Study On Radioactive Source Term Of Liquid Fuel Molten Salt Reactor

During the operation of a nuclear reactor,a large amount of radionuclides are produced by neutron-induced fission of fission material.The types and evolution of these radionuclides are extremely complex,and highly radioactive.In the design of nuclear reactor,multiple physical barriers will be designed to limit the release of these radionuclides to the environment,in order to meet the requirements of environmental radiation protection of nuclear power plant.In addition,biological shielding design will be carried out for nuclear reactor to reduce the radiation dose level of staff to a reasonable level,in order to meet the requirements of radiation protection of nuclear power plant operation.In the case of nuclear power accident,once the radioactive physical barrier is destroyed,a large amount of radionuclides will be released to the environment,which will bring great harm to the public and the environment.Therefore,it is of great significance to accurately determine the type and total amount of radioactivity produced in the reactor for the safe operation of the reactor and the safety protection of the environment and human beings.Compared with the traditional commercial reactor,the fuel composition and shape,the structure material and the design structure of the molten salt reactor are significantly different.For example,the conventional PWR uses a fixed type of nuclear fuel nd uses light water or heavy water as coolant,while the molten salt reactor uses liquid molten salt as the fuel and also as the coolant.During normal operation,a large amount of radionuclides flow out of the core with the fuel salt,passing through the upper chamber,the hot leg section,the main pump,the heat exchanger,the cold leg section,the lower chamber,and finally flow back into the reactor.For conventional PWR,the static static point-burnup code is usually used to simulate the evolution of radionuclides.However,for molten salt reactor,the flow of fuel salt will cause some radionuclides flowing out of the core to escape the absorption of neutrons in the reactor,thus breaking the evolutionary equilibrium relationship between radionuclides and their daughters in the fixed type reactor,which will have a certain impact on the concentration and the total amount of important nuclides.It is obvious that the static point-burnup code is not suitable for the radionuclides analysis of molten salt reactor.Based on the above discussion,the evolution equation of fission products in flow state has been derived based on Mathematica7.0.Along the flow direction,space nodes were divided,and flow terms were added,and flow relation between adjacent spaces were established.Thus,MSRFP 1.0,a dynamic model of fission product source term in the primary loop system of molten salt reactor was established.At the same time,a special model MAST1.0 was established for the source term analysis of activation products.The MSRFP 1.0 and MAST 1.0 were compared with the reference program ORIGEN-S,and the maximum relative deviation of single nuclide activity is less than10%which indicates that they are in good agreements.Taking a loop type 2MW molten salt experimental reactor as the research object,the effect of flow on the source term concentration and total amount of key fission products was analyzed.The results show that the flow will lead to the uneven distribution of short-lived fission products in the primary loop system,which is significantly affected by the flow rate.The migration and evolution of fission gas and its daughter in off gas process system were analyzed.The results show that different removal rate of the fission gas have significant influence on the fission gas daughter production in off gas process system.Flow will also have a significant impact on the total amount of fission neutron poisons such as ¹³⁵Xe and fission products that produced by neutron capture such as ¹³⁴Cs and ^110mAg.At the same time,the radioactivity of covering gas system,off-gas system,graphite component,control rod system,main container alloy material and the potential biological toxicity after discharging were analyzed.The calculation results and theoretical methods provide important reference for molten salt reactor shielding design,radioactive waste management,reactor in-service maintenance and reactor decommissioning.