Control Rod Calculation in Axially-Heterogeneous Fast Reactors. Part I: Influence of the Absorber Environment

Journal article, 2017

In axially heterogeneous fast reactor concepts, such as the ASTRID CFV core, the accurate neutronic prediction of control rods is a challenge. In such cores, the performance of the classical 2D equivalence procedure, used for control rod homogenization in homogeneous fast reactors, is questionable. In this work (Part I of II), a number of axially heterogeneous environ- ments, representative of a CFV-type core are investigated using 2D (X-Z ) models, with the objective to distinguish regions where the classical equiva- lence procedure is valid from those where it is not. It is found that the environments that affect the control rod absorber the most, and are likely to invalidate the procedure, are the internal control rod interfaces, such as the absorber/follower interface and the interface between zones of different boron enrichments. The range of the main spectral impact could be seen within 0-10 cm from the material interfaces studied. In a companion Paper (Part II), a full core investigation is performed, which builds upon the results of this paper.