Research On The High-precision Burnup Calculation Methods And Thorium-Uranium Fuel Breeding For The Liquid-Fueled Molten Salt Reactor

Molten Salt Reactor(MSR)is the only liquid fuel reactor among the six candidates of Gen-IV advanced nuclear power system,and it is the most ideal type for Thorium-Uranium(Th-233U)fuel cycle.However,the characteristics of on-line continuously refueling also makes the burnup calculation of MSR much different from the traditional solid-state reactor,which presents primarily in two aspects:the first is that the non-homogeneous external feed term must be introduced into the burnup calculations to simulate the on-line continuously refueling;the second is that during the operation of MSR,the feed rate needs to be adjusted to maintain the reactor critical at any time.In this work,the particularity of burnup calculation of liquid molten salt reactor is studied based on the basic burnup model and algorithms.The fuel burnup calculation methods for molten salt reactor are developed firstly,and then Th-233U breeding performance of MSR is studied.First,MSRs generally adopt a closed fuel cycle mode and have extremely deep burnup,which puts forward higher requirements for the accuracy of point-depletion calculation.In view of this,a molten salt reactor specific depletion code MODEC is developed based on several advanced depletion algorithms.The code implements three algorithms,including recursive form of generalized Transmutation Trajectory Analy-sis(TTA),Quadrature-based Rational Approximation Method(QRAM)and Chebyshev Rational Approximation Method(CRAM),and then,based on the complex nuclide sys-tem and a series of efficient programming skills,the high precision and high efficiency of point-depletion calculation can be guaranteed.The three depletion algorithms are compared in calculation accuracy and efficiency,and then a detailed error analysis on ORIGEN-S is carried out.The analysis shows that in addition to the error source from-secular equilibrium assumption for the short-lived nuclides,the incomplete classifica tion of nuclides in ORIGEN-S causes some nuclides to be in the incomplete burnup chain,which can also bring significant errors.Second,in order to solve the nonhomogeneous burnup equations caused by on-line continuously refueling of MSR,this work proposes two new algorithms,extended QRAM(Ext-QRAM)and extended CRAM(Ext-CRAM),based on Laplace transform method.The comparison with other existing nonhomogeneous burnup algorithms shows that the new algorithms maintain the high accuracy and efficiency of QRAM and CRAM.And the new algorithms have the unified construction method for different functions of the external feed term,are thus more applicable for the complex feed functions than other nonhomogeneous burnup algorithms.Then,based on the Monte Carlo(MC)neutron transport code SCALE6.1/KENO-VI and the point-depletion code MODEC,an MSR specific burnup analysis code TM-CBurnup is developed and then verified.A special MC burnup procedure is used to simulate the the characteristics of on-line continuously refueling for MSR.In addition,for solving the problem of time consuming in liquid-fueled MSR(MSR-LF)burnup transition calculation to equilibrium state,an equilibrium burnup analysis code for MSR(MESA)is also developed.It is shown that MESA can search the equilibrium state with less than 10 iterative MC burnup steps.Compared with the conventional MC burnup codes,which need hundreds of MC burnup steps to calculate the equilibrium state of MSR,MESA can save computing resources and computation time greatly.Finally,the optimization of Th-233U breeding performance of thermal MSR is stud-ied by the developed burnup analysis tools above.Based on the Th-233U breeding per-formance on the equilibrium state,the graphite cell is optimized.The results show that the optimal graphite cell is independent of the power density and the critical level kinf,and the corresponding pin pitch is 5 cm,and the volume fraction of molten salt is 21.5%.And then,based on the equilibrium state of thorium uranium breeding performance,the power density of Molten Salt Breeder Reactor(MSBR)with the optimal graphite cell is optimized.The results show that when the power density is 70 MW/m3,there exists minimal equilibrium doubling time of 35.45 years.Compared with the MSBR doubling time of 43.05 years,the optimized IMSBR has a better Th-233U breeding performance.And IMSBR has a negative temperature coefficient of reactivity both in the initial state and in equilibrium state,so IMSBR is more capable of meeting the requirements for safe operation than MSBR which has a positive temperature coefficient.Then,the tran-sition burnup to the equilibrium state of IMSBR is analyzed,and the time evolution of the heavy metal nuclides and the parameters of Th-233U breeding performance are em-phatically analyzed.Finally,the radiation parameters of fuel salt and radioactive waste are analyzed to provide a parameter basis for the shielding design of the reprocessing system and nuclear waste disposal system.