热管堆固态堆芯燃料辐照-热-力耦合性能分析

Coupled irradiation-thermal-mechanical analysis of fuel in solid core of heat pipe cooled reactor

热管冷却反应堆(简称:热管堆)具有高可靠性和固有安全性、体积小、模块化和全固态堆芯等特点。固态堆芯燃料服役过程在高温、强辐照、固态约束多因素作用下堆芯的传热和力学性能受到严重影响,基体接触导致应力与间隙换热都随燃耗加深而发生较大非线性改变,且两者相互影响,因此基体在服役过程中的多物理场耦合的辐照-热-力行为复杂。本文基于有限元多物理场分析软件针对固态堆芯燃料开展辐照-热-力耦合分析,考虑UO2芯块与316不锈钢基体的辐照变形效应以及蠕变效应,并在固态堆芯间隙中引入间隙传热模型,探究固态堆芯寿期内间隙变化特点以及传热和力学耦合作用特性。结果显示:基体与燃料包壳的完全接触会导致芯块温度上升以及基体与包壳蠕变现象加强,燃料棒周围平均热管数量较少会导致附近区域较高的温度和应力分布,且寿期中该区域包壳因燃料棒内压和基体-包壳接触压力具有蠕变失效风险。分析结果表明间隙接触会对热管堆固态堆芯的传热和力学性能造成影响,甚至提高包壳的失效风险。

[Background]Heat pipe cooled reactor(HPR)has many characteristics,such as reliability,inherent safety,small volume,modularity,and solid core.The nuclear fuel of solid core is seriously affected by high temperature,strong irradiation,and solid constraint when operating,which affect the heat transfer performance and mechanical properties of the core seriously.The stress and gap heat transfer caused by the contact between monolith and other components change nonlinearly with the increase of burnup,and they influence each other.Therefore,the coupled irradiation-thermal-mechanical behavior of the monolith is a complex multi-physics phenomena.[Purpose]This study aims to develop a coupled irradiation-thermal-mechanical model to explore the characteristics of gap variation,heat transfer and mechanics during the lifetime of solid core.[Methods]First of all,based on the geometric parameter and material of a typical solid core of HPR with fuel rod composed of UO2 pellets and 316 stainless steel cladding,a coupled irradiation-thermal-mechanical model was developed and applied to the finite element multi-physics field analysis software COMSOL.The calculation parameter settings mainly referred to the design parameters of the MegaPower reactor.Then,a thermal conductivity model changing with the increase of burnup for UO2,the gap heat transfer model and mechanical contact were introduced in the gaps in the solid core,and both irradiation-induced deformation effect including densification and fission product swelling,and creep effect of UO2 pellets and 316 stainless steel monolith were taken into account.Finally,the model was applied to calculating the typical HPR and the characteristics of gap variation,heat transfer and mechanics were analyzed.[Results]Analysis results show that pellet temperature and creep of monolith and cladding increase after complete contact between monolith and cladding.A smaller average number of heat pipes around the fuel rod result in higher temperature and stress distribution in the nearby area,and the cladding in this area has a risk of creep failure during its lifetime caused by internal pressure of the fuel rod and contact pressure between the monolith and cladding.[Conclusions]The gap contact can affect the heat transfer and mechanical properties of the solid core of HPR,and even result in an increase in the risk of cladding failure.