Reflector features and physics consideration issued from the Jules Horowitz Reactor design analyses

Mechanic solicitations induced by neutron and photon interactions have to be featured for components lifespan determination. TechnicAtome is in charge of both the design and building on behalf of CEA of the 100 MW Jules Horowitz Reactor (JHR). This modular Material Testing Reactor is under construction in southern France, with radioisotope production and material testing capabilities. Inner core components have been designed based on mechanical and thermohydraulic considerations. Both studies require neutronic physical quantities like the neutron flux and deposited energies. The JHR reflector is outside the primary loop and is composed of beryllium. Gamma shields are partially positioned between the reflector and the core to reduce photon heating on aluminum structures. The design is completed and this paper deals with the neutronic and photonic impacts on the reflector. A Monte Carlo methodology based on the MCNP code was developed to model the reactor and enhance fluxes and energy deposited maps. MCNPs mesh options are used over the detailed geometry model. The convolution with mechanical meshes enables to determine neutronic parameters on local structures, material by material. Time required for such modeling is very long if one requires results on every mesh with a maximum uncertainty of 2% (1σ). To reduce time calculation by a factor 3.5 on refined meshes, MCNP biasing methods have been used. Spatial distribution of the gamma heating shows the importance of the interface with the surrounding area. For example, photon and neutron interactions close to the gamma shield create numerous photons with lower energy adding heating at the shield interfaces. In order to keep high flux in the experimental part of the reflector, gamma shields are not continuously set around the reactor vessel. Consequently, some photon leakage arises in the reflector area, with limited impact on aluminum structures. The overall thermal flux map shows local effects and gradients that have to be taken into account by the physics studies. Material swellings are deduced from the fluxes on all reflector structures