事故容错热导率增强型UO2核燃料的研究进展

Research Progress on Enhanced Thermal Conductivity Uranium Dioxide for Accident Tolerant Fuel

UO2+Zr合金燃料元件为当前轻水核反应堆应用最广泛的核燃料体系。然而,福岛“311”核事故的突发揭露了UO2+Zr合金燃料体系在事故状态下的重大安全隐患,研发事故容错核燃料计划被提上议程。事故容错燃料是为提高核燃料元件抵抗严重事故能力而开发的新一代燃料系统。对现有核燃料形式进行设计改进,即在UO2基体中添加一定量高热导第二相,开发热导率增强型UO2核燃料,此方法对工业体系的改动小,为近期事故容错核燃料的主要研究方向。现阶段,在热导率增强型UO2核燃料开发历程中,已取得应用性研究进展的候选体系主要为UO2-SiC、UO2-BeO、UO2-金刚石以及UO2-Mo。其中,在UO2-SiC和UO2-金刚石体系中,对SiC以及金刚石与UO2的界面反应认识还不足,在堆内辐照条件下SiC和金刚石性质的演变对UO2热物理性能的作用规律尚未明晰。电场辅助快速烧结技术是抑制界面反应、制备UO2-SiC和UO2-金刚石的有效途径。在UO2-BeO体系中,前期大量实验研究和堆内模拟表明BeO与UO2具有优异的化学相容性以及良好的增强效果,UO2-BeO被视为具备工业应用前景的燃料体系,然而,铍材料作为战略资源的稀缺性和BeO的剧毒性以及对乏燃料后处理流程的变革是工业化应考量的。在UO2-Mo体系中,Mo作为金属中最具潜力的添加材料,呈现三维网状分布,展现出优异的热导率增强作用,这种微结构还兼具持留裂变产物的优势;与其他几种添加材料相比,Mo的中子吸收截面较高,添加量应合理调控,相应的基础研究需持续跟进。目前,上述候选燃料体系尚缺乏堆内辐照考核数据。可将高通量制备、机器学习等引入UO2系核燃料的研制中,以加快热导率增强型UO2的工业化应用进程。本文归纳了添加第二相的热导率增强型UO2核燃料的研究进展,分别对制备方法、微观结构、导热性能等进行介绍,分析了热导率增强型UO2面临的问题并展望了其应用前景,以期为研发轻水堆用事故容错燃料提供参考。

The UO2+Zr alloy fuel assembly is the most widely used nuclear fuel system for the current light water nuclear reactor. However, the Fukushima Daiichi nuclear disaster revealed the primary safety risks of UO2+Zr alloy fuel system under accident, thereby various international programs have been launched to develop accident tolerant fuel (ATF). ATF is a new generation of fuel system developed to enhance the ability of nuclear fuel assembly under severe accident. The primary domain research of accident tolerant fuel is to design and improve current nuclear fuel forms and develop enhanced thermal conductivity UO2 nuclear fuel on the basis of little change in industry. At present, the candidate fuel systems for thermal conductivity enhanced UO2 that have achieved progress are the UO2-SiC, UO2-BeO, UO2-diamond and UO2-Mo. For the UO2-SiC and UO2-diamond systems, the details of the interfacial reactions between SiC/diamond and UO2 have been poorly understood. The role of the evolution of properties for SiC and diamond on the thermos-physical performance of UO2 under irradiation has yet to be fully defined. Nevertheless, electric field assisted sintering (FAST) technology is an effective method to suppress the interfacial reaction and fabricate UO2-SiC and UO2-diamond. In the UO2-BeO system, the excellent chemical compatibility between BeO and UO2, and the enhancement effect for thermal conductivity of UO2 has been demonstrated through the extensive work. Accordingly, UO2-BeO is regarded as the candidate fuel system for industrial application. However, the Be presenting as the scarcity material for the strategic resource, and the toxic nature of BeO as well as the transformation of spent fuel reprocessing processes, should be considered for industrialization. In the UO2-Mo system, Mo has been considered as the most potential additive among the metals, and indicates outstanding thermal conductivity enhancement for the three-dimensional network distribution. Moreover, such microstructure is beneficial for retaining fission products. Compared with other additives, Mo has the property of higher neutron absorption cross section, and thus the amount of addition should be reasonably regulated. While such fuel system also should be investigated in detail. It is well accepted that the candidate fuel system above is still lacks of irradiation data for assessments. High-throughput preparation and machine learning could be introduced into the development of thermal conductivity enhanced UO2 to accomplish the industrial application at an earlier time. This review offers a retrospection of the research efforts with respect to the thermal conductivity enhanced UO2, and provides the fabrication process, microstructure and thermal conductivity respectively. Then the current problems confronting the thermal conductivity enhanced UO2 are concerned as well as the application prospects are presented to the benefit for ATF in light water reactor.