Integral large scale experiments on hydrogen combustion for severe accident code validation

P. Pailhories, J.P. L'hériteau, M. Petit,

Version brève du rapport de synthèse du Projet européen HYCOM septembre 2003,

Rapport DSR/28

The HYCOM project, an EC Cost Shared Action, was carried out with contributions from six organisations, which included research and expert organisations as well as industry. The project aimed at extension of the experimental data base which is needed for the verification of newly developed analysis methods and codes to predict hydrogen combustion behaviour and corresponding loads on representative scale. An experimental programme in medium and large scale facilities has been performed with combustion modes, ranging from slow to fast turbulent deflagration, that were not yet covered by previous experiments. The main focus was on complex, multi-compartment geometry and on inhomogeneous hydrogen concentrations in dry test atmospheres and at ambient temperatures, which allowed precise definition of the initial and boundary conditions. Detailed data were obtained revealing specific effects of scale, multi-compartment geometry and venting. It was observed that the flow geometry has some influence on critical conditions for fast combustion regimes, however applicability of the s-criterion was confirmed also for complex enclosures.

The data base has been used for the validation of criteria, models and codes which were developed by the partners. For several tests, blind predictive calculations were performed. Lumped parameter codes performed reasonably well in cases with slow flames, CFD codes showed better performance for fast combustion. Some phenomena like flame quenching or oscillation are not yet modelled, also description of heat losses needs improvement.

Post-test calculations were performed for selected tests, which gave valuable information on code capabilities and on the range of validity of models and code control parameters. A number of suitable tests were identified for benchmarking purposes and relevant data are made available to interested users outside the project.

After the validation stage, a scaling-up exercise was performed in order to evaluate the applicability of the codes to real-scale plants. The exercise was carried out on a simplified PWR containment. The assessment of the results of the validation phase and of the challenging containment calculation exercise allows a deep insight in the quality and capabilities of the CFD tools which are currently in use at various laboratories.

The HYCOM project contributed significantly to the establishment of a meaningful method to assess hydrogen risk in a nuclear plant containment. It consists of three steps:

• Calculation of the time dependent gas- and temperature distribution using CFD codes.
• Assessment of the potential combustion mode using state-of-the-art combustion criteria.
• If combustion criteria are met, the impact of the combustion needs to be calculated.

The codes under investigation in this project are suited to predict the overall course of combustion events and of the containment impact within their validation range.

This project demonstrates that the quality of validation work is of prime importance. In addition to validation runs, reasonable physical models for input parameters, depending on basic mixture properties, should be validated. This could help to further reduce conservatism and uncertainties of plant application calculations.