The demise of zero power reactors: From concern to action

In the early decades of nuclear reactor development, many countries built and operated zero-power facilities in which many different critical arrangements of materials were studied. These flexible experimental facilities, which included zero power reactors (ZPRs), criticality-safety assemblies and shielding facilities, have produced large quantities of physics data, such as average neutron cross sections or integral reactor physics quantities, which were needed to ascertain the calculational techniques used for reactor design. Over the years, with the progress of reactor modelling and computer simulations, the use of such facilities progressively shifted from studying engineering mock-ups to producing benchmark-quality experimental information (both separate-effect and combined-effect tests) for nuclear data and computer code validation. Today, many of the unique experimental data measured in these facilities have been curated and stored in databases, in particular the International Handbook of Evaluated Reactor Physics Benchmark Experiments (IRPhE) and the International Criticality Safety Benchmark Evaluation Project (ICSBEP) databases, and are routinely used for benchmarking activities.

As of the beginning of the 1980s, the number of ZPRs in operation started to decrease progressively, and this trend continues ever since. In 2009, the NEA report on Research and Test Facilities required in Nuclear Science and Technology alerted on the shortage of facilities for performing nuclear and neutron physics measurements and for new reactor development. The report concluded that there is “a need for versatile zero (or low) power reactors and sub-criticality assemblies for basic reactor physics experiments”. Today, nearly all of the original ZPRs have been permanently shut down, without being replaced by correspondingly new facilities. This demise has resulted in a drastic reduction in experimental capability. Only a handful of facilities remain in operation worldwide and there is little prospect for an improvement in the near future. What is at stake is not only the loss of the experimental infrastructure and expertise, but also the future capacity at acquiring new data to support verification, validation and uncertainty quantification (VVUQ) of the simulation tools, as well as experimental investigation, such as new phenomena and materials to foster innovation.

This has caused concerns among the international reactor physicists’ community, all the more as they anticipate that many new experimental data will be needed to back the VVUQ process in performance and safety demonstrations for fission, fusion, and accelerator type systems. Indeed, such demonstrations rely almost systematically on computer code simulations, which recent regulatory documents (e.g. IAEA, US NRC, French IRSN, UK ONR) require to support with appropriate experimental evidence.

In order to address this concern, the Nuclear Science Committee has decided to create an international specialists’ Task Force, with the objective of:

  1. reviewing the projected needs for new reactor physics validation data and elaborating on their motivation; and
  2. recommending a consensual course of action for acquiring such data, including minimal functional specifications of the needed facilities and expertise.

Detailed specifications, economics considerations, and possible implementation plans are beyond the scope of the Task Force. The expected outcome of the Task Force activities is a report summarising the findings and recommendations, intended for use by decisionmakers.



  • 22 June 2023, 09:00-18:00 CET: Lessons learned &  ZPR needs
    • Discussion focused on lessons learned and the projected needs for new reactor physics validation data
  • 23 June 2023: 09:00-16:00 CET: The Future of ZPRs
    • Discussion focused on possible courses of action for acquiring reactor physics validation data, including minimal functional specifications of the needed facilities and expertise

A detailed agenda is provided on MyNEA SharePoint.

Workshop objective

This NEA Working Party on Scientific Issues and Uncertainty Analysis of Reactor Systems (WPRS) Task Force workshop provided an opportunity to discuss the needs for new reactor physics experimental data and to evaluate possible courses of action for acquiring such data.

In preparation of the workshop, the Task Force conducted around 50 interviews with international experts representing different stakeholder communities (research, industry, technical support organisations, regulators, government) and different job profiles (ZPR operators, ZPR data users,  experimentalists, data evaluators). 

Each workshop session started with a summary of the key findings derived from the interviews. Workshop participants were then invited to react and contribute to in-depth discussions. The objective was to collect the views and expectations of all workshop participants. The discussions and conclusions served as input for the Task Force report, which will summarise the stakeholders’ positions and needs, and will provide rerecommendations.


The Task Force on Zero Power Reactors operates under the auspices of the Working Party on Scientific Issues and Uncertainty Analysis of Reactor Systems (WPRS).

Meeting material and the detailed agenda is available on MyNEA SharePoint.

Workshop venue

The workshop was hosted by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) at Fontenay-aux-Roses in the Paris region in France. 


31, avenue de la Division Leclerc
92260 Fontenay-aux-Roses

(Restricted access media)  


By plane

  • Paris Orly Airport: at 18 km
  • Paris Roissy Charles de Gaulle Airport: at 40 km

By car

  • From Paris/Porte de Châtillon: 
    take D906 direction CLAMART/VERSAILLES. After 4 km, you arrive at Place de la Divison Leclerc.
  • From Versailles:
    take highway A86, Exit CLAMART PARIS PORTE DE CHÂTILLON. Take D906, after 3 km, you arrive at Place de la Division Leclerc.

By train

  • RER B (Charles de Gaulle/Robinson):
    • Then Bus 394: stop DIVISION LECLERC.
  • RER C (Versailles Chantier/Massy Palaiseau):
    • Station ISSY-VAL-DE-SEINE.
    • Then Bus 394: stop DIVISION LECLERC.

By metro

  • Line 4:
    • Station PORTE D’ORLEANS. Bus 388 or194
    • Then Tramway T6, stop DIVISION LECLERC.
  • Line 13:
    • Then Tramway T6, stop DIVISION LECLERC


A selection of hotels in the vicinity of the IRSN:

22 - 23 June 2023
IRSN, Fontenay-aux-Roses, France

Task Force Chair: Robert JACQMIN, CEA (France)

Local Organising Commitee: Aurelie BARDELAY, IRSN (France)

NEA Secretariat: Oliver BUSS and Elena POPLAVSKAIA