1. Introduction This is a report on the work performed at the Federal University of Rio Grande do Sul in Brazil on the conceptual design of the Fixed Bed Nuclear Reactor (FBNR) concept that constitutes a part of the IAEA Project on Small Reactors without On-Site Refuelling and is performed under the IAEA Research Contract No. 12960/ Regular Budget Fund (RBF). The Small Reactors without On-Site Refuelling are defined by IAEA “As reactors which have a capability to operate without refuelling and reshuffling of fuel for a reasonably long period consistent with the plant economics and energy security, with no fresh and spent fuel being stored at the site outside the reactor during its service life. They also should ensure difficult unauthorized access to fuel during the whole period of its presence at the site and during transportation, and design provisions to facilitate the implementation of safeguards. In this context, the term “refuelling” is defined as the ´removal and/or replacement of either fresh or spent, single or multiple, bare or inadequately confined nuclear fuel cluster(s) or fuel element(s) contained in the core of a nuclear reactor`. This definition does not include replacement of well-contained fuel cassette(s) in a manner that prohibits clandestine diversion of nuclear fuel material.“