Towards a more integrated approach to safety research in thermal-hydraulic processes and experimental data


The NEA joint project called Experimental Thermal Hydraulics for Analysis, Research and Innovations in Nuclear Safety (ETHARINUS) helps member countries build the knowledge base for safety assessments and to provide data for the development and validation of thermal-hydraulic computer codes used to analyse safety issues.

The group is wrapping up one multi-year project and considering a proposal for a new one, and discussed these at the latest meeting of its programme review group on 27-28 June 2024 in Erlangen, Germany.

The current project, which began in October 2020 and is due to end in September 2024, involves experimentation in the modified PKL facility (PKL stands for Primärkreislauf-Versuchsanlage, or Primary Coolant Loop Test Facility) at Framatome GmbH, Germany, and in the PACTEL facility at the Lappeenranta-Lahti University of Technology, LUT University, Finland.

The new project, called SYStem THERmal-hydraulics (SYSTHER), is a joint effort of three operating agents – Framatome, LUT University, and France’s CEA – and is due to start in early 2025. In all, five experimental facilities would be used in the project for experiments on passive phenomena. The experiments cover a range of topics related to PWRs, boiling water reactors and water cooled-small modular reactors and have been arranged into three focus areas:

  • parametric studies on thermal-hydraulic phenomena and plant data;
  • the effectiveness of passive heat removal systems; and
  • the interaction of systems/processes and thermal hydraulic phenomena during complex (accidental) transients.

This new project is essential to promoting a more integrated approach to running experimental facilities in the technical areas of nuclear safety, specifically thermal-hydraulics.

The PKL and PACTEL facilities have for many years been used for extensive experimental investigations into the system response of pressurised water reactors (PWRs) under accident conditions. The various phases of the experiments have always reflected and given priority to current safety issues and have covered a broad spectrum of topics. The PKL and PACTEL tests have contributed to a better understanding of the sometimes highly complex thermal-hydraulic processes involved in various accident scenarios, to a better assessment of the countermeasures implemented for accident control and have supplied valuable information regarding safety margins available in the plants.

The test results have also led to concrete applications in the validation and further development of system thermal hydraulic computer codes and were largely used by the working groups of the NEA Committee on the Safety of Nuclear Installations (CSNI), including the Working Group on the Analysis and Management of Accidents (WGAMA). For example, the International Standard Problem (ISP) 10 addressed a refill and reflood experiment in a simulated PWR primary system and was conducted in the PKL in 1981; the ISP 33 addressed a natural circulation stepwise coolant inventory reduction experiment in the PACTEL facility in 1994; and the new ongoing ISP-52 addresses the assessment of code capabilities in predicting multiple steam generator U-tube ruptures.

Looking ahead, this area of work will be affected by the closure of the PKL facility beginning in April 2024. With the loss of access to other critical thermal-hydraulics facilities dealing with reactor coolant system thermal-hydraulics, the LUT University nuclear safety laboratory will be one of the few laboratories left in Europe able to address nuclear safety issues.

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