NEA Mandates and Structures

Expert Group on Scientific Issues in Reactor Core Thermal-hydraulics

Chair(s): Upendra ROHATGI, United States
Secretary:  Ian HILL
Member(s):All NEA member countries
Full participant(s): European Commission
Under the NEA Statute
Observer(s)(International Organisation): International Atomic Energy Agency (IAEA)
By agreement
Date of creation:30 June 2019
End of mandate:30 June 2021

Mandate (Document reference):

  • Draft Summary Record of the 16th meeting of the Working Party on Scientific Issues of Reactor Systems (WPRS) held on 22 February 2019 [NEA/NSC/WPRS/DOC(2019)5/PROV]
  • Mandate of the Working Party on Scientific Issues of Reactor Systems [NEA/NSC/WPRS/DOC(2019)11]
  • Mandate of the WPRS Expert Group on Scientific Issues in Reactor Core Thermal-hydraulics (EGRCTH) [NEA/NSC/WPRS/DOC(2019)15]

Mandate (Document extract):

Extract from document [NEA/NSC/WPRS/DOC(2019)15]



Under the guidance of the Nuclear Science Committee (NSC) and under the mandate of the Working Party on Scientific Issues of Reactor Systems (WPRS), the expert group will perform specific tasks associated with multi-scale core thermal-hydraulics of present and future nuclear power systems. Coupling aspects of neutronics, fuel performance, coolant chemistry, and fluid structural mechanical interactions for normal and/or deformed core configurations will be considered, as will problems requiring high fidelity multi-scale multi-physics core simulation.  Reactor types considered include, but are not limited to, present generation light-water reactors (LWRs), heavy-water reactors (HWRs), small modular Reactors (SMRs), sodium-cooled fast reactors(SFRs), high-temperature reactors (HTRs), molten salt Reactors (MSRs) and other advanced nuclear systems.


1. To provide expert advice to the WPRS and the nuclear community on the development needs (data and methods, validation experiments, scenario studies) for multi-scale core thermal-hydraulics modelling and simulation of existing and proposed nuclear reactor systems.

A key activity associated with this objective is the identification and preservation of appropriate experimental data. In addition, the expert group will provide member countries with the guidance and processes for certifying the experimental data for its use as a stand-alone core thermal-hydraulic validation or for uses as part of validation pyramid of multi-physics modelling and simulation tools.

2. To provide specific technical information regarding:

  • The status of national and international programmes on reactor core thermal-hydraulics including experimental capabilities;
  • State-of-the-art reactor core thermal-hydraulics methodologies on different modelling scales;
  • Provision of experimental data for model development and validation to The International Experimental Thermal HYdraulicS (TIETHYS) database;
  • To provide TIETHYS with a methodology for a systematic and standardized process for collecting facility hardware and experimental test data;
  • Techniques for high to low fidelity/resolution model data exchange (Hi2Lo);
  • Values and distributions of local safety-related parameters and margins.To monitor, steer and support the continued development of the TIETHYS database. The expert group also will facilitate the dissemination of technical information and knowledge through activities such as workshops, benchmark studies and training activities.


  • State-of-the-art report on reactor core thermal-hydraulics multi-scale modelling and simulation and experimental data requirements.
  • Maintenance and further expansion of TIETHYS thermal-hydraulics database with core thermal-hydraulic experiments including expansion of computational fluid dynamics (CFD)-related section.
  • Development of benchmark specifications and organizing benchmark activities for blind benchmark on McMaster Core Thermal-hydraulics (McMaster CTH).
  • Development of sodium-fast reactor (SFR) thermal-hydraulics benchmark specifications based on the Thermal Hydraulics Out of Reactor Safety (THORS) experimental data (SFR THORS).
  • Organisation of workshops on preservation of thermal-hydraulics experimental data.

Links to other groups

The expert group will report to the Working Party on Scientific issues in Reactor Systems (WPRS) and will liaise closely with other relevant NEA bodies, including those operating under the guidance of the NSC, in particular the Expert Group on Uncertainty Analysis in Modelling (EGUAM), the Expert Group on Multi-physics Experimental Data, Benchmarking and Validation (EGMPEBV) as well as the Working Group on Analysis and Management of Accidents (WGAMA) under the Committee on the Safety of Nuclear Installations, in order to ensure that the respective programmes are complementary, to provide advice and support where required and to undertake common work where appropriate.


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