压水堆核电厂乏燃料水池γ剂量率变化分析

Analysis onγradiation variation of the spent fuel pool in PWR NPP

58 Co是压水堆核电厂活化腐蚀产物的核心γ源项核素,受pH值和温度变化影响,含58 Co的活化腐蚀产物溶解度将持续发生变化。福清核电厂在执行某次机组调停小修过程中,一回路冷却剂中的58 Co活度浓度,随冷却剂温度下降而持续上升;在完成某次换料大修卸料工作后,乏燃料水池水温上升,池内58 Co活度浓度也随之升高,导致乏池表面最高γ剂量率达到了设计值的10倍左右。通过分析两个案例中,58 Co活度浓度、γ剂量率水平和温度变化趋势,对比工艺系统的运行记录,可以确认:两次58 Co活度浓度的升高,均与溶液温度密切相关。分析结果表明,在酸性环境下,含58 Co的活化腐蚀产物,其溶解度在一定温度范围内具有正温度系数,溶解度将随温度上升而增大;达到最大值后,溶解度表现出负温度系数,溶解度随温度上升而减小。根据该结论,通过启动乏燃料水池备用冷却回路,降低乏池温度,成功减小了池内的58 Co活度浓度,乏池表面γ剂量率迅速恢复至正常水平,避免了后续燃料操作人员的额外剂量照射。该实践的成功,对抑制和去除压水堆核电厂活化腐蚀产物中的58 Co,提供了新的思路。

58 Co is the keyγradiation source of activated corrosion products of PWR nuclear power plant.Influenced by pH value and temperature variation,the solubility of activated corrosion products containing 58 Co will change continuously.During a shutdown process in Fuqing NPP,the specific activity of 58 Co in primary coolant was found to increase continuously as the coolant temperature drops.Meanwhile after completing fuel unloading within an outage period,the water temperature raised,leading an increase of the 58 Co specific activity in the spent-fuel pool,of which the maximum surfaceγdose rate was reaching about 10 times of the designed value.By analyzing the specific activity of 58 Co,γdose rate level and temperature variation trend in the two cases,as well as the system operation records comparison,it can be confirmed that the increase of 58 Co specific activity for both cases is closely related to the solution temperature.Under acidic environment,the solubility of activated corrosion products containing 58 Co has a positive temperature coefficient within a certain temperature range,which means the solubility will increase with the temperature rise.After reaching the maximum value,the solubility,decreasing with the temperature raise,shows a negative temperature coefficient.According to the conclusion,the specific activity of 58 Co was successfully reduced by starting the spare cooling circuit for the spent-fuel pool,of which the tem-perature drops.Finally,the surfaceγdose rate of the spent-fuel pool quickly returned to the normal level,avoiding additional radiation exposure for the subsequent fuel operators.This successful practice provided a new idea for inhibiting and removing 58 Co in the activated corrosion products of PWR nuclear power plants.