NUCLEAR REACTOR WITH BI-LEVEL CORE
A reactor core (104) includes upper and lower matrices of fuel bundles. Fuel rods in the upper matrix (126) have their plenums oriented upward, while fuel rods in the lower matrix (124) have their plenums oriented downward. Refueling involves removal of a first bundle in the upper matrix (126), removal and retirement of the bundle in the lower matrix (124) directly below the original position of the first bundle, inversion and installation of the first bundle in the lower matrix, and installation of a new bundle in the upper matrix. The new bundle is installed plenum-side up. The bi-level core (104) provides greater flexibility in repositioning fuel bundles for longer burnups and lower high-level waste. In particular, problems with axial spectral variations in neutron flux can be compensated using the disclosed core arrangement and refueling procedure.