The effect of water quality on reduction of dose rate

The corrosion products in primary cooling system are one of the radioactive substances in light water-cooled nuclear reactors and can be suppressed by the reduction of impurities in the primary cooling water. The installation of a condensate filter and oxygen injection (as corrosion inhibitor) were effective to reduce the dose rate of the primary cooling pipes, due to the reduction of iron crud in the reactor and also due to the control of the generation of radioactive corrosion products, in the Shimane Nuclear Power Station generating electric power of 460 MWe since 1974. The operation record, the main flow diagram of the plant, the plant specification, the concentration distribution of iron crud and iron ions in the primary cooling system, the iron crud concentration at the outlets of the demineralizer and the condensate filter, and the flow diagram of the oxygen injection facility in the Shimane Nuclear Power Station are presented. The dissolved oxygen, the iron crud concentration and the iron ion concentration in feed water at the exit of No. 5 feed heater are 30 ppb after oxygen injection and about 3 ppb in case of no oxygen injection, about 5 ppb and about 4 ppb after oxygen injection and from 20 to 45 ppb in case of no oxygen injection, respectively. The radiation dose rate of recirculating pipes is about 50 mR/h due to the control of corrosion products and the value is far less as compared with the dose rate of from 400 to 600 mR/h in BWRs in the USA. This dose rate was calculated separately by estimating Co60 deposition on the inner surface of pipings. Comparing the calculated dose rate with the measured value of recirculating pipes, good agreement was obtained. (Nakai, Y.)