Study on a corrosion products behavior with zinc injection in PWR primary circuit based on the crud investigation in actual plant

Zinc injection into the reactor coolant of Pressurized Water Reactor (PWR) is one of the most effective techniques for removing radioactive cobalt from the chromite type spinel oxides in the inner layer of reactor coolant component surfaces. Many documents have reported that zinc injection applied plants have generally achieved positive dose rate reduction effects for the reactor coolant components and piping. Tsuruga Nuclear Power Plant unit 2 (hereafter Tsuruga unit 2) which applied zinc injection in 2005 has also achieved good dose reduction effects in most primary coolant piping. However, some piping without dose-rate reduction effects exist, it has come to an issue to plan radiation exposure reduction measures for the works around relevant piping. To plan a reasonable dose-rate reduction measures for respective components and piping in primary systems, it is considered necessary to understand the dose-rate reduction effects of zinc injection mechanically. Therefore, the surface oxides/deposits on piping and fuel cladding tubes with operational histories under zinc injection were investigated. As a result of the investigation, the changes in specific aspects of oxide layers and CRUDs are found as follow: - Owing to long-term zinc injection, deposits and oxides on the primary coolant piping are extremely reduced, the boundary between the outer oxide layer and the inner oxide layer become unclear with the thinning of the outer oxide layers. - At a Pressurizer Spray Piping, the dose-rate increases than before zinc injection. The oxide layers have the same 3-layer construction as before zinc injection, and the surface concentration of the radioactive cobalt is high. - At the fuel cladding tube surfaces, what is observed is the amount of metal nickel decrease and the chromium composition increase in the spinel type oxide From these results, it is suggested that, the oxide layers composed principally of zinc chromite spinel grow up on the pipe surface with zinc injection, nevertheless the oxide film on the Pressurizer Spray Piping is similar oxide layers before zinc injection because the zinc feed rate is insufficient at this piping. Therefore zinc injection has effect on the reduction in cobalt uptake by the oxide layers, however such effects cannot be achieved the piping where the zinc feed rate is insufficient. And it is suggested that, on the piping surface with zinc injection, the oxide layers are composed of the chromite type spinel oxides, and that what is fallen away and flaked down from the oxide layers deposits on the fuel cladding tube surfaces. The investigation results obtained this time for the oxide layers and the fuel cladding tube deposits are reflected to our CRUD Evaluation Code, and the Code is optimized. The evaluation results of radiation source obtained from this optimization and the actual data concerning the radiation source behaviour of Tsuruga unit 2 are in good consistency, therefore, the evaluation is confirmed to have sufficient practicality effective for evaluating radiation source after the zinc injection. The radiation exposure evaluation with high-accuracy will become available anytime soon. Using this modified Code, the evaluation results of dose-rate reduction effects of the zinc injection with a cumulative 34 months at Tsuruga unit 2 shows 32% reduction, compared to the evaluation value for the case without zinc injection. (authors)