Illustration of the multi-physics nature of nuclear
reactor cores, where neutron transport, fluid dynamics, and heat
transfer need to be simultaneously modelled.
The design, licensing, and operation of nuclear
reactors heavily rely on state-of-the-art modelling tools. These tools
are used to demonstrate that such systems can be run safely and
economically during normal and off-normal situations. This applies to
both the existing fleet of nuclear reactors, as well as future nuclear
reactors.
By essence, nuclear reactors are complex systems,
involving different strongly-coupled physical phenomena at different
scales. More precisely, the temporal and spatial distribution of the
neutrons is strongly influenced by the temporal and spatial distribution
of the density and temperature of the different media constituting such
systems. Likewise, the distributions of the density and temperature
depend on the energy released by fission, which in turn depend on the
distribution of the neutrons throughout the nuclear core. In order to
guarantee a reliable prediction of the behaviour of nuclear systems, the
different physical fields have to be solved simultaneously at all
scales, and calculations of the transport of neutrons, of the dynamics
of the fluid cooling the system, and of the transfer of heat throughout
the system have to be performed in a concurrent manner.
With the
rapid development of the computer capabilities in the 70ies, different
sets of computers codes and modelling strategies have emerged. For
historical reasons, they nevertheless all rely on solving the different
physical fields and different scales separately, i.e. different codes
are used to separately resolve the different physical fields and
different scales. Such codes are only thereafter coupled in an a
posteriori fashion in order to handle the inter-dependence of the
different physical fields and their different scales. The current
modelling tools all rely on such an approach.
Although this
modelling strategy works satisfactorily for steady-state conditions, it
is challenged by the new designs of the fuel assemblies being introduced
in the existing fleet of nuclear reactors, and by the new reactor
concepts being investigated for near-term and long-term deployment.
Moreover, in case of non-steady-state conditions, such a strategy leads
to simulations of rather poor reliability, since the non-linear coupling
terms existing between the different physical fields are actually not
resolved.
The present project tackles the modelling of nuclear
reactor systems from several innovative viewpoints. The multi-physics
and multi-scale aspects of these systems are taken into account right
from the start in the modelling process, by accounting for the
interdependency of the different physical fields and their different
scales. The latest advances in numerical methods are used for resolving
the strong non-linear coupling existing between the different physical
fields. In order to use a set of consistent methods for performing both
steady-state and non-steady-state simulations, fast running coarse-mesh
methods are used to resolve the different physical fields. The
multi-scale phenomena are handled in two different manners: for neutron
transport, a fine-mesh solver is embedded into the coarse-mesh solver,
and both solvers work in tandem in order to model the impact of large
scale phenomena on small scale ones and vice-versa. For fluid dynamics,
subgrid models are used to provide the necessary small scale information
the coarse-mesh solver cannot by itself recover.
In contrast
with how the existing codes being used were developed, the present
project gathers a team of experts in nuclear reactor simulations,
neutron transport, fluid dynamics, heat transfer, and numerical methods,
working in an integrated and collaborative manner. With the innovative
approach highlighted above to resolve the different physical fields and
their different scales, they try to bring the simulation of nuclear
reactor systems to an unprecedented level of reliability and
sophistication.
Professor Christophe Demazière, Subatomic and plasma physics