Technical experts from the seven partners involved in the so-called the International Thermonuclear Experimental Reactor (ITER) project - the European Union, Japan, United States, China, Russia, South Korea and India - tied up the agreement over the weekend in Barcelona.
Following years of wrangling, the partners finally agreed last June on a site for the ITER in France, after Japan dropped its demand to host the project.
The accord finalised more technical elements of the agreement.
A spokeswoman for the ITER said on Monday: "This weekend saw an important step that brings us even closer to the realisation of the ITER International Fusion Energy Research project, which will be hosted by the EU in Cadarache in southern France."
She said that a draft text, the ITER Agreement, would be finalised by chief negotiators before being submitted to ministers for ratification by the partners.
Negotiations over the project began in June 2002 and took three years to complete, with the six original partners split down the middle between Japan's and the EU's bid to host it.
Of the total staff posts, 20% have been given to Japan, including the director general's job, in exchange for dropping its proposal to build the reactor in northern Aomori prefecture.
The $12 billion (€10 billion) programme, seen as one of the most exciting ventures in world science, is expected to run over 30 years.