快堆瞬态中子学计算中堆芯变形的显式模拟方法研究

The inherent characteristics of high temperature and sharp temperature gradient in fast reactors result in inevitable geometric deformation such as the expansion of fuel and structural materials and the bowing of fuel assemblies. The deformation will lead to significant changes of core status, and it even becomes the main source of negative feedback in the transient process in some types of fast reactors, especially in those reactors loaded with metal fuel. However, the complex geometric deformation cannot be explicitly modeled by the traditional neutronics computational methods based on assembly homogenization. They can only obtain the total deformation reactivity by approximate methods such as the perturbation theory. These methods usually have problems of low accuracy and cannot be coupled with three-dimensional multi-physics processes because of single output response. Therefore, how to evaluate the neutronics effect caused by deformation accurately has become a key physical problem to be solved urgently in the design and safety analysis of fast reactor. For this purpose, the heterogeneous variational nodal method is proposed to directly describe the assembly axial elongation, bowing, and the expansion of the structural materials, realizing the explicit simulation of core deformation. Thus, it will eliminate the errors of existing approximate methods completely and tackle their problems of having difficulty in coupling with three-dimensional multi-physics processes. This project has important academic value for perfecting the core-analysis theory of fast reactor, and also has important application value for improving the safety of fast reactor design.

快堆堆芯温度高、温差大的固有特点使其在瞬态过程中存在不可避免的几何变形，表现为燃料、结构材料的膨胀以及燃料组件的弯曲。这些变形会导致堆芯状态的显著改变，在某些堆型如装载金属燃料的快堆的瞬态过程中甚至成为最主要的负反馈来源。然而，传统基于组件均匀化的堆芯中子学计算方法无法对复杂的几何变形进行显式建模，只能通过微扰理论等近似方法获取变形引入的总反应性。这些方法普遍存在精度低和输出响应单一以致无法进行三维多物理耦合的问题。因此，如何准确计算变形引起的中子学效应成为了快堆设计和安全分析中亟待解决的关键物理问题。为此，本项目提出采用非均匀变分节块法来直接描述堆芯各组件在轴向的伸长、弯曲和结构材料的膨胀，实现对堆芯变形的显式建模，从而彻底消除现有近似方法的误差，同时解决其无法进行三维多物理耦合的问题。本项目对于完善快堆堆芯计算理论具有重要的学术价值，也对提高快堆设计的安全性具有重要的应用价值。

快堆的研发设计在军、民两方面都具有重要战略意义。一方面，快堆具有功率密度高、寿期长的特点，容易实现小型化，是海、陆、空、天等各种军用场景下重要的备选堆型，开发一批创新型、多用途的核反应堆型号是各国先进核能技术竞争的高地；另一方面，快堆是解决当前压水堆铀资源利用率低、废料嬗变困难的重要途径，对核能的可持续发展具有重要意义，我国已经确立了“压水堆-快堆-聚变堆”三步走的核能发展战略，快堆在这一战略路线中发挥着承上启下的关键作用。然而，由于快堆独有特性，导致燃料多普勒温度负反馈和冷却剂温度负反馈减弱，变形成为了瞬态中最重要的负反馈来源，对瞬态过程中功率随时间的变化过程具有决定性的影响作用，而此效应在现有国内外设计软件中是忽略的或考虑得极为粗糙的。因此，准确考虑堆芯变形效应对提升快堆瞬态数值模拟精度具有重要意义。.面对以上需求，本项目建立了快堆燃料轴向伸长、堆芯径向膨胀、组件弯曲这三类主要变形方式的数值计算方法，对于其中比较复杂的组件弯曲反应性，提出了基于通量重构的新计算思路。最终获得了能够在瞬态中考虑各类变形效应的反应性反馈，为瞬态计算提供了关键数据。同时，开发了相应的计算软件，对理论模型进行了验证，证明了变形反应性计算的正确性；并通过耦合物理、热工和力学模块进行了美国实验装置EBR-II的 SHRT-45R实验的模拟验证，比较了裂变功率、温度、各类反馈反应性等重要参数，证明了软件在瞬态计算中的计算精度能够满足设计分析的要求。因此，本项目研究成果对提升快堆设计分析计算精度和设计可靠性、从而助力我国先进快堆系统发展具有明显的应用价值和现实意义。.

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