI is the enhancing of operational reliability of the primary equipment and the core, as well as reducing the dose commitments related to accumulation of activated corrosion products and reducing the amount of the generated radioactive processing waste. Due to the above-mentioned, the design of VVER-TOI reactor provides for proportioning of the reagent with zinc ions, depleted in zinc-64 isotope, into the primary coolant for reducing the intensity of the processes of rise in deposits on the heat exchange surfaces and of accumulation of activated corrosion products on the primary equipment surfaces under power operation. (authors)