On Dec. 9, leaders of Devens-based Commonwealth Fusion Systems joined 13 other fusion-focused firms in a meeting with the U.S. Department of Defense to discuss public-private cooperation to accelerate the use and commercialization of fusion energy, which still must be proved as a viable way to generate net energy. Fusion energy is different than the more common fission nuclear energy used in today's nuclear power plants. Nuclear fusion releases energy when two atomic nuclei combine, forming one nucleus, and the process doesn't produce any greenhouse gases or the hazardous waste that fission does. Proponents of fusion energy say its use is a safer, greener, and more sustainable energy source.