IMPROVEMENT OF SHUTDOWN CHEMISTRY THROUGH NON-VOLATILAZATION OF RADIOIODINE IN THE CASE OF DEFECTED FUEL

摘要

The fuel using in nuclear power plants is inclined to increase fuel defect rate as a result of an increase of efficiency and long-term operation. Highly volatile radioiodine generated by fuel defect is a main cause of user's internal irradiation at a time of refueling work and this could lead to decrease operation reliability efficiency. Accordingly, iodine is removed by shutdown chemistry at a minimum level prior to reactor drain process in the case of defected fuel. Westinghouse Inc. and EPRI recommend that the radioiodine level be reduced to 370 Bq/cc and 37 Bq/cc, respectively. Although radioiodine which is low in permissible dose for internal radiation (1 DAC = 1.0 × 10~(-4) Bq/cc) is present in water in an amount of 0.1 Bq/cc, DAC is higher than 1 DAC. Yonggwang5 and Yonggwang6 have experienced 3 times of fuel defects currently. Throughout these cases, condition for non-volatilization of radioiodine has been found out that radioiodine in the water which is low in pH and high in oxidation-reduction potential and lithium of 1 ppm has not been volatilized. It is directly opposite idea that it is usually known as iodine which is not volatilized above pH 7 by conducting a strong alkali process in reactor core melting. In order to verify this phenomenon, iodine was treated with hydrogen peroxide and lithium hydroxide then examined result of color change and concentration of iodine remains after heating. Through this experiment, it is found out that iodine was not considerably volatilized. This result has been extended. In this event, iodine has not been volatilized with an amount of 150 Bq/cc in the water. For this consequence, iodine normality in air within Containment Vessel (CV) has been kept below 1 DAC and refueling operating smoothly.