The System optimization and pre-design Laboratory of CEA/IRESNE at Cadarache works on innovating nuclear reactor concepts in order to decarbonize all industry and urban sectors (flexible electricity, heat, cool, synthetic fuel, hydrogen). One of those innovating concept is the ARCHEOS passive water reactor dedicated to heat supply and designed to be intrinsically safe and simple to operate. The main challenge of this research is to understand and analyse the thermal-hydraulic behaviour of this reactor that fully operates in natural circulation, which is clearly an innovation in the domain. The PhD student will first identify normal and accidental scenarios and simulate them at the reactor scale. Thoughts for design improvements could emerge as a result of this research. Those simulations will be associated to a deep physical analysis of thermal-hydraulic phenomena that can play a role during the studied scenarios. An appropriate modeling (from 1D to porous 3D to 3D CFD) is to be found to capture the thermal-hydraulic phenomena of importance. This will be performed using CATHARE3 and Neptune_CFD tools. Working on such an innovating nuclear reactor concept represents a great opportunity for a PhD student. This experience will be provide the student with a solid background on various topics such as: nuclear safety, innovating reactor design, multi-scale thermal-hydraulic simulation, reactor physics in transient regimes as well as a solid knowledge on the CATHARE3 code which is widely used in French nuclear industry and research and the reference system code for many projects in the nuclear industry.