Numerical Simulation Of UF₆ Source Term Leakage And Hydrolytic Diffusion Process In Atmosphere

UF₆ is an essential working medium in uranium conversion plant,uranium enrichment plant and nuclear fuel elements manufacturing.Its accidental leakage accident may occur in any link of the nuclear fuel cycle,and due to its radioactive and chemical toxicity,the massive release of UF₆ represents one of the most potentially harmful accident in nuclear fuel cycle facilities.The advanced UF₆ monitoring system needs to be developed to monitor the concentration of UF₆ in real time after the occurrence of a leakage accident to minimize the harm to workers and ensure the safety of nuclear fuel production facilities.However,the development of monitoring system firstly requires a detailed understanding of UF₆ leakage characteristics.At present,RASCAL,HGSYSTEM/UF₆ and other calculation models have some problems such as inaccurate source term parameters and hydrolysis rate.Therefore,this paper aims to study the typical UF₆ leakage types and their characteristics.For two of the problems with existing models,numerical simulation is carried out to simulate the UF₆ phase change process during the leakage from the 3m³ container and the hydrolysis diffusion process in the atmosphere,so as to provide input for the estimation of the consequences of leakage accident and optimize the analysis of UF₆ leakage accident.Firstly,Fluent UDF is used to establish the evaporation,solidification and sublimation models to simulate the phase change and the heat and mass transfer process of UF₆ gas leakage in the 3m³ container.Based on the Lee model and Solidification and Melting model,the numerical models of the three phase change and mass and heat transfer are established to simulate the phase change process in the container during UF₆ leakage.The mass transfer rates are calculated from the gas density,temperature and saturation temperature corresponding to the pressure in the three-phase diagram,and the phase change criterion determined by gas partial pressure is added.The changes of average pressure,temperature and UF₆ leakage rate with time during phase change are simulated.The calculation results show that the UF₆ gas leakage can be divided into three stages by triple point and sublimation point.The leakage rate is 4.718 kg/s initially,then decreases with the decrease of temperature and pressure in the container,and is roughly stable at 2.451 kg/s at the triple point.After the UF₆ liquid completely solidifies and evaporates,the leakage rate continues to decrease until the pressure in the container is in equilibrium with the external environment.At that point,UF₆ is no longer leaking and there is only UF₆ solid in the container,with a leakage of 5004 kg UF₆ and a leakage time of 1927 s.Numerical simulation of UF₆ leakage can reflect the influence of pressure and temperature changes on leakage rate in real time.With more accurate leakage rate and longer leakage duration,numerical simulation is more conservative than empirical calculation to some extent.Secondly,Euler Model and Species Transport Model are used to simulate the hydrolysis and diffusion process of UF₆ leaking into the atmosphere.Euler Multiphase Chemical Reaction Model is used to realize the UF₆ hydrolysis reaction process.The hydrolysis reaction rate is determined by Arrhenius reaction equation.Species Transport Model is used to simulate the diffusion of UF₆ and its hydrolysates HF and UO₂F₂ in the atmosphere.Under different humidity and wind speed,the time and space changes of each substance are calculated after one-second leakage of 13.403 mol UF₆ in the atmosphere.The simulated results show that UF₆has hydrolyzed at 0.002 s.UF₆ and hydrolyzed products HF and UO₂F₂ have diffused to the top of the 10×10×6 m³ room at 0.5s,and have diffused to 300 m³ at 1 s.At 2 s,the leaked UF₆has almost completely hydrolyzed in the atmosphere,and HF and UO₂F₂ have almost diffused to 300 m³.The UF₆ hydrolysis degree is different under different humidity and wind speed.The higher humidity,the faster the hydrolysis reaction reaches equilibrium,but the residual amount of UF₆ in the atmosphere is larger.When humidity is 50%,60%and 70%,UF₆ residual amounts are 1.720 mol,2.971 mol and 3.645mol respectively.The higher the wind speed,the slower the hydrolysis reaction,and the smaller UF₆ residual amount in the atmosphere.When the wind speed is 0 m/s,2 m/s and 4 m/s,the residual amount of UF₆ is 2.971 mol,0.626 mol and 0.555mol,respectively.