Abstract A benchmark activity on Two-Phase Critical Flow (TPCF) prediction was conducted in the framework of the Forum & Network of System Thermal-Hydraulics Nuclear Reactor Thermal-Hydraulics (FONESYS). FONESYS is a network among code developers who share the common objective to strengthen current technology. The aim of the FONESYS Network is to highlight the capabilities and the robustness as well as the limitations of current SYS-TH codes to predict the main phenomena during transient scenarios in nuclear reactors for safety issues. Six separate effect test facilities, more than 90 tests, both in steady and transient conditions, were considered for the activity. Moreover, two ideal tests were designed for code to code comparison in clearly defined conditions. Overall eight System Thermal-Hydraulic (SYS-TH) codes were adopted, mostly by the developers themselves, ensuring the minimization of the user effect. Results from selected tests were also compared against Delayed Equilibrium Model, not yet implemented in industrial version of SYS-TH codes. Generally, the results of the benchmark show an improvement of the capability of SYS-TH codes to predict TPCF in the last three decades. However, predicting break flowrate remains a major source of uncertainty in accidental transient simulations of Water-Cooled Nuclear Reactors (WCNR). A set of possible actions is proposed to go beyond the current limitations of choked flow models. More detailed guidelines for using 0-D choked flow models is possible by using the experience gained by the benchmark results as well as all available validation results. Progress in understanding and 1-D modelling of flashing and choked flow might be achieved by a deeper physical analysis leading to more mechanistic models based on specific flow regime maps for high speed flow. Also the use of advanced 3-D numerical tools may help to understand and predict the complex 3-D geometrical effects.

中文翻译：

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通过系统代码预测临界流量——最近在 FONESYS 网络中进行的分析

摘要 在系统热工水力核反应堆热工水力论坛和网络 (FONESYS) 的框架内进行了两相临界流 (TPCF) 预测的基准活动。FONESYS 是代码开发人员之间的网络，他们的共同目标是加强当前技术。FONESYS 网络的目的是突出当前 SYS-TH 代码的能力、稳健性以及局限性，以预测核反应堆瞬态场景中的主要现象以解决安全问题。六个独立的效果测试设施，超过 90 次测试，在稳定和瞬态条件下，被考虑用于活动。此外，还设计了两个理想的测试，用于在明确定义的条件下进行代码与代码的比较。总共采用了八个系统热工液压 (SYS-TH) 代码，主要由开发者自己完成，保证了用户影响的最小化。选定测试的结果还与延迟均衡模型进行了比较，该模型尚未在工业版本的 SYS-TH 代码中实现。总体而言，基准测试的结果表明 SYS-TH 代码在过去 30 年中预测 TPCF 的能力有所提高。然而，预测中断流量仍然是水冷核反应堆 (WCNR) 意外瞬态模拟中不确定性的主要来源。提出了一组可能的操作，以超越阻塞流模型的当前限制。通过使用基准结果以及所有可用验证结果所获得的经验，可以为使用 0-D 阻塞流模型制定更详细的指南。通过更深入的物理分析，可能会在对闪蒸和阻塞流的理解和一维建模方面取得进展，从而基于高速流的特定流态图产生更多的机械模型。此外，使用先进的 3-D 数值工具可能有助于理解和预测复杂的 3-D 几何效应。