The mission of FONESYS is to keep the code limitations ‘under control’ and to provide guidance for code improvements.

Main objectives

  • To create a common ground for discussing envisaged improvements in various areas of system thermal-hydraulics, promoting a cooperation aimed at the improvement of the SYS-TH codes and their application in the licensing process and safety analysis.

  • To identify the area of improvement and share experience on the Graphical User Interface, SYS-TH code coupling with other numerical tools as 3D neutron kinetics, fuel pin mechanics, CFD, CMFD.

  • To share the experience on code inadequacies and cooperate in identifying experiments and/or code-to-code benchmarks for resolving the deficiencies.

  • To share the user experience on code scalability, applicability, and uncertainty studies.

  • To establish the acceptable and recognized procedures and thresholds for the V&V processes.

  • To maintain and improve the user expertise and the user guidelines for applying the code.

  • To share and resolve the safety issues and new licensing guide.

Working modalities

  • Developing a common understanding (e.g. by collecting different opinions and achieving a consensus document) about: SYS-TH codes (the definitions, the requirements, the capabilities, the current status), and limitations for SYS-TH codes (balance equations, numerical solution, user effect, from applications).

  • Identification of specific code limitations not covered in the validation process in order to address the areas of investigations.

  • Establishment of validation procedures for 3D SYS-TH codes for assigned phenomenon based complementary experiments performed in IET facilities and 3D separate effect test facilities.

  • Running and collecting results from ‘specific additional’ V&V – Benchmark activities.

  • Attending regular workshops, possibly creating ‘ad-hoc’ groups for special topics.

  • Addressing the possible skepticism from international community and answering questions.

  • Providing recommendations to prioritize code improvements and required experiments.

Road map for FONESYS - List of topics and subtopics for the operation for FONESYS


  • Virtual mass and interfacial pressure difference terms in relation to the hyperbolicity, and the pressure and void wave propagation.
  • Use of the Best Estimate SYS-TH codes for licensing.
  • Scaling of thermal-hydraulic phenomena.
  • System codes benchmarking on Critical Heat Flux (CHF) prediction.
  • Acceptability of errors in code predictions.
  • Comparison of capabilities of various approaches as “drift flux”, “2-fluid 6-equation model”, and “multi-field”.
  • Loop seal clearance: the physical process, the possible distortions in iets, and the prediction capabilities of system codes.
  • The various approaches for break flow and choked flow prediction and system codes benchmarking on Two-Phase Critical Flow
  • Dispersion and diffusion processes and the modelling in 1D and 3D approaches.
  • Transport of Interfacial Area and turbulence modeling in system codes.
  • The issue of convergence in time and in space.
  • Three-field equations separating liquid into droplets and liquid films: modeling and experimental validation.
  • Relevance of 3-field modeling in system codes and impact on code predictions.
  • Extension of system codes capabilities for super critical water, gas, sodium and lead-bismuth reactors.
  • Reflood.
  • Radiation heat transfer.
  • 3D modeling capabilities of system codes.
  • Codes portability and “mesh convergence” issues.
  • Natural convection heat transfers.
  • Scalability of codes closure laws supported also by benchmark activities (horizontal stratification, droplet entrainment, etc.)
  • Natural circulation.
  • Jacobian and solution methods.
  • Coupling between SYS-TH & CFD codes, fuel performance codes, etc.
  • Critical flow, sonic velocity, effect of sharp edge cavitation.
  • 3D core thermal-hydraulics.
  • Dry-out.
  • Share and/or develop the experiments together.
  • Counter Current Flow Limitation (CCFL).