Abstract Fuel of fast reactors is designed in the form of cylindrical pellets of solid or annular geometry. The annular configuration provides advantages in terms of higher achievable energy extraction and lower fuel-clad mechanical interaction. Another advantage of this configuration is the hydrodynamic flow of molten fuel inside the annular pellets during accidental meltdown, known as in-pin fuel motion. This motion can provide safety benefits during an unprotected transient overpower accident (UTOPA), if the molten fuel is dispersed significantly away from the core mid-plane. The physics behind the flow is complex. Intricate theoretical modelling and detailed experimental validation are pre-requisites for the reliable estimation of safety benefits. To meet this requirement, a M ulti-phase I n-pin T hermal hydraulic R elocation A lgorithm (MITRA) is developed under the purview of the pre-disassembly analysis code, PREDIS. This paper presents a theoretical foundation of the algorithm followed by experimental verifications, burnup and top blanket design sensitivity analyses and whole core UTOPA simulations. Results show that regardless of the fuel burnup level, the melt tends to agglomerate into a column, slightly below the core mid-plane. Upon further melting, this column grows slowly, devoid of the rapid dispersion associated with fuel squirting. Minor deviations in the melt position arise due to variations in thermal parameters with burnup. UTOPA analysis shows a reduction in the safety feedback and increased melting. Modifying the conventionally solid top axial blanket to annular geometry enhances the safety feedback only after 34.2% melting.

中文翻译：

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严重事故代码 MITRA 环形快堆燃料棒的建模、验证和安全反馈评估

摘要 快堆燃料被设计成圆柱状的固体或环形几何形状的芯块。环形配置在更高可实现的能量提取和更低的燃料包覆机械相互作用方面提供优势。这种配置的另一个优点是在意外熔化期间，环形芯块内的熔融燃料的流体动力流动，称为柱内燃料运动。如果熔融燃料显着远离堆芯中平面，则这种运动可以在无保护的瞬态超载事故 (UTOPA) 期间提供安全优势。流动背后的物理学是复杂的。复杂的理论建模和详细的实验验证是可靠估计安全效益的先决条件。为了满足这个要求，a 多相 In-pin 热液压再定位算法 (MITRA) 是在预拆卸分析代码 PREDIS 的范围内开发的。本文介绍了该算法的理论基础，随后进行了实验验证、燃耗和顶部覆盖层设计灵敏度分析以及整个核心 UTOPA 模拟。结果表明，无论燃料燃烧水平如何，熔体都倾向于聚集成一个柱状体，略低于堆芯中平面。在进一步熔化时，该柱缓慢生长，没有与燃料喷射相关的快速分散。熔体位置的微小偏差是由于热参数随燃耗的变化而引起的。UTOPA 分析显示安全反馈减少和熔化增加。