3D thermo-hydraulic multiphase flow code mainly used for fuel-coolant interaction (or steam explosion)
MC3D is a general purpose thermo-hydraulic multiphase flow code. The IRSN currently uses it to model steam explosion following fuel-coolant interaction in nuclear reactors (principally water reactors), something which is theoretically possible in the event of a severe reactor accident with fuel meltdown. With its modular structure, it has also been used for a wide range of specific applications such as sodium fires in fast neutrons reactors and modelling the porous media transfer kinetics of long term nuclear waste storage.
The current standard version of MC3D offers two applications. PREMIX is the first of these and is dedicated to studying the fuel-coolant mixing phase. As a general purpose application, it is also used to study other aspects e.g. to assess the phenomenon of direct cladding heating. The second application, known as EXPLO, models the explosion propagation phase.
Current MC3D code development has two aims:
to provide a suitably reliable tool for evaluating pressure loads due to steam explosion,
to develop a sufficiently general-purpose, global tool capable of processing complex multiphase flows.
MC3D is currently available at the following institutes: IKE, FZK (Germany), KAERI, KINS (South Korea), JSI (Slovenia) and AECL (Canada).
Fuel-coolant interaction can occur during an hypothetical severe accident when the molten fuel (corium) interacts with a volatile liquid used as a coolant or moderator. The phenomenon’s initial phase, known as premixing, entails the two liquids roughly mixing together, along with more or less extensive vaporisation and possibly, oxidation. Under certain conditions, the premix may become unstable and provoke a violent, detonation-like explosion. MC3D is one of the most advanced and reliable tools for modelling this phenomenon.
Software description and methodology
The MC3D computer software is based on a Eulerian approach using a mixed finite-volume (mass and energy balance) and finite-difference (momentum balance) method together with a 3D structured mesh (Cartesian or cylindrical). A modular structure defines applications with a random number of components grouped together in momentum and energy fields.
A specific feature of this software is that the PREMIX function includes a field for modelling a non-dispersed liquid using a VOF-PLIC method, in order to model molten fuel flow and mass amongst other elements.
MC3D is used to compute the steam explosion consequences for level-2 probabilistic safety assessments (PSA) developed by the IRSN. It is also used to deal with explosion impact following a neutron transient state in experimental nuclear reactors.An additional use of this software is to assess dynamic and thermal phenomena in the event of a nuclear reactor’s molten fuel discharge under pressure (direct cladding heating).
3D calculation of ex-vessel premixing. Central break, diameter = 60 cm. © IRSN