Analyse des barres de compensation au cobalt du reacteur CANDU-6 en geometrie exacte en utilisant le code DRAGON

Adjuster rods are part of the CANDU-6 reactor's reactivity mechanisms. In the Gentilly-2 reactor, as in a few other CANDU reactors, these rods are made of Cobalt pencils, a material that absorbs thermal neutrons to help flatten the power distribution in order to achieve optimal power delivery while maintaining safe margins to maximum channel power. They also act as a reserve of positive reactivity in case a reactor trip occurs. The Cobalt adjuster rods exhibit a cluster geometry that was difficult to model accurately. As a result of the limitations in the lattice codes available in the past, the geometry of the various types of rods was cylinderized in order to compute the incremental cross sections. Moreover, the fuel bundles surrounding the reactivity device in the super cell couldn't be modeled in their exact geometry. Therefore, a prehomogenization of the cell in 2D was required for the supercell calculations. These approximations were used in the reference methodology used in the Canadian nuclear industry for the past years.;The results of the study showed high relative variations in the incremental cross sections for the two models. However, the global effect in the reactivity of the core was very small. We found that the exact model lowered the reactivity by 1,8 mk compared to the annular model. The transport calculations performed in DRAGON were pretty much similar in both WINFRITH (69 groups) and ENDF-B/V libraries (89 groups).;The aim of this study is to introduce a new methodology that takes into account the progress achieved in the DRAGON lattice code. The new simulations will allow the comparisons of the two-groups incremental cross sections (DeltaSigma) generated with the exact models and the annular models. Moreover, we will be able to evaluate the effect of these two models in the reactor core using the DONJON reactor code. The new data are expected to better represent the neutron flux and power distribution in the reactor.