Investigation On In-Core Fuel Management For Pebble-Bed Fluoride Salt-cooled High Temperature Reactor

Energy security and climate change are the top two reasons for pursuing nuclear energy,but developing nuclear energy inevitably faces the risks of traditional challenges of nuclear energy such as cost,safety,waste and proliferation.In order to minimize those risks,Fluoride Salt-cooled High Temperature Reactor(FHR)was one of the proposed new nuclear reactor types and was developed rapidly.FHR had made progress to the stage of baseline designing of Pebble-Bed Fluoride Salt-cooled High Temperature Reactor(PB-FHR)after more than ten years diverse pre-conceptual designing.Restricted by limited development of pebble-bed reactors,the circulation of fuel management codes of pebble-bed reactor based on deterministic method was also limited.On the other hand,regular commercialized fuel management codes for Pressurized Water Reactor(PWR)did not suit pebblebed reactor,and general Monte Carlo particle transport code could provide highaccuracy neutronic solution for pebble-bed reactor.Consequently investigation on fuel management for PB-FHR at present focused on equilibrium cycle search of pebble-bed reactor by coupling Monte Carlo particle transport code and burnup calculation code,which unavoidably suffered the extremely low efficiency of Monte Carlo code.Deterministic method for solving flux distribution is more efficient but still on study for that of PB-FHR.Comparing with fuel management calculation for PWR,fuel management calculation for pebble-bed reactor based on deterministic method had to confront the difficulties in geometric modeling,double heterogeneity,leakage effect between nodes,control rod in diffusion calculation and significantly historic effect in burnup calculation.In this paper a brief introduction to two kinds of numerical computation of neutronic physics were presented,and they were Monte Carlo method and deterministic method.Results from Monte Carlo method were usually referred as reference in this study.With regard to deterministic method,we introduced some equivalent methods to address the above problems in order to make it suitble for fuel management calculation for PB-FHR:(1)Collision probability method was applied to solve the neutronic system of pebble cell of PB-FHR,and attained its infinite multiplication factor,cell-homogenized few-group cross sections including total,capture and fission cross sections,spectra and spectral indices.The evaluation of accuracy of the output was based on comparison to a continuous-energy Monte Carlo reference model.Resonance calculation routine solved slowing-down equations of lattice with microscopic cell on the ultra-fine lethargy mesh by collision probability method,through which the doubly heterogeneous system could be treated accurately.The cell-homogenized capture cross section of resolved resonance energy region was only 0.08% lower than reference result,the existence of TRISO structure would reduce that cross section by 20%,and increase infinite multiplication factor by 7%.Results show that outputs from our method are consistent with that of reference model,and our method is capable of generating few-group cross sections for PB-FHR.(2)Spectra modification method was studied,which was a basic scheme that could allow for leakage effect between nodes for generating few-group cross sections.Whether applying eigenvalue mode or fixed source mode iteration scheme,spectra modification method could reach the accuracy of flux-volume homogenization method without knowing transport reference solution in advance,and was applicable to multi-dimensional reator models.Fixed source mode iteration scheme had better adaptation to models than eigenvalue mode iteration scheme,for example models with separate thorium zones.(3)The control rods of PB-FHR are located in the side reflector.Dealing with those control rods in diffusion calculation had to face the difficulties of no fission source in the control rod region and strong effect from the core leakage spectrum.Therefore we introduced two calculation models: spectra modification method-blackness material method and spectra modification method-super homogenization(SPH)method.Results shows that reactivity worth and flux distribution could be calculated accurately by both methods compared with transport reference solution,and spectra modification method-SPH method was more suitable for the situation with interference effect between different control rods compared with regular SPH method.(4)The demands of burnup calculation for fuel management of PB-FHR were described,which is different from those of PWR.Regular lattice burnup calculation is difficult to take notable historic effect in online refueling of pebble-bed reactors into consideration.Therefore neutron flux calculation was coupled with zero-dimensional burnup calculation,in order to burn a pebble cell in the situation with specified cell-homogenized one-group cross sections,fluxes and time steps.Curves of infinite multiplication factor and concentrations of some important actinides versus burnup could fit the results from MOBAT code,which showed the accuracy of our coupled code.The fuel management of pebble-bed reactor was online refueling scheme,through which fuel pebbles circulated the core in the way of granular flow.On the basis of studying equivalent methods for neutronic physics of PB-FHR,partition coefficients method was adopted to describe the online refueling scheme,and a code COBBLE for searching equilibrium cycle of pebble-bed reactor was developed aiming at analyzing fuel management of PB-FHR.A simplified version of pebble bed modular reactor(PBMR)model was referred to as a benchmark test.Power and burnup distributions were calculated by COBBLE and compared with those obtained by PBRE code based on Monte Carlo neutronic transport program.Results showed that COBBLE was suitable to analyze the equilibrium cycle of pebble-bed reactor.Fuel management calculation for the equilibrium cycle of Thorium Molten Salt Reactor-Solid Fuel 1(TMSR-SF1)with online refueling scheme was carried on,which based on the code COBBLE developed in this paper.Three refueling schemes,which were single-zone scheme with recirculated pebbles randomly refueled,two-zone scheme with recirculated fuel pebbles randomly refueled beside dynamic inner reflector and two-zone scheme with recirculated pebbles burnup-dependently refueled,were studied to analyze their influence on the maximum power density of the core at equilibrium cycle.On the condition that two-zone scheme with recirculated pebbles burnup-dependently refueled was implemented,if OUT-IN fuel management policy was applied to TMSR-SF1,it would damage fuel efficiency on some extent,however power flattening would benefit from it and power peaking factor was reduced.The maximum power density would decrease first then increase in response to increasing the ratio of outer zone,and the minimum value of maximum power density occurred at outer zone ratio of 80%,and is 7.8% lower than that of single-zone scheme.After that,for the purpose of researching how to increase fuel efficiency by using thorium pebbles in PB-FHR,reactivity comparison method was proposed to analyze the influence of thorium zone ratio,the pass times of thorium pebbles and thorium loading capacity in a pebble on effective discharge burnup,and TMSR-SF1 was choosed as the object of study.It turned out that only if uranium pebbles reached a high level discharge burnup in model with all uranium pebbles and thorium pebbles accumulated burnup to some extent would fuel efficiency increased in the case of using thorium pebble in online refueling scheme in which uranium pebbles and thorium pebbles were separated into two zones and recirculated pebbles were randomly refueled according to their fuel types.Increasing the thorium loading capacity in a pebble would obviously enhance the fuel efficiency and the effective discharge burnup would increase first then decrease as the ratio of outer thorium zone increase.