Abstract: 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 a first core, a plurality of steam separators disposed above the first core, and a second core disposed above the steam separators all inside a single pressure vessel. The steam separators receive a steam and water mixture from the first core and separate the water from the steam. The separated steam is channeled to the second core which cools the second core resulting in the generation of superheated steam. Preferably, fuel bundles of the second core are arranged in vertical alignment with fuel bundles of the first core. This permits a fuel bundle of the first core to be accessed by removing only the adjacent fuel bundle of the second core. During refueling operations, fuel bundles can be shifted from one core to the other, providing additional flexibility in arranging units at various states of burnup.