NEA Hydrogen Mitigation Experiments for Reactor Safety (HYMERES) Project

The main objective of the HYMERES Project is to improve the understanding of the hydrogen risk phenomenology in containment in order to enhance its modelling in support of safety assessment that will be performed for current and new nuclear power plants. With respect to previous projects related to hydrogen risk, the HYMERES project will introduce three new elements.

Consequently, investigations for safety relevant system behaviour related to BWRs, PWRs or PHWRs are proposed in the HYMERES Project. The knowledge expected to be gained from the HYMERES project will contribute to the improvement of the Severe Accident Management (SAM) measures for mitigating hydrogen risks. The test series proposed within the HYMERES Agreement, have been carefully defined, considering the Operating Agent's experience in other NEA projects (e.g. SETH and SETH-2).

The needs for further research as identified in the SETH-2 Seminar, and the important issues and questions raised by the different countries and organizations during the HYMERES Expert meeting, which took place at the NEA Headquarters on 14 September 2011, were taken into account.. The HYMERES project is 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, will enable 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 PSI and is a unique, large-scale, multi-purpose facility that is well suited for performing thermalhydraulics 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 was 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 all 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 that will be created 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. The Operating Agents would also consider during the Project new experiments in response to specific participant requests


Canada, China, Czech Republic, Finland, France, Germany, India, Japan, Russian Federation, Spain, Sweden and Switzerland.

Project period

January 2013 to December 2016


EUR 4.0 million

Steps to implement the programme

The programme has been elaborated on the basis of the needs expressed in the conclusions of the Expert Meeting for the OECD HYMERES Project held on 14 September 2011. Discussion amongst international experts continued during the first half of 2012. The draft Agreement was circulated for comments on 10 April 2012. The project agreement was finalised and distributed for signature in the last quarter of 2012.

Horizontal collaboration

NEA Committee on the Safety of Nuclear Installations (CSNI).

Related links

Last updated: 9 February 2018