nuclear reactors represent multi-physic systems, in which different
fields of physics are inter-related. In best-estimate approaches,
such interdependent fields are usually restricted to the physics of
neutron transport, the physics of fluid dynamics and heat transfer.
The simultaneous modelling of these three fields is important both in
steady-state conditions as well as in transient conditions. The
addition of other fields (fuel behaviour, chemistry, material
physics, etc.) might also be necessary in some specific situations.
modelling of each physics is done in a segregated manner. One code is
used per field at a time, assuming that the other fields are
“frozen”, and iterations are performed to resolve the
interdependences until convergence on all physical fields is
achieved. Such a solution strategy is the result of the use of legacy
codes developed for modelling each physical field separately, codes
that were à posteriori coupled to model multi-physics problems. The
main advantage of such an à posteriori coupling lies with the
absence of internal code modification when the code is externally
coupled to another software. This results in the preservation of the
Verification and Validation (V&V) work for each code.
Nevertheless, achieving tight convergence on all physical fields
might be difficult and numerical oscillations are often encountered.
In addition, when considering transient calculations, the
non-linearities are often never fully resolved, resulting in
inconsistent coupling algorithms.
In this project,
a monolithic coupling approach relying of the so-called Jacobian-Free
Newton Krylov (JFNK) method is studied. This technique considers
solving the entire multi-physics problem as one single problem, in
which all the non-linearities are inherently taken into account
without any need to iterate between each physics.
Illustration of a workbench developed for testing the Jacobian-Free Newton Krylov (JFNK) method in the case of a coupled one-dimensional BWR model – the animation shows how the coupled solution evolves during the application of the JNFK algorithm
Sebastian Gonzalez-Pintor, Post-Doc, Subatomic and Plasma Physics, Department of Physics
Manuel Calleja, Post-Doc, Subatomic and Plasma Physics, Department of Physics
Anders Ålund, Fraunhofer-Chalmers Centre