NEUTRONIC STUDY OF AN INNOVATIVE BWR THORIUM-URANIUM FUEL

This paper presents the neutronic design of an innovative fuel design proposed to be used in a Boiling Water Reactor (BWR). The study was carried out assuming different infinite lattice configurations calculated with HELIOS. The idea behind the lattice design is to use as much as possible the thorium conversion capability in a BWR spectrum, taking advantage of the U233 build-up in a once-through cycle. Other of the main goals is to reduce the production of long-live actinides and of course to improve the fuel cycle economy searching to reduce de U-235 enrichment. The blanket-seed concept was used, and a triangular pitch lattice was designed including the blanket sub-lattice (ThO2 rods) and the seed sub-lattice (U/Zr rods) in a heterogeneous arrangement. The blanket sub-lattice is first loaded in the core (one cycle) to produce its own fissile fuel. At this step a dummy zircaloy rod instead of the seed rod occupies the center of the triangular lattice. At the next cycle the blanket sub-lattice will be assembled with the fresh seed sub-lattice to form the blanket-seed lattice. A fuel lattice was designed with a U-235 enrichment of 5.5% in the seed rods, which produces a reactivity performance similar to typical UO2 and MOX fuel used in BWR’s. Regarding the reactivity void coefficient, the proposed blanket-seed lattice shows a negative value: -240 pcm at BOL and –54 pcm at 60,000 MWd/T.