MOX乏燃料在碱性熔体中热化学、电化学还原分离的机理研究

In order to solve the problems of high temperature chemical processing of MOX spent fuel (i.e. unable to realize the separation of actinide by thermal reduction resulting in more secondary pollutants and low reduction rate of electrochemical reduction of solid oxide), it was proposed that MOX spent fuel were dissolved in alkaline melts, actinide oxides can dissolve and other insoluble oxides were removed from the melts utilizing the difference in solubility of metal oxides in the melts. And then, the dissolved actinide compounds were reduced to corresponding metals by thermochemical or electrochemical reduction in order for separation and recovery of nuclear fuel. UO2 and rare earth oxides were used as simulated MOX spent fuel. The mixed oxides burnt before and after were investigated in Na(Na2O2)-NaOH melts: (1) dissolution behavior (the solubility of UO2, UO3 and U3O8 and change of composition and valence), separation factor of U/RE, thermodynamics and kinetics of dissolution reaction; (2) thermodynamics and kinetics and mechanism of thermal reduction reaction; and (3) electrochemical behavior, thermodynamics and kinetics and electrochemical mechanism of electrolytic reduction. The mechanism of acid-base effect on the dissolution and separation of oxide and reduction reaction were studied under high temperature in order to improve separation factor of U/RE and the reaction rate and conversion rate of uranium oxide reduced to uranium metal and solve the key problems of low reduction efficiency of MOX spent fuel.

针对目前高温化学处理MOX乏燃料存在的问题（热还原无法实现分离而造成二次污染物多，电化学还原固态氧化物还原速率低），本申请项目提出利用金属氧化物在碱性熔体中溶解性的不同，将MOX乏燃料中的锕系氧化物溶解在熔体中，去除其他不溶的氧化物，再利用碱金属将溶解的锕系化合物通过热化学或电化学还原为相应的金属，达到分离回收核燃料的目的。以氧化铀和稀土氧化物为模拟MOX乏燃料，研究灼烧前后的混合氧化物在Na (Na2O2)-NaOH熔体中：(1)溶解行为（UO2、UO3、U3O8溶解度及组成价态变化）、U/RE分离因子、溶解反应热力学和动力学；(2)热还原反应热力学和动力学及还原机理；(3)电解还原的电化学行为、热力学和动力学及电化学机理。探讨高温下酸碱效应对氧化物溶解分离、还原反应影响的机制，提高U/RE分离因子及氧化铀还原为金属铀的反应速率和转化率，以期攻克困扰MOX乏燃料还原效率低的关键问题。

高温氧化挥发技术被称为先进首端处理技术。在氧化过程中 UO2 芯块转化为U3O8 和 UO3 粉末。为了使U3O8 或 UO3与裂变产物分离，开展了碱性熔体为介质的研究。本研究以UO2、稀土氧化物以及BaO或SrO为模拟氧化物乏燃料，首先研究了UO2-CeO2的灼烧反应，CeO2对UO2高温氧化的影响；研究了单一UO3或U3O8以及混合氧化物UO2-RE2O3、UO2- (BaO或SrO)灼烧后的产物在 NaOH、 Na2O2-NaOH 熔体中的溶解反应，溶解过程中铀的化学组态的变化，以及溶解反应的热力学；研究了掺杂裂变产物氧化物对U3O8在NaOH熔体中反应机理的影响，以及U3O8在NaOH熔体中的钠热化学还原过程的还原机理，探索了U3O8在NaOH熔体中电解还原的电化学行为、电化学机理。结果表明，当掺杂5wt% CeO2后，UO2的氧化（160-443℃）受到抑制，U3O8的转化率从100%降低到85%。对UO3与U3O8在NaOH熔体中溶解行为进行了比较：UO3在NaOH熔体中溶解一步生成Na4UO5；而U3O8在有氧气存在时先生成中间产物Na4UO4，然后生成Na4UO5而溶于NaOH中；而在无氧条件下会生成Na4UO4和Na4UO5两种产物。UO3在熔体中的铀元素含量为8.17 wt.%，而U3O8为7.99 wt.%。确定了U3O8、UO3在NaOH、Na2O2-NaOH熔体中的溶解反应如下：.2U3O8(s)+24NaOH(l)+O2(g)→6Na4UO5(s)+12H2O(g)； .UO3(s)+4NaOH(l)→Na4UO5(s)+2H2O(g) ； .10NaOH(l) + Na2O2(l) + U3O8 = 3Na4UO5 + 5H2O(g)。