The “Institut de Radioprotection et de Suˆrete´ Nucle´aire”, as the technical support of the French Safety Authority, carries out studies and research to analyze and assess the safety of all nuclear plants. In this frame IRSN studies the feasibility of modeling Material Testing Reactor core with SIMMER-III code, for simulation of reactivity initiated accidental transients. The SIMMER-III multi-physics code system was initially developed for mechanistic safety analyses of liquid metal cooled fast reactors while employing coupled spatial neutron kinetics and thermal hydraulics models. Neutronics and thermal-hydraulics SIMMER-III models have been extended to safety analyses for water cooled and moderated reactors. The use of a code like SIMMER-III requires approximations; it computes a simplified R-Z geometry and chemistry description of the core that must be validated. The methods applied consist here in developing models of the same reactor on several scales of detail. The first step is the validation of the cross section condensation for deterministic APOLLO2 calculation against Monte Carlo TRIPOLI4 2D model. Temperature effects, kinetic parameters and void coefficients on the whole core are then calculated on a 2D APOLLO2 model, using the Method of Characteristics. These parameters are also computed with a 3D combined transport and diffusion calculations by means of APOLLO2/CRONOS2 calculations, validated against a TRIPOLI4 3D precise reference model. The final step is the validation of the simmer-like R-Z geometry in APOLLO2 Sn and Pij. Finally, an R-Z geometry has been computed in SIMMER-III, for the calculation of the kinetic parameters and temperature coefficients. This validation method has been applied to Jules Horowitz Reactor, a French Material Testing Reactor currently in commissioning by the CEA. This leads to conclude that discrepancies due to simplifications are acceptable. Moreover SIMMER-III shows quite a good agreement with CEA ring calculation on the kinetic parameters. Concerning neutronics feedbacks coefficients, further analyses remain necessary.

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