Exhaustive 3D modelling of the ILL High-Flux Reactor (HFR) has recently been carried out with MCNPX 2.6d code and ENDF/ B-VII library. All in-pile elements have been taken into account, including beam tubes (H,IH,V), safety rods, cold and hot neutron sources and so on. . . Material densities, compositions and fuel specificities comply with the Safety Report or have been updated if needed. The HFR fuel element has been detailed with a high degree of accuracy. All 280 fuel plates have been modelled with the closest geometry available (involutes have been modelled by cylinder sections). Control and safety rod positions can be adjusted to fit experimental conditions.The cross-section library used in these calculations is ENDF/ B-V II at 300K. We
employed the latest available release of thermal structure factors for low energy neutrons.

HFR has 5 safety rods (BS), which have all been weighted by subcritical approaches.The weight has been determined by substracting the keff of the "all BS up" configuration from the considered keff. Most calculations match the experimental values with a shift less than roughly 500 pcm. We can thus conclude that we have a very good agreement between simulations and experiments for these configuration series. When all BS are up, the fit is excellent with only 31 pcm of difference between simulation and experiment. For a better view of the neutron distribution in-pile, we scored mesh tallies for several configurations in the median plane.

We performed a benchmark of ILL in collaboration with ILL's Science Division for MCNP and TRIPOLI codes. In the framework of this study, we matched our simulation results in the heavy water tank with measurements and calculations carried out in the past by P.Ageron. We obtained good agreement, except perhaps for epithermal neutrons.