During an hypothetical severe nuclear in a nuclear reactor, core damage occurs and may lead to the formation of the corium (a mixture of oxide and metals at 3000 K), followed by relocation, by gravity, to the vessel lower head for pressurized water reactors, or on a core-catcher for sodium fast reactors.
The strategy adopted is then to cool the corium in order to avoid the vessel, or core-catcher, breakdown and to stop its progression. The evaluation of the heat fluxes at the pool boundaries is an important issue . These corium pools (volume heating by the fission products and cooling at the pool boundaries) are characterized by very turbulent natural convection flows. These turbulent flows are characterized by chaotic changes of the velocity, temperature and pressure fields. This complexity makes turbulence one of the unsolved problems in physics.
The increase in capacity of the computing now offers the opportunity to improve our knowledge on these flows with the use of numerical simulations. However, the current computing powers do not yet allow directly resolving all the fluctuating scales of the flow fields (velocities, temperature and pressure). A turbulence modelling is then required.
In this context, the PhD work will consist in developing a turbulence model specific to these anisotropic pools and under the influence of buoyancy effects [2, 3]. The developments will occur in the TrioCFD code  developed in the Service of Thermalhydraulics and Fluid Mechanics at CEA/Saclay. The validation of developments will be done on analytical cases and experimental results. These developments will eventually extend to the reactor case with a stratified pool (oxide and metallic layers).
The single-phase turbulent flows are found in many situations (nuclear, aeronautics, space, environmental, biophysics…).
During this PhD, the PhD student will be able to develop his competences in the field of thermohydraulics and turbulence, in both physical (new turbulence model) and numerical (developments and simulations) aspects. The simulations will be performed using TRUST/TrioCFD software (C++) in Linux environment and large scale calculations will be launched at the TGCC supercomputing center (CEA-GENCI-PRACE).
1 H. Tuomisto & T.G. Theofanous, “A consistent approach to severe accident management”, Nuclear Engineering And Design, vol. 148 (1994)
2 A. Shams, F. Roelef, E. Baglietto, S. Lardeau, S. Kenjeres, “Assessment and calibration of an algebraic turbulent heat flux model for low-Prandtls”, Int. Journal f Heat And Mass Transfer, vol. 79 (2015), pp 589-601
3 A. Shams et al, “Status of computational fluid dynamix for in-vessel retention: Challenges and achievements”, Annal of Nuclear Energy, vol 135 (2020) pp107004
4 Bienvenue sur le site TrioCFD - Le code TrioCFD (cea.fr)