Advanced Thermal-hydraulic Test Loop for Accident Simulation (ATLAS) Project
Joint project

View of the ATLAS experimental loop. KAERI, Korea

The Advanced Thermal-hydraulic Test Loop for Accident Simulation (ATLAS) Project was referenced in The Fukushima Daiichi Nuclear Power Plant Accident: OECD/NEA Nuclear Safety Response and Lessons Learnt report as one of the three new joint projects based on existing research facilities which address safety issues related to the accident.

The ATLAS project is aimed at topics of high safety relevance for both existing and future nuclear power plants.

It is important because the use of computer codes is required in safety evaluation of light water reactors (LWRs) in order to simulate plant behaviour during design basis accidents (DBAs) and design extension conditions (DECs). This involves complex multi-dimensional single-phase and two-phase flow conditions.

Although current thermal-hydraulic safety analysis codes have achieved very high predictive capability especially for one-dimensional phenomena, there is a strong need for experimental work and code development and validation for these complex flow conditions.

Moreover, the increased use of best-estimate (BE) analysis methods in licensing, which is replacing traditional conservative evaluation model (EM) approaches, require the validation and quantification of uncertainties in the simulation models and methods.

Many experimental facilities have contributed to the thermal-hydraulic databases available today which have been extensively used for the validation of EM and BE computer codes. However, most of the current data are insufficient for future codes that are to incorporate multi-dimensional simulation capabilities, mainly because the spatial resolution of measurement is not enough to assess the simulation models and methods, especially in system-integral testing.

ATLAS phases

First phase (2014-2017)

The main objective of the first phase was to provide experimental data for resolving key LWR thermal-hydraulics safety issues related to multiple high-risk failures and highlighted in particular from the Fukushima Daiichi nuclear power plant accident, by using the ATLAS facility at the Korea Atomic Energy Research Institute (KAERI).

The first phase focused in particular on the validation of simulation models and methods for complex phenomena of high safety relevance to thermal-hydraulic transients in DBAs and DECs:

  • Generated a system-integral and separate-effect experimental database to validate the predictive capability and accuracy of computer codes and models. Thermal-hydraulic phenomena coupled with multi-dimensional flows that may have included mixing, stratification, counter-current flows, parallel-channel flows and oscillatory flows were the main focus of the investigations.
  • Facilitated the assessment of codes currently in use for thermal-hydraulic safety analyses, as well as advanced codes under development, including three-dimensional computer codes, through active involvement of the project partners, who maintained and improved the technical competence in thermal-hydraulics for nuclear reactor safety (NRS) evaluations.

The experimental programme was intended to provide a valuable and broadly usable database to achieve the above objectives. In phase one, a total of eight to ten tests at the ATLAS facility were proposed to be conducted within five different research topics:

  • prolonged station blackout (SBO)
  • small break loss-of-coolant accident (SBLOCA) during SBO
  • total loss of feedwater (TLOFW)
  • medium-break LOCAs
  • scale-up issues – related to assessing the applicability of small-scale experimental data to full-scale reactors.

ATLAS Members' area (password protected | reminder)

Second phase (2017-2020)

ATLAS-2 is a follow-up to phase one and is focused on topics of high safety relevance for both existing and future nuclear power plants that have been identified by the participants. The following topics will be addressed:

  • long-term coolability with partial core blockage
  • passive core makeup during station blackout (SBO) and small break loss of coolant accident (SBLOCA)
  • intermediate break loss of coolant accident (IBLOCA), including risk-informed break size definition
  • design extension condition (DEC) scenarios such as steam line break (SLB) followed by steam generator tube rupture (SGTR) and shutdown coolability without residual heat removal system (RHRS)
  • open test to address scaling issues by performing counterpart test to previous Integral Effects Tests (IETs).

The experimental programme is devised to allow for an open test, which is to be defined in consultation with project members and which might cover the above issues or other safety-relevant issues. The experimental programme is to provide an integral-effect experimental database, which will be used to validate code predictive capability and accuracy of models. The experimental programme and associated analytical activities will help to create a group among NEA member countries which share the need to maintain or improve the technical competence in thermal hydraulics for nuclear reactor safety evaluations.

ATLAS-2 Members' area (password protected | reminder)


ATLAS: Belgium, China, Finland, France, Germany, Hungary, India, Japan, Korea, Russian Federation, Spain, Sweden, Switzerland, United Arab Emirates and the United States.

ATLAS-2: Belgium, China, Czech Republic, France, Germany, Korea, Spain, Switzerland, United Arab Emirates and United States.

Project period

ATLAS: April 2014 to March 2017
ATLAS-2: Oct 2017 to Sept 2020


ATLAS: EUR 2.5 million

ATLAS-2: EUR 3 million