Control Rod Homogenization in Heterogeneous Sodium-Cooled Fast Reactors
The sodium-cooled fast reactor is one of the candidates for a sustainable nuclear
reactor system. In particular, the French ASTRID project employs an axially
heterogeneous design, proposed in the so-called CFV (low sodium effect) core, to
enhance the inherent safety features of the reactor.
This thesis focuses on the accurate modeling of the control rods, through the
homogenization method. The control rods in a sodium-cooled fast reactor are used
for reactivity compensation during the cycle, power shaping, and to shutdown
the reactor. In previous control rod homogenization procedures, only a radial
description of the geometry was implemented, hence the axially heterogeneous
features of the CFV core could not be taken into account. This thesis investigates
the different axial variations the control rod experiences in a CFV core, to determine
the impact that these axial environments have on the control rod modeling.
The methodology used in this work is based on previous homogenization
procedures, the so-called equivalence procedure. The procedure was newly implemented
in the PARIS code system in order to be able to use 3D geometries, and
thereby be take axial effects into account.
The thesis is divided into three parts. The first part investigates the impact
of different neutron spectra on the homogeneous control-rod cross sections. The
second part investigates the cases where the traditional radial control-rod homogenization
procedure is no longer applicable in the CFV core, which was found to
be 5-10 cm away from any material interface.
In the third part, based on the results from the second part, a 3D model of the
control rod is used to calculate homogenized control-rod cross sections. In a full
core model, a study is made to investigate the impact these axial effects have on
control rod-related core parameters, such as the control rod worth, the capture
rates in the control rod, and the power in the adjacent fuel assemblies. All results
were compared to a Monte Carlo-based model which served as the reference.
It was demonstrated that the treatment of the radial environment, surrounding
the control rod, has a small impact on the core parameters, and the traditional
radial homogenization procedure yields reliable results. For axial interfaces within
the control rod itself, the traditional radial homogenization procedure could no
longer capture the large environmental impact, hence 3D modeling is recommended.
PJ Lecture Hall, Kemigården 1
Opponent: Professor Kord Smith, Department of Nuclear Science & Engineering, Massachusetts Institute of Technology, Cambridge, USA