核热推进反应堆燃料元件热工应力安全分析

Thermal stress safety analysis of nuclear thermal propulsion reactor fuel elements

核热推进采用的超高温气冷堆堆芯长期运行在高热流密度、大功率梯度、大温差、高速冷却剂冲刷的严苛服役条件下,其内部的六棱柱型燃料元件可能发生应力集中导致引发结构失效风险,影响核反应堆燃料元件的运行安全性能。为探索核热推进燃料元件的热工应力行为规律及热工安全边界,以火箭飞行用核引擎(nuclear engine for rocket vehicle applications,NERVA)型核热推进反应堆为对象,选取反应堆内部密排燃料组件基本单元,建立对称模型,针对堆内(U,Zr)C石墨基复合燃料元件开展高温高流速氢气推进模式下的流-热-应力行为研究,评估燃料元件的高温熔化与断裂失效风险。研究结果表明:核热推进反应堆运行工况下,燃料元件受自身冷却剂通道排布方式与连接管元件冷却作用影响导致内部热流分配不均;径向大温差带来的热膨胀差异在轴向上积累是引发燃料元件结构断裂失效的主要原因;综合分析燃料元件内部的温度-应力场分布情况与影响因素,可为核热推进系统的运行安全设计提供优化思路与参考依据。

In nuclear thermal propulsion,the very-high-temperature gas-cooled reactor core operates under challenging service conditions including high heat flux,steep power gradients,significant temperature difference,and intense coolant flow erosion.In this environment,there is a risk of structural failure due to stress concentration in the hexagonal fuel elements,which can potentially impact the safety performance of the nuclear reactor fuel elements.To explore the thermal stress behavior and thermal safety boundaries of nuclear thermal propulsion fuel elements,with a focus on NERVA-type nuclear thermal propulsion reactors,a symmetrical model is established using the basic unit of densely packed fuel components inside the reactor,which is conducted for(U,Zr)C graphite-based composite fuel elements within the reactor,under high-temperature,high-flow hydrogen propulsion conditions,evaluating the risk of high-temperature melting and structural failure.The research findings indicate that under the operating conditions of nuclear thermal propulsion reactors,fuel elements experience non-uniform internal heat distribution due to the arrangement of their coolant channels and the cooling effects of connecting pipe elements.The accumulation of radial temperature differences leading to different thermal expansion along the axial direction is the primary cause of structural failure in fuel elements.Combining the analysis of temperature-stress field distribution within the fuel elements and the factors influencing it,this research can provide optimization ideas and guidelines for the safety design of nuclear thermal propulsion systems.