The Parties, signatory of the ITER Agreement [1] -Euratom and the governments of Japan, the Russian Federation and the United States of America- are implementing fusion programs directed ultimately towards the development of commercial magnetic fusion energy. Depending on each Party's strategy, ITER may be considered, in some cases, the last experimental step before building a commercial fusion reactor producing electricity economically.

From the results reported in the ITER Outline Design [2], it is possible to define a route towards the construction of a fusion power reactor that would produce large amount of power (~1 to 2 GWe in a single unit) at a capital cost of around $5 per watt for the fusion plant.

If some technologies developed for ITER are extrapolable to the reactor, such as the concept of a self-supporting breeding blanket; a low pressure coolant; no manifolding inside the machine; bending free toroidal field coils; and a fully welded vacuum vessel, some issues still remain to be addressed before a fusion reactor can be considered for construction. These issues involve mainly technological issues, coupled with the uncertainties of plasma behavior, and require adapting the present R&D programs, and a coherent fusion development program plan.

The main technological constraints of a fusion reactor results from economics which favors large a large neutron flux at the reactor first wall. This constraint has an impact on the viability, reliability, and life time of the blanket and divertor components which are subject to important mechanical and thermal stresses, and to a large neutron fluence.

Furthermore, the Tokamak topology is complex, and makes the remote assembly and maintenance of the device more difficult than in other available commercial energy sources.

In the following, the parameters of the reactor will be defined by extrapolating from the ITER Outline Design, and the issues of the reactor physics and of the blanket, divertor and magnet systems will be reviewed, with a view towards balancing the constraints resulting from economics, safety and maintenance.