Post-doc Position
Topic : SIMULATION OF FLOW DEVIATION INDUCED BY BALLOONED FUEL RODS IN A NUCLEAR 
REACTOR CORE - HYDRODYNAMICS AND THERMAL STUDY 

Duration: 18 months 
Employer: IRSN 
Specificity: reserved for foreign (non-french) applicants 
Workplace: Cadarache research center, France 
Laboratory :LSMA (Laboratoire de Statistique et de Méthodes Avancées) - 
Laboratory of statistical and advanced simulation methods in mechanics and fluid 
dynamics
Profile: PhD in the field of thermal-hydraulics with an experience in CFD 
Keywords: CFD, convective heat transfer, upscaling 

Context:
IRSN (Institut de Radioprotection et de Sûreté Nucléaire) is the technical 
support organization of the French regulator ASN for nuclear safety. The 
activities of the IRSN SEMIA department covers especially the assessment of 
safety studies of industrials and the corresponding research in the field of 
design basis accidents in nuclear power plants. This concerns the Loss of 
Coolant Accident scenario that corresponds to the consequences of a hypothetical 
pipe break in the primary loop of the plant. 
During a hypothetical Loss of Coolant Accident within a nuclear power plant, the 
fuel rods within the core may balloon by creep and partially obstruct the 
coolant flow, potentially impeding their cooling. The flow deviation induced by 
the rods deformation has been experimentally studied in the mock-up MASCARA [1]  
using Reynolds analogy and MRI (magnetic resonance imaging), allowing to get a 
fine mapping of the flow velocity. 

Work: 
The main objective of the work is to study the consequences of the flow 
deviation on the cooling of the fuel rods at the assembly scale. Based on the 
validation of the Code-Saturne CFD computational tool [2] on the MASCARA 
experimental results, models will be defined to describe the impact of 
ballooning on a simplified flow model used in the DRACCAR software [3], [4] that 
describes the hydrodynamics at a so-called sub-channel scale. 
The study can be divided in different tasks: 
1/ CFD simulation of flow deviation 
CFD calculation of the flow deviation induced by different geometrical and flow-
rate configurations will be compared to experimental results in order to better 
understand the flow just downstream deformed rods, where the deterioration of 
heat transfer may be critical. 
2/ Upscaling model of flow deviation 
Cross-flows deduced from CFD in the region downstream deformed rods will be 
analyzed for prescribing models for a sub-channel scale analysis. 
3/ CFD simulation of convective heat transfer 
Thermal heat transfer between rods and flow will be studied thanks to CFD in 
order to identify the hot spots within the geometry and to relate their position 
to the previous hydrodynamics study. The fluid considered will be steam taking 
into account its thermal expansion that may induce an additional phenomenology. 
4/ Upscaling model of single-phase heat transfer 
Cross-flows and heat transfer intensity deduced from CFD in the region 
downstream deformed rods will be analyzed for prescribing models for a sub-
channel scale analysis. 
5/ Toward other geometries [optional] 
The hereinabove studies are limited to a set of configuration of an array of 4x4 
rods deformed within a 7x7 rods array. The ability of modeling the phenomena on 
a more generic fuel assembly geometry is required to study core geometry. Based 
on the previous analyzes, recommendation for the design of new experiments or 
for the performance of additional computations will be made. This work will be 
performed in interaction with both the experimental IRSN department SEREX [5] at 
Cadarache and the LEMTA lab at Lorraine University [6] where the MASCARA 
experiments have been performed. 
6/ Two-phase flow heat transfer
In more realistic conditions, droplets flow within superheated steam act as an 
additional cooling source of the rods. Previous studies will be complemented by 
modeling their flow within the geometry based on an Euler-Euler two-phase model 
with heat and mass transfer thanks to the Neptune-CFD software [7], [8]. 

Contact:
Candidates should send their CV, a motivation letter, a short abstract of their  
PhD thesis to the following person : Hervé Mutelle herve.mutelle@irsn.fr 
The review of the referees and/or a recommendation letter by the PhD 
supervisors/professors would be appreciable


References 
[1] A. V. S. Oliveira et al., “Velocity field and flow redistribution in a 
ballooned 7×7 fuel bundle measured by magnetic resonance velocimetry,” Nucl. 
Eng. Des., vol. 369, p. 110828, Dec. 2020, doi: 10.1016/j.nucengdes.2020.110828. 
[2] “Code_Saturne is the free, open-source software developed and released by 
EDF to solve computational fluid dynamics (CFD) applications.” [Online]. 
Available: https://www.code-saturne.org/cms/ 
[3] T. Glantz, T. Taurines, S. Belon, O. De Luze, G. Guillard, and F. Jacq, 
“DRACCAR: A multi-physics code for computational analysis of multi-rod 
ballooning, coolability and fuel relocation during LOCA transients. Part Two: 
Overview of modeling capabilities for LOCA,” Nucl. Eng. Des., vol. 339, pp. 202–
214, Dec. 2018, doi: 10.1016/j.nucengdes.2018.08.031. 
[4] “DRACCAR computer software.” [Online]. Available: 
https://www.irsn.fr/EN/Research/Scientific-tools/Computer-codes/Pages/DRACCAR-
software.aspx 
[5] “IRSN SEREX and corresponding experimental research lab.” [Online]. 
Available: https://www.irsn.fr/EN/Research/Research-organisation/Research-
units/nuclear-safety-unit/LE2M/Pages/mechanical-material-experiment-
laboratory.aspx 
[6] “LEMTA, Laboratoire Énergies & Mécanique Théorique et Appliquée.” [Online]. 
Available: http://lemta.univ-lorraine.fr/ 
[7] A. Guelfi et al., “NEPTUNE: A new software platform for advanced nuclear 
thermal hydraulics,” Nucl Sci Eng, vol. 156, pp. 281–324, 2007. 
[8] P. Ruyer et al., “Two-phase flow across a partially damaged core during the 
reflood phase of a LOCA,” Nucl. Eng. Des., vol. 264, pp. 187–194, 2013, doi: 
10.1016/j.nucengdes.2013.02.026.