[en] Studies are being carried out at General Atomic Company and Oak Ridge National Laboratory to develop conceptual designs of tokamak fusion reactors for the TNS (The Next Step) programme. The primary purpose of these studies is to identify technical problems and to develop solutions to them. The TNS studies, initiated in 1976 under the sponsorship of the US Energy Research and Development Administration, have been directed to the development of designs of reactors which achieve ignition and plasma burns sufficiently long to study burn dynamics. The ORNL study has resulted in the design of a reactor with a D-shaped plasma which has a major radius of 5.0 m, a minor radius of 1.2 m, and a plasma elongation of 1.6. A toroidal β of 5% is achieved by adopting a flux-conserving mode of operation. The General Atomic design is based on the use of a Doublet plasma cross-section to achieve high β. This reactor design has a major radius of 3.6 m, a minor radius of 0.95 m, and a plasma height-to-width ratio of 2.7. Both designs have a plasma volume of approximately 225 m³ and a peak power in the 700-1000 MW(th) range. The current ORNL TNS reactor is designed for a plasma burn of 16 seconds, and the GA reactor is designed for a burn of 30 seconds. While the ORNL reactor is designed for a comparatively modest duty factor of 0.05, the GA reactor is designed for a duty factor of 0.5 and includes a blanket and power conversion system to produce a nominal amount of electric power. The costs of the current TNS reactors are estimated to be in the range of US $400 - US $600 million without escalation and specific supporting research and development. (author)