Critical flow prediction by system codes – Recent analyses made within the FONESYS network

M. Lanfredini (Corresponding Author), D. Bestion, F. D'Auria, N. Aksan, P. Fillion, P. Gaillard, J. Heo, Ismo Karppinen, K. D. Kim, Joona Kurki, L. Liu, A. Shen, J. L. Vacher, D. Wang

Research output: Contribution to journalArticleScientificpeer-review

10 Citations (Scopus)


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.

Original languageEnglish
Article number110731
JournalNuclear Engineering and Design
Publication statusPublished - Sept 2020
MoE publication typeA1 Journal article-refereed


  • International cooperation
  • SYS-TH codes development
  • Two-phase critical flow


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