| Molten salt reactor（MSR）is a unique liquid fuel reactor among the six advanced Fourth Generation Reactors.Because of the properties of liquid fuel,the behavior and distribution of actinide nuclides and fission products have an important impact on the design,construction,operation and safety of MSR.Shanghai Institute of Applied Physics,CAS（SINAP）is developing thorium molten salt reactor nuclear energy system（TMSR）,the molten salt reactor chemistry aimed at studying the behavior and distribution of actinide nuclides and fission products,and their effect on the operation of MSR and pyroprocess separation has become an important work to be carried out as early as possible.Noble metal fission products,which are an important part of nuclear fission products,have complex properties,and their behavior and distribution are closely related to the important characteristic of molten salt,such as redox potential,in MSR.Therefore,the research on the behavior and distribution of noble metal fission products in molten salt has become the key research point.In this paper,the nat.UF4 was irradiated by photo-neutron source driven by a 15Me V electron linear accelerator built at SINAP,and the simulated fuel salt was prepared by melting irradiated UF4 and FLi Be melt,used as both coolant and carrier salt.The deposition and distribution of noble metal-containing fission products between the molten salt,Hastelloy and graphite crucible,were investigated by gamma-ray spectroscopy technique,and the influence of addition of metal Li and Ni F2 on the distribution of noble metal fission products was discussed.The main research contents and results obtained are as follows:（1）Establishment of method for studying the behavior and distribution of fission productsBased on the molten salt pyroprocessing technology,an experimental method for studying the behavior of fission products in molten salt under laboratory conditions was established.The general behaviors of volatile fission products,salt-seeking fission products and noble metal fission products observed in the experiment,and the conclusion that 95Nb could be used as redox indicator of molten salt were consistent with ORNL’s research on MSRE,which demonstrated the feasibility and credibility of the experimental method used in this paper,and the established method laid the foundation of technical support for the subsequent research.（2）The behavior and distribution of fission product 95Nb in FLi Be molten saltBased on the prospect of fission product 95Nb as an indicator of redox potential of molten salt,the behavior and distribution of 95Nb in FLi Be molten salt were investigated.The results indicated that the majority of 95Nb could maintained stably in molten salt,and addition of metal Li made 95Nb in salt to be reduced and settled,leading to the decrease in its activity.Degree of the decrease was found to be correlated with the time of the 95Zr-95Nb growth-decay relationship and the 95Nb concentration in molten salt.The technical limitation and solution of 95Nb as an indicator of redox potential in MSR were discussed.（3）Deposition of fission product 95Nb on Hastelloy in FLi Be molten saltThe results of ORNL pointed out that the measurements of 95Nb activity in molten salt were also affected by the correct distinction between soluble 95Nb and insoluble 95Nb metal particles in molten salt,which should be one of the reasons that the data given by ORNL were fragmentary,scattered,even anomalous sometimes.Above results were supported by our experiments.In this part,the deposition of fission product 95Nb on Hastelloy and the correlation of the deposition of 95Nb on Hastelloy with its specific activity in molten salt were investigated.The experimental results showed that the amount of 95Nb deposited on Hastelloy exhibited well correlation with the change of 95Nb activity in FLi Be salt,and the ratio of deposition rate of 95Nb to 103Ru on Hastelloy could quantitatively characterize the redox potential of molten salt.The above results provided two new solutions that are expected to be more accurate,more convenient and feasible for the use of noble metal fission products to measure the redox potential of MSR.（4）The distribution and behavior of other noble metal fission products in FLi Be molten saltThe specific activity of 99Mo,103Ru and 132Te in molten salt,and their deposition and distribution on graphite and Hastelloy were studied.In addition,the influence of the redox potential of molten salt on the deposition behavior of 99Mo,103Ru and 132Te on Hastelloy was discussed.Because 99Mo has a huge demand in the development of radiopharmaceuticals,a feasible method of separation based on the behavior of 99Mo in molten salt was proposed.The findings mentioned above indicated that it is not only feasible to start the research of molten salt reactor chemistry by using photo-neutron source to obtain irradiated sample in the laboratory before the completion of thorium molten salt reactor system,but also many meaningful research results could be obtained.After the thorium molten salt reactor system begin operation,the results from laboratory will provide scientific and technical support,guarantee and guidance for the monitoring and diagnosis of reactor system on-site. |
