Study On Real Time Simulation Model For Continuous Dissolution Of Spent Fuel Pellets

Spent fuel reprocessing is one of the most critical links in the closed nuclear fuel cycle.It is the most extensive treatment method for spent fuel treatment at present.As the key unit of head end treatment in spent fuel water post-treatment,dissolution is particularly important.The composition and content of its solution are very important to the subsequent post-treatment process.The establishment of dynamic simulation model of spent fuel post-treatment dissolution process has the characteristics of safety,economy and allowing multiple repetition.It can play an important role in improving the quality of large post-treatment plants and ensuring operation safety.To carry out computer simulation research on the process and establish its dynamic simulation model can carry out process analysis and optimization with less human and material resources and in a more general range,and find out the best process conditions,which has certain practical significance and theoretical value.In this paper,by analyzing the mass transfer process of the dissolution reaction between spent fuel pellets and nitric acid and the mass transfer process of solute in the solution,a dynamic dissolution model of spent fuel pellets dissolution process is obtained based on mass balance calculation.The model includes three parts,namely,chemical reaction module,solute transfer module and solution flow field analysis module,which are used to calculate the mass transfer caused by chemical reaction,the mass transfer caused by liquid convection and diffusion in liquid phase and the analysis of fluid flow state in solution system.The turbulent flow state of the dissolved liquid in the dissolver is simulated by Comsol software,and the flow field cloud diagram of the dissolved liquid flow is obtained,which provides the analysis data of the liquid phase flow state for the established dynamic dissolution model of the spent fuel pellet dissolution process.By using Matlab software,a numerical simulation program for solving the real-time simulation model of spent fuel pellet dissolution process is established,and the feed dynamic simulation of spent fuel dissolution process in continuous dissolver is carried out under specific dissolution process conditions.The simulation results show that the change trend of the simulation results is basically consistent with the actual process.The uranium concentration in the dissolver generally increases rapidly at first,then decreases rapidly,and finally decreases slowly,while the overall change trend of nitric acid concentration is just opposite to the change trend of uranium concentration.Both uranium concentration and nitric acid concentration have extreme values at about 282.5 min,which are 265.65 g·L^-1 and 2.44 mol·L^-1 respectively.The dissolution process takes 270min to reach the uranium discharge requirement of 250 g·L^-1,but it takes about 2000 min to reach dynamic equilibrium;After equilibrium,the concentration of nitric acid in the solution discharge is 2.78 mol·L^-1 and the concentration of uranium is 253.23 g·L^-1.The relative error of uranium material balance calculation in the whole dissolution process is generally in an acceptable range,and there are large errors and fluctuations only after nitric acid feeding.In the overall trend,the relative error is gradually decreasing as the dissolution process reaches equilibrium.When it reaches equilibrium,the relative error of overall mass balance is only 2.57%.In general,the dynamic dissolution simulation model of spent fuel pellets in the rotary continuous dissolver established in this study can better describe the actual dissolution behavior of spent fuel in the dissolver and provide simulation data support for the spent fuel dissolution process in the actual production process.