Numerical investigations of turbulent natural convection between concentric, cylinders has been carried out. Large Eddy Simulations (LES) have been performed on three different geometries. The first two consider a single inner cylinder at gap-based Rayleigh numbers of 1.18×109 and 2.38×1010 and an aspect ratios Ro/Ri=4.87 and 3.37, respectively. The last LES run considers three internal cylinders. It reveals that the flow patterns in the decay heat pen are more complex than the one internal concentric cylinder cases. The two-equations RANS models predictions for this test case are less satisfactory than in the simplified test cases, while second moment closure results are closer to the LES predictions.