核电厂H3BO3-NaOH-H2O溶液结晶影响因素研究

Study on Crystallization Influencing Factor of H3BO3-NaOH-H2O System in NPP

为研究核电厂H3BO3-NaOH-H2O溶液结晶机理,选择钠硼比(K)、pH值、硼浓度以及晃动和杂质作为影响因素,通过实验研究各影响因素对溶液结晶的影响和不同K值范围内溶液的主要成分。结果显示:K<0.2(pH<7.5)时,溶液的主要成分是硼酸,溶液结晶温度随K值或pH值的增大显著下降;0.2<K<0.4(7.0<pH<8.5)时,溶液的主要成分是五硼酸盐,硼浓度低于25000ppm的溶液在5℃均不结晶;0.4<K<0.8(8.5<pH<11.0)时,溶液的平衡结晶物质是硼砂,结晶温度相对回升;K>0.8时,溶液的主要成分是偏硼酸钠。另外,晃动和杂质均加速溶液结晶。综合考虑各因素对溶液体系结晶的影响,在核电厂浓缩液硼浓度目标控制值为15000ppm、厂房最低环境温度5℃下,溶液不结晶的推荐K值控制范围为0.20~0.25,最佳K值为0.23。

The crystallization mechanism of H3BO3-NaOH-H2O solution in NPP was studied.The influences of factors such as sodium boron ratio(K),pH value,boron concentration,shaking and impurity on the crystallization of solution system were researched.The key compositions of the solution in different K values were also studied.The results show that the key composition of the solution is boric acid when K<0.2(pH<7.5),and the crystallization temperature of the solution decreases significantly with the increase of K or pH.The key composition of the solution is pentaborate when 0.2<K<0.4(7.0<pH<8.5),and the crystallization temperature is higher than 5℃for the solution of boron concentration below 25 000 ppm.The key composition of the solution is borax when 0.4<K<0.8(8.5<pH<11.0),and the crystallization temperature relatively increases.The key composition of the solution is sodium metaborate when K>0.8.In all of the experiments,shaking and impurity accelerate the crystallization of the solution.According to the comprehensive analysis of the experimental results,when the minimum ambient temperature of the NPP is 5℃and the target control value of boron concentration is 15 000 ppm,the recommended K value is 0.20-0.25 and the optimal K value is 0.23 to avoid solution crystallization.