| The nuclide 99Mo is a widely used radioactive medical radionuclide that is used in more than 32 million nuclear medicine procedures are performed each year in the worldwide.At present,99Mo is mainly produced by irradiating High Enrichment Uranium(HEU)targets in experimental reactors.However,this production method has the characteristics of less utilization of uranium,more three wastes,unfavorable to nuclear proliferation and high cost.In addition,a large part of the 99Mo production reactors have been built for a long time,and often have unplanned shutdowns during operation,which will soon be decommissioning.As a result,the world will face a serious shortage of 99Mo supply in the future.In order to overcome the above shortcomings of target production of 99Mo,a method of producing 99Mo by molten salt reactor has been proposed.The 99Mo produced in the molten salt reactor is insoluble in molten salt and will spontaneously enter the off-gas along with the entrainment gas.Therefore,the production of 99Mo by using molten salt reactor has the characteristics of simplified process,short production cycle,high production efficiency,nuclear non-proliferation,high utilization rate of uranium,and less waste.The extraction and utilization of 99Mo in the off-gas will produce huge additional economic value.In order to develop this technology,a 400MWt small modular thorium based molten salt reactor(SM-TMSR)was used as a reference.The fission produce and migration of 99Mo in thorium based molten salt reactor were studied in this paper.It can be divided into three parts:generation and migration in the reactor,the deposition of gas pipeline and extraction from the off-gas system,as follows:(1)Generation and migration in the reactorThe production and migration and deposition of 99Mo in the reactor under different conditions were studied.We firstly calculate the fission percentage of 233U,235U and 239Pu with the variation of burnup time by using the Molten Salt Reache-based Fuel Management Program(MOBAT),which was based on the characteristics of molten salt reactor(On-line feeding,on-line removal of noble metals and insoluble gaseous),and the variation of fission yield of 99Mo with burn-up time was obtained.The minimum value of the fission yield of 99Mo was 1.13×10-3(6-day TBq/MW/s)at approximately 600 equivalent effective full power days.Secondly,the migration probability of 99Mo from the main loop to the off-gas system was calculated by using the equilibrium equation based on the generation and disappearance of noble metals and the experimental and literature data of MSRE.It is calculated that the equilibrium value of migration probability is 18.4%when the bubble volume fraction is 0.02%and the value is 36.6%when the bubble volume fraction is 0.6%.Thirdly,the production rate of major noble metal nuclides except99Mo and the migration probability of these nuclides from the main loop to the off-gas system are calculated.when the same amount of 99Mo was produced,the amount of fission products to be processed by the thorium-based molten salt reactor is about9.71%of that by the conventional uranium target fission method.Finally,we calculated the annual amount of 99Mo migrated to the off-gas system ranges from1.96×106(6-day TBq)to 3.90×106(6-day TBq)when the load factor of sm-TMSR(the design power was 400MW)is 0.75.As long as the 99Mo in the off-gas system is slightly utilized,the global demand for 99Mo for can be met.(2)the deposition of gas pipelineThe computational fluid dynamics software Fluent16.1 was used to calculate and analyze the deposition of 99Mo in the off-gas pipe.A Realizable k-?model(2 eqn)was adopted for gas-phase turbulence model.Enhanced wall treatment was used in the near wall area.Considering the drag force,thermophoretic force,gravity and Brownian force of 99Mo particles,the DPM model was adopted.The calculation results show that the particle deposition rate is closely related to the gas inlet velocity,the ambient temperature of the pipe,the particle size distribution and the pipe geometry size.Considering the variation of initial conditions and models,the deposition rate calculated in this paper ranges from 15%to 37%.In order to reduce the deposition of 99Mo in the pipe and improve its extraction efficiency,the diameter of the pipe should be increased and its length should be reduced when considering the design of off-gas pipe.The appropriate gas inlet speed should be selected and the temperature difference between the ambient temperature of the pipeline wall and the gas inlet temperature should be reduced during operation.(3)Extraction from the off-gas systemIn order to extracted 99Mo from the off-gas system online,refering to the spray method and electrostatic-adsorption method which are commonly used for purifying gas or air in industry at present and considering the characteristics of high temperature and high radioactivity of the off-gas,we give the structure,material and solution of the extraction device of the spray method and electrostatic-adsorption method.These devices have the following characteristics:maintaining the seal of the off-gas system to prevent the leakage of radioactive gas;cooling the off-gas to facilitate the subsequent treatment of off-gas;reducing the composition of off-gas and no new composition is added,which is also convenient for the subsequent treatment of off-gas;The current conventional 99Mo separation and purification process can be used to reprocess the extracted solution containing 99Mo.Considering the three-phase flow of solid(99Mo particles)-gas(helium)and liquid(Naoh solution),the VOF+DPM model was adopted to simulate the three-phase flow field in the spray tower.Firstly,the computational grid of the model was determined by grid irrelevancy verification.Secondly,the influence of different initial conditions on the sealing of off-gas is calculated and analyzed.The results show that the sealing of off-gas was greatly affected by the initial liquid level and almost not affected by the inlet velocity.Finally,the proportions of each state of 99Mo particles at a certain time(t=10s)were calculated under different initial conditions and the extraction efficiency of 99Mo in the spray tower was obtained.The calculated show that the minimum extraction efficiency was 65.17%when the inlet velocity is 1(m/s),the initial liquid level height is 0.6m and the particle size distribution is gradation 1.the maximum extraction efficiency was 92.25%when the inlet velocity is 10(m/s),the initial liquid level height is 0.5m and the particle size distribution is gradation 3.Based on the summary of the whole paper,it was concluded that the ratio of99Mo extracted from thorium based molten salt reactor sm-TMSR was in the range of7.8%-27.7%.If the sm-TMSR load factor was 0.75,the rate range of 99Mo migrating the outside of the reactor is 2.6×10-2-9.3×10-2(6-day TBq/s).The global demand for99Mo was about 23,000(6-day TBq)in 2012.If we calculate at a minimum rate,the global demand(2012)for 99Mo can be met with only 2.8%utilization of the 99Mo transferred outside the reactor. |
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