核石墨与熔盐相容性测试装置设计与实验

Equipment design and experiment of compatibility test of nuclear graphite and molten salt

钍基熔盐堆(Thorium Molten Salt Reactor,TMSR)是以核石墨为反射体及慢化体、2Li F-BeF_2(FLiBe)熔盐为主冷却剂的反应堆。在TMSR中,核石墨直接与熔盐接触。由于石墨的多孔特性,熔盐有可能渗入石墨的孔隙中,引发其力学、热学性能的变化。研究熔盐在TMSR环境下是否渗入候选核石墨及其浸渗量,对于反应堆的运行安全至关重要。基于自行研制的熔盐浸渗实验装置,采用静态熔盐浸渗试验方法,测试TMSR候选核石墨T220在不同压强下的熔盐浸渗量,并研究了温度、时间对T220、NBG-18及IG-110石墨材料熔盐浸渗行为的影响。研究结果表明:T220石墨的临界浸渗压强介于600~700 kPa之间,这说明在TMSR工况下(<500 kPa)该石墨不发生FLiBe熔盐浸渗。温度(600℃和700℃)及时间(20~2 000 h)对三个牌号石墨熔盐浸渗行为影响不大。

[Background] Thorium molten salt reactor（TMSR） is a nuclear fission reactor using nuclear graphite as neutron reflector and moderator and molten 2 LiF-BeF2（FLiBe） salt as primary coolant. In TMSR, graphite is directly in contact with the molten salt. Given the porous nature of graphite, molten salt may penetrate into its pores in molten salt reactor environments, which may change graphite’s thermal and mechanical properties. [Purpose] This study aims to verify whether the FLiBe salt penetrates into candidate nuclear graphite and to test the amount of infiltrated molten salt, which is of vital importance to the safety in operation of the reactor. [Methods] An equipment used for static infiltration tests was developed to investigate the infiltration rate of molten FLiBe salt into graphite materials under different pressures. Effects of temperature and duration on the molten salt infiltration behaviors of graphite materials were also studied. [Results] The threshold pressure for FLiBe salt infiltration for TMSR candidate graphite T220 was between 600 k Pa and 700 k Pa at 700 ℃. The higher the infiltration pressure, the greater the infiltration rate, while the increase amplitude decreased gradually. The infiltration rate of three grades of graphite after infiltration tests in different temperatures（600 ℃ and 700 ℃） and durations（202 000 h） showed no significant difference. [Conclusion] FLiBe salt could not penetrate into graphite T220 in TMSR environments（〈500 kPa）. Both factors of temperature and duration had little effects on infiltration behaviors of graphite materials.