Thorium Breeding Investigation Of A Solid-fuel Fluoride-Salt-Cooled Fast Reactor

The research on thorium breeder reactors are important for China to accelerate the application of thorium fuel.The large-scale application of thorium fuel in nuclear energy systems is of important significance in the sustainability of nuclear resources.In recent years,there has been a rapid growth in research and development on the use of thorium-based fuel in nuclear systems.MSRs and MSFRs are the best candidate thermal reactors and fast reactors for thorium–uranium breeding,but the study of Nagy et al.showed that a self-breeder MSRs with FLiBe as coolant requires a fast removal fission products?FP?and actinide metals?MA?online and the extraction of ²³³Pa and the re-introduction of the formed ²³³U.It is a great technological challenge for fast online processing of fuel salts.Thorium-based molten salt fast reactor MSFR has better neutron economy and security advantages than other FRs.However,the main challenges of MSFR operation are as follows:?1?using a canister structure with no internal moderation and structure materials,the outside of the core is exposed to strong neutron radiation at high temperature;?2?the fuel salts need online processing at high temperature and high neutron flux.In this paper the thorium breeding research of a liqiud-salt-cooled solid-fuel fast reactor LSFRs was studied.The solid-fuel form has the following key advantages:?1?radioactive inclusion;?2?increasing the fuel loading capacity to obtain a harder energy spectrum and high discharge burn up with good economy;?3?low technical requirements and the breeding capability is independent of the on-line processing.The LSFR core fuel adopts UC/ThC as fuel,SiC as cladding material and Zr₃Si₂ as reflector material.Each fuel assembly is hexagonal with pin-type fuel elements in a triangular array.The 3-dimensional core layout uses the experience of the design of the UCB breed and burn reactor.This paper studied the feasibility region of thorium-uranium breeding in LSFR,the LSFR core characteristics at equilibrium,and the characteristics of the closed fuel cycle.At last,the reference core designs were given.Firstly,the selection of fluoride salt in fast spectra and the feasibility window of thorium-uranium breeding in LSFR were investigated.Under a fuel assembly model with infinite boundary conditions,among five candidate fluoride salts,FNaBe salts were selected as LSFR coolant for its excellent performance and reasonable cost.In the study of a design window for flexible self-sustaining core design,the accuracy and validity of the combination of neutron balance analysis with least square method in the parametric study of LSFR system in 0D simplified core model were firstly demonstrated;Subsequently,the effects of fuel volume fraction,the removal of fission gases,the neutron loss term and the power density level on the neutron balance evolution were investigated systematically with this method.This study proved that the design of a self-sustaining core for fluoride-salt-cooled fast breeder based on thorium fuel is achievable.The results showed that the fuel volume fraction strongly affects the concentration of ²³³U,the discharged burnup BUuni and the FIR?Fissile Inventory Rate from EOC to BOC,FIR>1 indicates the reactor core could achieve a self-sustaining core at least?at discharges etc;the removal of fission gas can significantly deepen the discharged burnup and the discharged burnup of the core with 60%fuel volume can be up to 40.84%FIMA;the unique solution of neutron equilibrium equation is very sensitive to the neutron loss term,and it is recommended that the neutron loss term be no more than 6%,otherwise it is difficult to maintain the self-sustainability of ²³³U in the core.Based on the above basic analysis,the equilibrium physics characteristics of thorium-based LSFR with 3D homogeneous model were further studied.Firstly,2D and 3D fuel management schemes were used to analyse physics characteristics at equilibrium.Compared with 2D shuffling,the 3D shuffling has the following advantages:3D refuelling scheme can reduce ²³³U inventory by 4%,increase the FIR at discharge from 1.013 to 1.090,decrease the fuel discharge burnup peak factor from 1.25 to 1.09,reduce the leak rate from the active core,reduce the reactivity swing at equilibrium and the cladding peak radiation damage and improve neutron economy.Subsequently,the effects of fuel volume fraction,the removal of fission gases,and the power density level on the LSFR core characteristics at equilibrium were investigated systematically using 3D shuffling scheme.The results showed that with the increasing of the fuel volume fraction,the ²³³U inventory is reduced and a better breeding performance is achieved.However,the reactivity swing,the discharge burnup peak factor and the irradiation time are increased and the cladding peak radiation damage exceeds the limit:the irradiation time of core fuel is up to 77.9 years at the 50MW/m³ power density with the fuel volume of 60%,resulting in the cladding peak radiation damage up to 285.23 DPA exceeding the limit of silicon carbide.The power density level strongly affects the cycle length of the core,and the cycle length is reduced by more than half with the power density increasing from 50 WM/m³ to 100 MW/m³.Based on the 3D homogeneous core model and multi-generation reactor concept,the closed fuel cycle characteristics and the spent fuel characteristics from the first generation to the eighth generation were analysed under U recycle scheme and U-Pu-MA recycle scheme,respectively.The research showed that the recycle nuclides can partially replace the ²³³U which can reduce the²³³U inventory and improve the breeding capability of LSFR core.Compared to uranium cycle,U-Pu-MA cycle has a slightly better performance.However,the recycled nuclides have higher absorption cross section than that of ²³³U and ²³²Th,leading to a higher absorption and lower leakage in the active core,and the core power peak factor increases to 3.22–3.29,increasing the neutron economy,but posing a challenge to the reactor operation.Trans-uranium elements gradually accumulate with the fuel recycling for thorium-based LSFR core cannot transmute trans-uranium nuclides.Taking the spent nuclear fuel of eighth generation core with U-Pu-MA scheme which has the largest amount of trans-uranium nuclides as an example,its radioactivity and decay heat level are equivalent to those of light water reactors,but the radioactive toxicity level is much lower than that of LWRs.Finally,the 3100 MWth and 6200 MWth LSFR reference cores were given.The heterogeneous core model with 3D shuffling scheme was used to analyse the core breeding performance,neutronics and burnup characteristics.Detailed neutron physics information of every fuel element in the core can be given based on these analyses.The LSFR core at equilibrium based on thorium fuel has the following characteristics:?1?high discharge burnup²0%FIMA;?2?the acceptable cladding peak radiation damage:the peak radiation damage of3100 MWth and 6200 MWth reference cores are 171.5 DPA and 112.1 DPA respectively;?3?small reactivity swing during the reactor operation at equilibrium:the reactivity swing of 3100MWth and 6200 MWth reference cores are 0.530%?k/k and 0.534%?k/k respectively,which is far less than that of UC Berkeley breed and burn reactor?4.83%?k/k for 2D shuffling and 6.96%?k/k for 3D shuffling?;?4?the breeding performance is not as good as that of fast reactors under U-Pu cycle scheme,but ²³³U self-sustaining can be achieved,which indicates that it can make full use of thorium without adding fissile ²³³U,beneficial to the sustainable development of nuclear energy;?5?at the beginning and at the end of the equilibrium,the total temperature reactivity coefficient is negative.