The KLT-40S is a small modular reactor developed by Russia based on the KLT-40 reactor with two fuel assembly designs: a four-ring and a five-ring. Few studies have been published on fuel assembly and power-flattening designs for the KLT-40S. In this paper, the effects of different fuel assembly designs on burnup and power flattening are investigated. This paper compares the effects of the two fuel assembly designs of the KLT-40S on its burnup characteristics, analyzes the effects of fuel rod diameter on burnup characteristics, and conducts a c...