Patent Number: 051620979
Section: abstract

In accordance with the present invention, a nuclear reactor with a recirculating heat transfer fluid has a bi-level core which provides enhanced flexibility in fuel arrangement. The bi-level core includes two sets of fuel units, one set arranged on a first level, the other set arranged on a second level. Preferably, fuel units of the second level are arranged in vertical alignment with fuel units of the first level. This permits a fuel unit of the first level to be accessed by removing only the adjacent fuel unit of the second level. During refueling operations, fuel units can be shifted from one level to the other, providing additional flexibility in arranging units at various stages of burnup. Preferably, fuel units of the first level are inverted relative to the fuel units of the second level. The inversion provides for placing plenum sections of fuel rods in different levels away from each other so that the plenums do not introduce a discontinuity in neutron generation, and allows for more uniform axial fuel burnup. The bi-level core allows fuel to be initially positioned in the second level for conversion of fertile fuel to fissile fuel, and then repositioned to the first level for more complete axial burnup. In a preferred embodiment the first level boils water to generate saturated steam, and the second level is cooled by the saturated steam and generates superheated steam.