Officials from six nations and the European Union signed a long-awaited, ¤10 billion (US$12.8 billion) pact Tuesday to build an experimental nuclear fusion reactor aimed at developing a cheaper, cleaner and safer energy source. French President Jacques Chirac hosted officials from the EU, the United States, India, China, Japan, South Korea and Russia in launching the project. «This mobilization is indispensable,» Chirac said at the signing in Paris, the Associated Press reported.