Hydrogen risk assessment methodology for small containments using a commercial CFD code.

• A multiphase CFD model for containment analyses was implemented in FLUENT. • A thermal-hydraulics theoretical zero-dimensional model is proposed for pseudo-verification. • FLUENT user-defined functions used to model condensation released. • Strategies to avoid numerical instability from condensation source terms. • Both hydrogen risk and overpressure risk sensitivity to wall condensation model assessed. Marine nuclear reactors, especially when powering submarines, take advantage of most of the benefits which nuclear technology can provide. Due to nuclear fuel high power density, small storage volume, and the long or even dismissed interval of refueling, marine reactors provide good applicability of nuclear power. However, the cladding of the reactor fuel elements employs materials that can generate large amounts of hydrogen posing combustion risks and threats to the containment integrity. Space is limited in a marine reactor containment, and a small amount of hydrogen released can become a potential combustion source. In this study, a computational simulation using the commercial multi-purpose code ANSYS Fluent was performed to provide local distributions of temperature, pressure, hydrogen, and steam concentrations in a reactor containment during an accident. These parameters were used to assess hydrogen combustion risk during an accidental scenario. The utilization of a multipurpose code presented several inconveniences as the necessity of an accurate discretization of the fluid–structure interface, the absences of in-built phase change models for condensation and of a library of hydrogen recombiners to model the functioning of the hydrogen mitigation system. All these capabilities are typically provided by specialized codes used in nuclear industry, and were overcome by the proposed simulation methodology through the implementation of external subroutines to model condensation, heat transfer and the hydrogen recombiners. For wall condensation, the subroutines were calculated based on empirical correlations and heat and mass transfer analogy for steam condensation in the presence of non-condensable gases. Special emphasis was given to steam phase change, as, during the progression of an accident, condensation plays an important role in limiting pressure increase and removing heat in the containment structures. Moreover, condensation also affects the hydrogen combustion risk as it reduces the fraction of steam in the containment atmosphere and allows the expansion of the flammable cloud. Hydrogen combustion risk and the possibility of slow deflagration, flame acceleration, and deflagration to detonation transition were assessed from the utilization of flammability limits, Sigma and Lambda criteria. A theoretical zero-dimensional model has been proposed for the comparison of the simulation results, due to the absence of experimental data. [ABSTRACT FROM AUTHOR]