BERLIN (AP) — A vast international experiment designed to demonstrate that nuclear fusion can be a viable source of energy is halfway toward completion, the organization behind the project said Wednesday.

Construction of the International Thermonuclear Experimental Reactor, or ITER, in southern France has been dogged by delays and a surge in costs to about 20 billion euros ($23.7 billion).

ITER's director-general, Bernard Bigot, said the project is on track to begin superheating hydrogen atoms in 2025, a milestone known as "first plasma."

"We have no contingency plan," he told The Associated P...