CFD analysis of flow deviation induced by ballooned fuel rods : hydrodynamics and thermal study

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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 rod deformations has been experimentally studied in the mock-up MASCARA (see Figure 1) using Reynolds analogy and MRI (magnetic resonance imaging), allowing to get a fine mapping of the flow velocity.

Figure 1 : Diagram of the MASCARA test-facility and the typical geometry for a ballooned 7x7 array of rods

The main objective of this research 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 [1] 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 [2,3] that describes the hydrodynamics at a so-called sub-channel scale (a sub-channel corresponds to the fluid volume located between 4 neighbors fuel rods). The study is divided in the main following tasks :

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. Typical experimental results of the flow field through deformed geometry are shown in the Figure 2 below.

Figure 2 : Fluid velocity in the MASCARA set-up

Cross-flows deduced from CFD in the region downstream deformed rods will be analyzed for prescribing models for a sub-channel scale analysis.

Thermal heat transfer between rods and fluid will be studied 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.

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.

The hereinabove studies are limited to a set of configurations 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 [4] at Cadarache and the LEMTA lab at Lorraine University [5] where the MASCARA experiments have been performed.

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 a Euler-Euler two-phase model with heat and mass transfer thanks to the Neptune-CFD software [6,7].

Applicant should possess a Ph.D degree in CFD and have a good knowledge of turbulence modelling, heat transfer processes and multiphase flow simulation. Knowledge of the C and python programming languages will also be an asset. The work will take place at the Cadarache research center (France).

For consideration, applicants should submit a resume, a publication list along with the name of referees and a cover letter outlining research interest to M. Herve MUTELLE at the following address : herve.mutelle@irsn.fr.

1. "Code_Saturne is the free, open-source software developed and released by EDF to solve computational fluid dynamics (CFD) applications." https://www.code­saturne.org/cms/
2. Glantz, T., Taurines, T., Belon, S., De Luze, O., Guillard, G., Jacq, F. "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. 2018 Dec;339:202–14. https://doi.org/10.1016/j.nucengdes.2018.08.031
3. "DRACCAR computer software." https://www.irsn.fr/EN/Research/Scientific­tools/Computer-codes/Pages/DRACCAR-software.aspx
4. "IRSN SEREX and corresponding experimental research lab." https://www.irsn.fr/EN/Research/Research-organisation/Research-units/nuclear­safety-unit/LE2M/Pages/mechanical-material-experiment-laboratory.aspx
5. "LEMTA, Laboratoire Énergies & Mécanique Théorique et Appliquée." http://lemta.univ-lorraine.fr/
6. Guelfi, A., Bestion, D., Boucker, M., Boudier, P., Fillion, P., Grandotto, M., Hérard, J.- M., Hervieu, É., Péturaud, P. "NEPTUNE: A new software platform for advanced nuclear thermal hydraulics." Nucl Sci Eng. 2007;156:281–324.
7. Ruyer, P., Seiler, N., Biton, B., Lelong, F., Secondi, F., Baalbaki, D., Gradeck, M. "Two­phase flow across a partially damaged core during the reflood phase of a LOCA." Nucl Eng Des. 2013;264:187–94. https://doi.org/10.1016/j.nucengdes.2013.02.026