Control rod calculation in axially-heterogeneous fast reactors. Part II: Impact of 3D homogenization on core parameters

Advanced sodium-cooled fast reactors with improved safety features, such as the French ASTRID CFV-core concept, is characterized by an axial heteroge- neous core, which will present a challenge on the homogenization procedures used today, taking into account all the different axial material transitions. Reliable modeling of the control rod and accurate prediction of the control rod worth are essential to determine the shutdown margins and to ensure safe operation. In this work (Part II of II), two different homogenization schemes are com- pared. One is based on the traditional reactivity-equivalence procedure in 2D, and the other a newly implemented 3D version of the reactivity-equivalence procedure, with approximations based on the results in a companion pa- pers (Part I). The deterministic results are compared with a Monte Carlo reference. Both of the cross section sets, from the two homogenization schemes, yielded results within the requested 5% error margin in reactivity. The largest discrepancy was found for the classical procedure for the case with a slightly inserted control rod (normal operating conditions). Both sets of cross sections yielded similar power profiles in the fuel sub- assembly neighboring the control rod within the 2 Monte Carlo standard deviation. Neither of the cross section sets were able to predict the large gradients in capture rates close to the internal control rod interfaces. The study showed that the traditional 2D reactivity-equivalence proce- dure produces homogenized cross sections which yield reliable results in a CFV-type core. One exception from this was found for slightly inserted con- trol rods, where the effect of the follower/absorber interface could not be fully captured by the 2D scheme, and for such cases, 3D modeling is recommended.