Radionuclide Volatilization During Evaporation Of Simulated Radioactive Waste Liquid And Safety Analysis Of Evaporation Residue

The high-level liquid waste(HLLW)produced by spent fuel reprocessing plant shall be reduced by evaporation concentration method.In the evaporation process,some radionuclides will volatilize into the condensate with the entrainment of water vapor,so that the radioactivity of radionuclides in the condensate exceeds the standard.Therefore,it is necessary to pay attention to the variation law of the volatilization behavior of common radionuclides with the change of evaporation conditions.A purification material was explored to adsorb radionuclides in water vapor in order to improve the purification effect of condensate.There is a certain risk of thermal runaway reaction in the residual liquid TBP system in the evaporator.Once the thermal runaway occurs,it will lead to a large release of radionuclides to the environment.Therefore,it is necessary to study the thermal risk of the residual liquid TBP system.In this paper,a laboratory evaporation and concentration device was built to carry out evaporation and concentration experiments on the prepared simulated high-level liquid waste,and explored the effects of different evaporation temperature,air pressure in the device,nitric acid concentration and TBP concentration on the volatilization rate of Cs,Sr,Ru and Ce.The results showed that in the range of 120℃-140℃,with the increase of evaporation temperature,the volatilization rate and radioactive concentration of the four nuclides gradually increased,and the purification coefficient gradually decreased.When the pressure in the evaporation device decreased from 91 k Pa to 51 k Pa,the volatilization rate and radioactive concentration of the four nuclides gradually increased and the purification coefficient gradually decreased.When the concentration of nitric acid in simulated HLLW increased from 0.5 mol/L to 4 mol/L,the volatilization rate and radioactive concentration of the four nuclides gradually increased,and the purification coefficient gradually decreased.As the concentration of TBP in simulated HLLW increased from 0 to 0.2%,the volatilization rate and radioactivity concentration of the four nuclides gradually increased,and the purification coefficient gradually decreased.The concentration of TBP increased from 0.2%to 2%,the volatilization rate and radioactivity concentration of the four nuclides gradually decreased,and the purification coefficient gradually increased.Melamine sponge has three-dimensional carbon skeleton structure,high porosity(>99%)and secondary processing performance,which is suitable for adsorption material.In this paper,melamine sponge was carbonized at different temperatures as a purification material for simulating the trace nuclides entrained in water vapor in the evaporation and concentration process of high-level liquid waste.The adsorption effect was judged by comparing the volatilization rate,radioactive concentration and purification coefficient of radionuclides in the condensate.The results showed that the adsorption effect of melamine sponge on radionuclides was the best after it was carbonized at 400℃.The adsorption effect on Ru was the most prominent,and the purification coefficient was 6.7×10~6.When the temperature in the evaporator is high and there is a certain concentration of nitric acid,nitrate and TBP,it may form"red oil",an explosive organic substance.Through adiabatic accelerated calorimetry experiment,this paper explored the effects of different evaporation time,different residue TBP ratio and nitrate on the thermal safety of residue TBP system.The results showed that the longer the evaporation time,the higher the concentration of nitric acid and nitrate in the residue,the greater the thermal risk of the residue TBP system.When the evaporation time was 6 h,the lowest initial exothermic temperature of the system was 120.9℃,the maximum reaction heat was 591.7 J/g,and the largest comprehensive thermal risk assessment parameter THI was 0.22;When the mass ratio of steam residue to TBP increased from 1:2 to 5:1,the greater the proportion of steam residue,the greater the thermal risk of the system.When the mass ratio of steam residue to TBP was 5:1,the initial exothermic temperature of the system was the lowest,which was 100.9℃,the maximum reaction heat was 729.1 J/g,and the comprehensive thermal risk assessment parameter THI was the largest,which was 0.27;The THI of pure HNO3-TBP system was 0.18,which was lower than 0.22 of steam residue TBP system,that was,the presence of nitrate can increase the thermal risk of self exothermic reaction.