The TONUS CFD code for hydrogen risk analysis: physical models, numerical schemes and validation matrix

S. Kudriakov, F. Dabbene, E. Studer, A. Beccantini, J.P. Magnaud, H. Paillère (1), A. Bentaïb, A. Bleyer (2), J. Malet (3), C. Caroli (2),
(1) CEA Saclay, (2) IRSN/DSR, (3) IRSN/DSU,
CFD4NRS Benchmarking of CFD codes for application to nuclear reactor safety, Garching, Munich, 5-7 septembre 2006,
Rapport DSR 137

The French Atomic Energy Commission (CEA) and the Radiation protection and Nuclear Safety Institute (IRSN) are developing a hydrogen risk analysis code (safety code) which incorporates both lumped parameter (LP) and computational fluid dynamics (CFD) formulations. In this paper we present the governing equations, numerical strategy and schemes used for the CFD approach. Typical numerical studies
will be presented for hydrogen distribution and combustion applications in realistic large geometries.

The TONUS code is the French hydrogen risk analysis code developed by CEA and IRSN over the last decade, to model hydrogen release, distribution and combustion in a PWR reactor containment. The code has both multi-compartment lumped-parameter and CFD formulations, but the present paper will only focus on the CFD part of the code, describing the physical models that have been implemented,
the numerical algorithms that have been developed, and the extensive matrix of experiments used for validation purposes. Since the objective of the code is to be applied to real plant applications such as EPR studies, numerical considerations such as mesh size and CPU run-time constrained the choice of physical models and numerical algorithms. On the other hand accuracy requirements and the need to
apply to the greatest extent possible ”best practice guidelines” when performing calculations were also taken into account when developing and validating the code.

The first part of the paper will briefly describe the physical models and numerical schemes implemented in the version v2006.1 of the TONUS CFD code. For the distribution part, a low Mach number multicomponent Navier-Stokes solver was developed, incorporating two types of turbulence models - mixing length and standard k-e models, and wall condensation models based on the heat and mass transfer analogy - Chilton-Colburn correlations. Heat transfer to the structures is modeled by coupling the CFD equations to 3D heat conduction equations. Mitigation systems such as passive autocatalytic recombiner systems are also modeled and coupled to the CFD solver. As far as combustion is concerned, different
types of models have been developed to cover slow deflagration regimes, accelerated flames and detonation and implemented in a fully compressible flow solver able to simulate shock wave propagation. Numerical algorithms suited to the various flow regimes - low Mach number flows characteristic of the hydrogen distribution phase up to high speed flows associated with hydrogen combustion - have been
developed and optimized to perform efficiently on the single processor platforms (Linux or Windows PC) on which the TONUS code was developed.

In the second part of the paper, we will review in detail the test cases which form the basis of the validation of the TONUS CFD code and identify the advantages and deficiencies of the models. The strategy for the validation of the code followed a progressive approach in terms of physical phenomena, from Separate Effect Tests (SET) to Coupled Effect Tests (CET). Much of the data used for the validation
of the distribution models was produced in the 7 m3 TOSQAN and 100 m3 MISTRA containment facilities operated by IRSN and CEA respectively and which were specially developed to produce high quality detailed data suitable for CFD code validation with well-controlled boundary conditions and a dense mapping of sensors in the volumes. Test data from the ISP-47 exercise is an example of such data. Besides TOSQAN and MISTRA data, tests from AECL’s Large Scale Gas Mixing Facility (LSGMF), PHEBUS, ThAI and PANDA facilities were used for validation purposes. Validation of mitigation models (spray, recombiners) will also be described in the paper. For the combustion part of the code, use was made of large scale hydrogen combustion tests which are representative of containment geometries such as Battelle, HDR and RUT tests which cover all the combustion regimes from slow deflagrations to detonations. More detailed data produced in the IRSN-sponsored ENACCEF facility is also being
used to investigate accelerated flame regimes. It is also to be noted that many of the TONUS validation cases have been selected in the OECD Containment Code Validation Matrix (CCVM) and are therefore considered as ”mandatory” validation cases for containment codes.

The paper will conclude with a description of on-going work including further code development and validation cases, interpretation of new MISTRA and TOSQAN tests and plant applications including EPR calculations.