Solutions of the neutronic design problems of small heating reactor cores based on the BWR principle

Small size, low power density and fuel temperature as well as the tendency to simplicity lead at first to a uniform design and a very simple fuel management strategy for the core of a small heating reactor following boiling water technology. Problems encountered in such a concept include high power and burnup peaking, resulting in poor fuel utilization, a strongly negative void reactivity coefficient, and high control rod reactivity worths exceeding the limit imposed by safety considerations. In order to solve them the following modifications of an original core based on standard BWR designs are investigated: poison zoning, separation of the moderating from the cooling functions of the water, different designs of the control rods according to their radial positions and application of a multiple batch loading scheme. The application of these measures leads to modified core designs which show a significant improvement of core performance as compared to the original uniform core, in particular flatter power and burnup distributions as well as reduced control rod worths and void reactivity coefficient.