The impact of the fuel bundle deformation on the thermal-hydraulics of ASTRID-like sub-assembly

Thierry Cadiou (1) , Francisco Acosta (1)

1 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)

The Sodium-cooled Fast Reactor (SFR) fuel sub-assembly irradiation leads to a substantial modification of its geometry and mechanical properties. Indeed, it produces void swelling and creep on the clad that increase the fuel pin diameter and its bending. As a result, the sub-assembly flowrate could be reduced, and hence the sodium temperature would increase. In order to guarantee the fuel pin clad mechanical strength in operation, which largely depends on its temperature, it is proposed to evaluate the maximum temperature reached by the clad for a deformed sub-assembly. The objective of this study is to provide the sensitivity of thermal-hydraulics to the fuel bundle deformation in order to propose criteria on the maximum admissible strain. To address this issue, the impact of a SFR fuel bundle diametral strain on the thermal-hydraulics is studied by means of Computational Fluid Dynamics (CFD) simulations with the industrial code STAR-CCM+. A nominal geometry of sub-assembly and a highly deformed one are considered. The modification on flowrate for this latter geometry is firstly determined by a method that estimates the relationship between the pressure loss and the flowrate in the sub-assembly. CFD simulations with the updated flowrate used as boundary conditions for the deformed geometry are then run and compared to the results obtained for the nominal geometry. By this way, the consequences of the fuel pin deformation on the clad temperature and velocity field are evaluated.This work, presented with conservative hypothesis and based on a geometry that is deformed beyond the recommended limits, concludes on the high sensibility of the deformation on the clad temperature. On the other hand, clad swelling and creep also depend on temperature, therefore the temperature increase in the sub-assembly resulting from the lower flowrate in sub-assembly could also significantly modify its geometry. Given the perturbation found on the clad temperature, it is suggested to adopt a coupled approach between thermal-hydraulics and thermomechanics to better assess the sub-assembly evolution during irradiation.

Mots clés: Liquid sodium; Sodium-cooled Fast Reactor; thermal-hydraulics in sub-assembly; CFD

Domaines: Physique Nucléaire Expérimentale [nucl-ex]; Physique Nucléaire Théorique [nucl-th]

Thierry Cadiou, Francisco Acosta. The impact of the fuel bundle deformation on the thermal-hydraulics of ASTRID-like sub-assembly. NUTHOS-12 (12th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety), Oct 2018, Qingdao, China. ⟨cea-02339116⟩