In axially heterogeneous fast reactor concepts, such as the Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID) CFV (French acronym of Coeur à Faible effet de Vide sodium, meaning low sodium void effect core) core, the accurate neutronic prediction of control rods is a challenge. In such cores, the performance of the classical two-dimensional (2-D) equivalence procedure, used for control rod homogenization in homogeneous fast reactors, is questionable.
In this work (part I of two companion papers), a number of axially heterogeneous environments, representative of a CFV-type core, are investigated using 2-D (X-Z) models, with the objective to distinguish regions where the classical equivalence 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 to 10 cm from the material interfaces studied.
In the companion paper (part II), a full-core investigation is performed that builds upon the results of this paper.