Specialists meeting on transient thermal-hydraulics in water cooled nuclear reactors

WGAMA meeting

Participants at a specialists’ meeting on transient thermal-hydraulics in water cooled nuclear reactors on 22-23 March 2022

Nuclear reactor thermal-hydraulics has developed over the past 60 years as an important discipline for nuclear reactor design, safety analysis and operational support. Despite the achievements in the field, certain issues remain. In order to summarise the current status of thermal-hydraulics and to formulate recommendations to the Committee on the Safety of Nuclear Installations (CSNI) for activities in the area, the NEA Working Group on Analysis and Management of Accidents (WGAMA) organised a specialists’ meeting on transient thermal-hydraulics in water cooled nuclear reactors on 22-23 March 2022.

Held virtually, the meeting was co-organised with the Spanish Nuclear Safety Council (CSN), the Centre for Energy, Environment and Technology (CIEMAT) and the University of Pisa (Italy). The meeting gathered nearly 70 experts from industry, safety authorities, research institutes and universities from 16 countries to discuss the achievements and the needs of safety research in nuclear reactor accident thermal-hydraulics.

Participants highlighted the following recommendations:

Data preservation

  • Preserving expertise, databases and facilities – and creating new ones, including improved and innovative measurement techniques – is key to maintain competences and ensure development. In this context, databases of past and current experiments should be properly gathered and presented. In addition, new and more stringent requirements concerning data should be set as new approaches such as 3D computational fluid dynamics are applied.


  • Multi-scale and multi-discipline (including multi-physics) modelling and simulation with applications to safety analyses, virtual reactors or digital twins are part of current practice.
  • New directions for research are proposed for possible improvements in analytical and numerical investigations that include thermal-hydraulics and computational fluid dynamics.


  • Code verification, validation, scaling and uncertainty are key BEPU (Best Estimate Plus Uncertainty) elements that require continued attention from research specialists and are expected to constitute topics for knowledge transfer.
  • The integration of deterministic and probabilistic approaches in a risk-informed context is being pursued for the quantification of safety margins. Additional research is recommended in this area, as well as the application of relevant findings to the design of non-water cooled reactors (gas, lead bismuth, sodium, etc.).
  • Fluids other than water and advanced reactors should be addressed as other fields in thermal-hydraulics might benefit from the experience of water cooled nuclear reactors.

The WGAMA will continue to advance the scientific and technological knowledge base needed for the prevention, mitigation and management of potential accidents in nuclear power plants, and to facilitate international convergence on safety issues and accident management analyses and strategies.

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