Radiochemical Study On Fission Product Iodine In LiF-BeF₂ Molten Salt

Molten salt reactor（MSR）is the unique type of reactors using liquid fuel in the fourth-generation advanced nuclear reactor proposed internationally.This feature makes radiochemistry play a larger role in the research of MSRs than in any other reactors.Therefore,MSR is also considered as“chemical reactor”or“chemist’s reactor”.The behavior and distribution of fission products in MSRs are closely related to the neutron economy,fuel utilization,operation efficiency and safety,and the fuel reprocessing.Fission product iodine,due to its wide mass distribution,high fission yield,and diverse physical and chemical properties,occupies an important position in all fission products,and accordingly,causes widespread concern.In this paper,UF₄ irradiated by the photo-neutron source was melted with FLiBe（LiF-Be F₂,67:33mol%）salt at 650°C in Ar atmosphere.Based on theγ-ray measurement and identification of salt samples,gas samples collected by Ni foils and the deposits on the specimens,the distribution and behavior of fission product iodine,and its relationship with the redox potential of FLiBe salt have been studied.The main investigations and results are listed as follows:（1）Distribution and behavior of iodine in FLiBe saltDuring the melting process of mixture of irradiated UF₄ and FLiBe salt at 650°C,a transient release of iodine was observed.The release of iodine has been attributed to the hot atomic effect,that is,the oxidation-reduction reaction occurred between iodine nuclides with different chemical valence produced by fission and the successive decay,resulting in release of iodine from FLiBe salt.The elemental iodine released was captured by Ni foils and reacted with Ni to form metal iodide at the high temperature.These experimental results reasonably explain the“mysterious”loss of about 1/4 to 1/3of iodine produced in MSRE reported by Oak Ridge National Laboratory.（2）Isotope effect of iodine activity and its application in the measurement of redox potential of molten saltBy adding reductant and oxidant into the FLiBe salt,the correlation between the activities of fission product iodine(¹³¹I,¹³²I and ¹³³I)in FLiBe salt and the redox property of molten salt has been investigated systematically.Based on the results,isotope effect of iodine activity has been observed,and the origin of isotope effect has been explained experimentally and theoretically to be the behavior of its parent tellurium in FLiBe salt and the different decay modes of Te-I with different mass number.This discovery suggests the activity ratio of iodine isotopes or their distribution could be used as an indicator of redox potential of fuel salt.Relying on the selective adsorption of iodine ions on the activated silver probe by ion exchange,a novel method for activity distribution measurement of the iodine isotopes in FLiBe salt has been established.The method is simple,fast,easy to operate,and would be suitable to monitor the redox potential of thorium molten salt reactor,where the redox potential should keep at relatively high level to avoid the deposition of ²³³Pa in the reactor.（3）New mode of on-line fuel reprocessing for molten salt reactorBy studying the behaviors of fission products in FLiBe salt under different redox condition,it has been concluded that the elevation of oxidizability of molten salt can promote the release of volatile fission product iodine and semi-volatile fission products Te,Mo,Tc,so that they can be stripped from the salt together with the noble gas fission products krypton and xenon.Meanwhile,the increasement of reducibility of salt encourages the reductive deposition of noble metal fission products and their removal from molten salt.Inspired by the AIROX（Atomics International Reduction Oxidation）processing,one kind of improved open-cycle fuel processing,a new mode for fuel reprocessing of molten salt reactors,namely Molten Salt Reduction and OXidation（MSROX）reprocessing,has been proposed.According to experiments and analysis,only one cycle of sequential oxidation and reduction treatment can remove about half of the fission products from the fuel salt.The fast-speed processing enables MSROX to achieve the goal of fuel online processing of MSRs.All the findings concluded above not only greatly enrich the basic knowledge of radiochemistry of MSRs,but also can provide a significant technical support for the diagnosis,monitoring and regulation of thorium based molten salt reactor in the future operation.