Panda facility under construction. Photo courtesy of the Paul Scherrer Institute

HYMERES phases

First phase (2013-2016)

The main objective of the first phase of the Hydrogen Mitigation Experiments for Reactor Safety (HYMERES) Project was to improve understanding of hydrogen risk phenomenology in containment in order to enhance its modelling in support of safety assessments that are performed for current and new nuclear power plants. With respect to previous projects related to hydrogen risk, the HYMERES project included three elements:

• Tests addressing realistic flow conditions – This provided crucial information in the evaluation of the basic computational and modelling requirements (mesh size, turbulent models, etc.) needed to analyse a real nuclear plant.
• Tests addressing the interaction of safety components – In general, previous investigations have focused on the activation of one safety component (spray, cooler, Passive Autocatalytic Recombiner (PAR), etc.) and showed their benefits and drawbacks. In the project, investigations of different combinations of safety elements, e.g. the thermal effects created by two PARs, spray and cooler, spray and opening hatches, etc., operating simultaneously, were performed. The specification for the design of the safety components (e.g. full cone vs. hollow cone for sprays, PAR simulator power source time history, cooler design, etc.) was defined based on consensus among the project participants.
• Reviews of system behaviour for selected cases – In certain reactor types (e.g. various BWR, PWR or PHWRs designs), the hydrogen concentration build-up in the containment depends on the responses of different components in the system.

Consequently, investigations for safety-relevant system behaviour related to BWRs, PWRs or PHWRs were conducted in the HYMERES Project. The knowledge gained from the project contributed to the improvement of the severe accident management (SAM) measures for mitigating hydrogen risks. The test series conducted within the HYMERES project, have been carefully defined, considering the operating agent's experience in other NEA projects (e.g. SETH).

The HYMERES project was specifically aimed at topics of high safety relevance for both existing and future nuclear power plants.

Facilities description

The unique and complementary features of the PANDA and MISTRA facilities, e.g. different size and configuration, enabled the full complement of measured parameters, configurations and scales to be explored, thus enhancing the value of the data in regard to code improvements.

PANDA is operated by the Paul Scherrer Institute (PSI) and is a unique, large-scale, multi-purpose facility that is well suited for performing thermal hydraulics experiments for investigating integral containment system response during accidents, and also for studying multi-compartment 3D effects related to LWRs. The design and scaling of the facility were originally based on the 670 MWe Simplified Boiling Water Reactor (SBWR) design from General Electric. Full vertical heights are preserved, at a volume scaling of 1:25. The complex containment volumes of the SBWR design together with its Reactor Pressure Vessel (RPV), are simulated by six cylindrical pressure vessels. Drywell and suppression (wetwell) pools are represented by two interconnected vessels. The total volume of the vessels and pools is about 520 m³, and the height of the facility is 25 m. All PANDA components are made of stainless steel. The facility is thermally insulated with 20 cm of rock wool, and the heat loss characteristics for the individual vessels were experimentally determined. The 115 heater elements installed in the lower part of the RPV provide a maximum power of 1.5 MW, also scaled to 1:25 (at decay heat level). The maximum operating conditions of PANDA are 10 bars and 200°C.

MISTRA is a large experimental facility belonging to the CEA and located at Saclay nuclear research centre, devoted to containment thermal-hydraulics and hydrogen risk. The containment is a stainless-steel cylindrical vessel with an internal volume of 97.6 m³ and comprises two shells, a flat cap and a bottom, which are joined by twin flanges. The height and inner diameter of the vessel are 7.38 m and 4.25 m, respectively. These dimensions correspond to a linear length scale ratio of 0.1 in relation to a typical French PWR containment. The thickness of the vessel walls varies from 25 mm at the bottom and 15 mm at the vertical walls to 120 mm for the lid of the vessel. The outside of the vessel is insulated by 20 cm of Rockwool. Three so-called condensers are inserted into the MISTRA vessel along the vertical walls. Each condenser is an open cylinder, with an inner diameter of 3.82 m, which is slightly less than the inner vessel diameter of 4.25 m. The condensers share the same vertical axis with the vessel walls and are located on top of each other, with some spacing in the vertical direction. Until 2004, MISTRA test series were based on a free gaseous volume configuration. The facility was then modified to accommodate compartments, in order to divide the internal volume of the MISTRA vessel into two distinct volumes — upper and lower. The compartment consists of a vertical cylinder, which is closed at the bottom and is fitted with a ring plate. The internal diameter of this cylinder is 1.906 m and its height is 4.219 m. The bottom of the compartment is at an elevation of 1.245 m from the vessel bottom, while the top of the compartment is at an elevation of 5.464 m. The compartment walls are about 3 mm thick. The ring plate is a steel plate positioned horizontally at an elevation of 3.658 m, with an outer radius of 1.728 m. As the condenser inner radius is about 1.910 m, there is therefore a gap between the ring plate and the middle condenser. Thus, gas can flow from upper to lower volume, or vice versa, but this flow path is partially obstructed by the presence of the lower part of the middle condenser. This compartmented configuration allows several possibilities for injection: two vertical in the lower area, centred and off-centred; one vertical at the level of the ring plate with a chimney; and several radial injection points at different locations (four per level). The maximum operating conditions of MISTRA are 6 bars and 200°C. Various auxiliary systems are available for controlling initial and boundary test conditions.

The PANDA and MISTRA instrumentation is comprehensive in term of both spatial and temporal resolution. Therefore, the high-quality experimental data generated in the Project can be used for improving the modelling capabilities of Computational Fluid Dynamics (CFD) and advanced Lumped Parameter (LP) computer codes designed to predict post-accident, thermal-hydraulics conditions in containments, and thus enhance the confidence in their use for plant analysis. 

HYMERES Members' area (password protected | reminder)

The data package for the first phase is available upon request through the NEA Data Bank at HYMERES-1, Hydrogen Mitigation Experiments for Reactor Safety Project, phase 1 (oecd-nea.org). 

Second phase (2017-2021)

The aim of the HYMERES-2 project was the detailed analysis of the containment phenomenology during postulated severe accidents with the release and distribution of hydrogen. The production and analysis of high-quality experimental data led to an improved understanding and to the enhancement of the modelling capabilities in support of safety assessments that are performed for current and new nuclear power plants. This further contributed to the improvement of Severe Accident Management (SAM) measures for mitigating hydrogen risks. With the experimental programme performed by the Paul Scherrer Institute (PSI) in Switzerland using the PANDA facility, the work from the first phase of the HYMERES Project (2013-2016) was continued and expanded.

In the HYMERES-2 project, the experimental investigations addressed four main topics:

• extending the database on flow impacting obstructions and containment internal structures;
• evaluating the importance of radiative heat transfer;
• solving remaining issues on suppression pressure pool and boiling water reactor (BWR) systems;
• extending the database on the performance of safety component operation (spray and cooler).

The experimental investigations were supported and completed by analytical activities from the project participants on a voluntary basis.

HYMERES-2 members' area (password protected | reminder)

External reference:

Domenico Paladino, Ralf Kapulla, Sidharth Paranjape, Simon Suter, Michele Andreani, PANDA experiments within the OECD/NEA HYMERES-2 project on containment hydrogen distribution, thermal radiation and suppression pool phenomena, Nuclear Engineering and Design, Volume 392, 2022, 111777, ISSN 0029-5493, https://doi.org/10.1016/j.nucengdes.2022.111777.