EXAFS和HERFD-XANES技术研究氧对液态FLiBe熔盐中钍铀物种的微观结构影响

Molten salt reactor with liquid nuclear fuel is one of the fourth-generation nuclear energy systems. The liquid LiF-BeF2-ThF4-UF4 molten salt was used as nuclear fuel in thorium-based molten salt reactor. The oxygen impurity in fuel salt will interact with both thorium and uranium and change the chemical constitution of fuel salt, influencing its solubility, thermal conductivity, diffusion coefficient and so on. In order to maintain the structural stability of liquid fuel within safe range when the liquid fuel is runing in salt circuit, it is necessary to study the effect of oxygen on the microstructure of both thorium and uranium in liquid fuel. The planned research work is proposed to change the fuel salt from a solid state into liquid by using an in-situ elevated temperature equipment, and to obtain the local structure on interaction between oxygen and both thorium and uranium in liquid fuel salt including valence state, local symmetry, unoccupied states of 5f-6d electron orbit, coordination number and bond length by synchrotron radiation EXAFS and HERFD-XANES techniques , and to study the effect of oxygen on microstructure of liquid fuel salt.

核燃料以液态形式存在的熔盐堆（热堆和快堆）是一类具有代表性的第四代核能系统，液态氟化物LiF-BeF2-ThF4-UF4熔盐是钍基熔盐堆用核燃料，燃料盐中的杂质氧会与钍和铀发生相互作用，改变核燃料的化学成分，影响其溶解性、热导率和扩散系数等宏观性能。为了使液态燃料在回路中循环时的结构稳定性长期保持在安全范围内，非常有必要研究氧对液态燃料盐中钍和铀物种微观结构的影响。本项目拟通过变温原位装置使燃料盐样品保持液态且处于目标气氛中，利用同步辐射EXAFS和HERFD-XANES技术获取液态样品中钍和铀与氧相互作用的局域结构信息，包括价态、局域对称性、非占据态5f/6d轨道、配位数和键长等，研究氧对液态燃料熔盐微观结构的影响。

核燃料以液态形式存在的熔盐堆（热堆和快堆）是一类具有代表性的第四代核能系统，液态氟化物LiF-BeF2-ThF4-UF4熔盐是钍基熔盐堆用核燃料，燃料盐中的杂质氧会与钍和铀发生相互作用，改变核燃料的化学成分，影响其溶解性、热导率和扩散系数等宏观性能。在本项目资助下，依托上海光源BL14W1线站，我们改进和发展了高温熔盐原位XAS技术和基于探测Lβ5荧光的HERFD-XANES技术；利用EXAFS和HERFD-XANES技术研究了液态LiF-BeF2-ThF4-UF4和LiF-BeF2-ZrF4-UF4等熔盐样品及其与氧反应的最终氧化物ThO2、Th1-xUxO2+y和U3O8的微观结构，发现燃料盐中的U4+无法避免与氧反应而最终生成U3O8。因此，如何确保生成的U3O8完全溶解于燃料熔盐而不会以沉淀析出是保证熔盐堆的液态燃料长期稳定安全运行的一个前提，具有重要的科学和实际工程意义。