IN Brief:
- ASNR inaugurated its PASTIS experimental platform at Cadarache on 4 November, with details published on 9 November.
- ALCINA will investigate pressure losses and natural circulation, while KoKoMo examines condensation and thermal stratification.
- The €12.8 million programme includes €9 million in French government support and will provide data for nuclear safety modelling.
France’s Autorité de Sûreté Nucléaire et de Radioprotection (ASNR) has inaugurated two experimental research facilities at Cadarache to investigate passive systems used to remove residual heat from nuclear reactors. The ALCINA and KoKoMo installations form the PASTIS research platform, which will provide measured thermal-hydraulic data for independent assessments of advanced reactor designs, including small modular reactors.
The opening ceremony took place on 4 November 2026 and was reported by ASNR on 9 November. PASTIS stands for Passive Systems Thermalhydraulic Investigations for Safety. The project has an estimated budget of €12.8 million, including €9 million in French state support administered through the National Research Agency under the France 2030 programme. The installations are research facilities, not prototype nuclear power plants.
Passive safety arrangements seek to perform protective functions through processes such as gravity, natural circulation, convection, and condensation, reducing dependence on powered pumps or operator intervention. They are relevant when conventional equipment or external electrical supplies are unavailable. Their effectiveness still depends on the physical conditions, component design, and accident sequences against which they are assessed.
Heat continues to be produced by radioactive decay after a reactor has shut down. That residual heat must be transferred away from the fuel and other relevant structures even though the main fission chain reaction has stopped. A passive cooling system may use differences in temperature and fluid density to sustain circulation without a mechanical pump, but the resulting flow is limited by the geometry and resistance of its circuit.
ALCINA, short for Analyse de la Circulation Naturelle, is a thermal-hydraulic loop designed to investigate those limits. Researchers will measure pressure losses in pipework during forced circulation and natural circulation, under single-phase and two-phase flow conditions. The facility can be arranged in different configurations to examine how changes in the circuit affect the movement of cooling fluid.
In natural circulation, heating changes a fluid’s density and can create a pressure difference that drives movement through a suitable loop. Flow is opposed by friction in pipes and by losses at bends, valves, and other components. The circulation rate results from the balance between the available driving force and those hydraulic losses, rather than from a pump imposing a fixed flow.
Two-phase flow introduces further uncertainty when liquid and vapour are present together. The distribution of the phases changes the density of the moving mixture and can alter pressure losses and heat transfer. Experiments that reproduce these conditions provide evidence about whether a proposed passive cooling circuit will sustain sufficient circulation across the conditions relevant to its safety case.
The second installation, KoKoMo, addresses a different part of the thermal-hydraulic problem. It reproduces aspects of convection and condensation inside a containment enclosure, including the principle of a cold-wall condenser in a vessel surrounded by water. The facility is intended to investigate how steam, cooler surfaces, and movement of gases interact during heat removal.
When steam contacts a sufficiently cool wall, it can condense and release heat to that surface. The effectiveness of the process depends on temperature differences and the local distribution of fluid and gases. Thermal stratification, in which layers at different temperatures remain partly separated, can change circulation patterns and the rate at which heat reaches the condensation surface.
KoKoMo will examine the coupled effects of condensation and stratification rather than treating them as independent processes. That distinction is important for reactor concepts whose containment cooling relies on natural movement of fluids and heat transfer to an external water volume. The facility does not reproduce every feature of an operating reactor or demonstrate the performance of one particular proprietary design.
ASNR intends the two installations to generate reference measurements against which numerical models can be assessed. The research programme covers the validation and development of thermal-hydraulic calculations, including work associated with CATHARE, used to assess normal and incident conditions, and ASTEC, used for severe accident analysis. Computational fluid dynamics methods are also within its planned scope.
Numerical calculations divide complex systems into models that approximate fluid motion and heat exchange. Their accuracy depends on the underlying physical relationships and the conditions for which those relationships have been tested. Experimental measurements can identify discrepancies between calculated and observed pressure loss, circulation behaviour, or condensation, allowing the models to be revised before they support a regulatory technical assessment.
The platform follows research begun by the former Institute for Radiological Protection and Nuclear Safety, IRSN, which examined the reliability of passive systems in 2016. IRSN became part of ASNR when France combined its nuclear safety authority and radiological protection research organisation in January 2025. The new platform extends that earlier work with dedicated experimental equipment.
ASNR expects the facilities to support national and international collaboration. Research links include EDF’s research and development teams and partners participating in the European EASI-SMR programme. The experimental arrangements are designed to be modular so that different geometric configurations can be examined as the requirements of new reactor concepts develop.
Construction of the PASTIS platform was scheduled between 2022 and 2025, with experimentation, model development, and code validation beginning in 2026. Its inauguration marks the availability of new research infrastructure, not completion of a full test programme. Results will need to be collected, evaluated, and incorporated into the assessment tools before they can inform specific reactor safety cases.
Passive heat removal may reduce reliance on active support systems, but it must still be assessed for operating limits and failure conditions. ALCINA and KoKoMo will provide measured evidence on circulation and containment cooling to support that examination, as developers submit new designs for independent nuclear safety assessment.
