 This allowed them to measure the nuclear cross section of various fusion reactions, and determined that the deuterium-deuterium reaction occurred at a lower energy than other reactions, peaking at about 100,000 electronvolts (100 keV).Accelerator-based fusion is not practical because the reaction cross section is tiny; most of the particles in the accelerator will scatter off the fuel, not fuse with it.These scatterings cause the particles to lose energy to the point where they can no longer undergo fusion.