Co-60 transport mechanism under ultra low crud control

Ni/Fe control had been performed in Japanese low crud BWR plants to reduce cobalt ion, including Co-60 and Co-58 in reactor water (RW). But, Co-60 ion conc. began to increase after the application of new type fuel bundles using high corrosion resistant zircaloy tubes. The behavior was clearly observed in plants where the fuel bundles were initially loaded. It was clarified from the analysis of fuel crud and the laboratory experiments that the cause was due to unstable chromium oxides assisting the release of Co-60 from crud. Another disadvantaged activity buildup behavior was observed. The deposition rates of Co-60 and Co-58 on electro-polished primary recirculation (PLR) pipings in some plants were widely scattered. It was assumed that high deposition was caused by large amount of crud due to deposited crud from RW or thick corrosion oxide films made by corrosion. On the other hand, such a high deposition rate was not observed after water chemistry change from Ni/Fe control to high nickel ion control obtained by ultra low crud control. In this report, a new concept, ultra low crud control substituted Ni/Fe control was introduced. The characteristics of cobalt transport behavior and the applicability to actual BWR plants were clarified. (J.P.N.)