TPTF horizontal flow prediction by SYS-TH codes – Recent analyses made within the FONESYS network

M. Lanfredini (Corresponding Author), D. Bestion, F. D'Auria, N. Aydemir, S. Carnevali, P. Fillion, P. Gaillard, J. J. Jeong, M. Junk, I. Karppinen, K. D. Kim, J. Kurki, J. H. Lee, P. Schoeffel, H. Sha, T. Skorek, J. L. Vacher, G. Waddington

Research output: Contribution to journalArticleScientificpeer-review


The horizontal stratification occurrence in water cooled nuclear reactors (WCNR) plays an important role in many transients such as loss of coolant accidents and loss of residual heat removal by influencing the liquid mass repartition, the natural circulation and reflux condenser modes, and the break or leak flowrate. A benchmark activity on horizontal flow prediction conducted in the framework of FONESYS is here presented. FONESYS is a network among code developers who share the common objective to strengthen current technology. The aim of the 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. One-hundred-twenty-six runs performed with the Two-Phase Test Facility (TPTF) are considered. Tests were selected aiming to support and complement an ongoing activity on the scalability of codes’ closure laws, currently focusing on the horizontal stratification criteria, the onset of droplet entrainment and entrainment fraction criteria. Therefore, experimental tests performed with the 4-inch and 8-inch tests sections, at pressure ranging from 30 bar to 118 bar, in different flow regimes are considered. Focus is put on the capability of ATHLET, CATHARE 3, MARS-KS, RELAP5, RELAP5-3D, SPACE and TRACE in predicting void fractions, the transition to slug flow and the onset of droplet entrainment. Overall, the benchmark results show reasonably good capabilities of codes in predicting horizontal flow. However, some noticeable limitations emerged related to flow regime transition criteria, interfacial friction, and to the prediction of non-established flow. The scalability of closure laws and possible ways for improving them are discussed.

Original languageEnglish
Article number112106
JournalNuclear Engineering and Design
Publication statusPublished - Feb 2023
MoE publication typeA1 Journal article-refereed


  • Horizontal flow
  • International cooperation
  • SYS-TH codes development


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