Fusion joins light nuclei into heavier products. Positive nuclei repel electrically, so sufficiently energetic close encounters and quantum processes are needed for fusion; high-temperature plasmas provide suitable conditions in stars and fusion systems. Not every fusion reaction releases energy. For the light-nucleus reactions used here, a positive difference between total reactant and product rest mass corresponds to released energy.
Balance total nucleon number and electric charge, including emitted neutrons or other particles. Then calculate E=Δmc2 using supplied complete masses. Fission splits a heavy nucleus; fusion combines light nuclei. Sustained energy production additionally requires reaction conditions, confinement and energy balance. A positive energy per reaction alone does not establish that an entire power system delivers net useful electricity.
Example 1
Deuterium (A=2,Z=1) and tritium (A=3,Z=1) fuse into helium−4(A=4,Z=2) and one particle. Identify the particle.
Initial totals are A=5 and Z=2. After helium−4, the remaining particle has A=1 and Z=0: a neutron.
Example 2
A fusion reaction has reactant rest mass 8.36×10−27kg and product rest mass 8.33×10−27kg. Use c=3.00×108m/s.
Δm=0.03×10−27=3.0×10−29kg. E=Δmc2=2.7×10−12J per reaction.
Watch out: Reaction energy must be distinguished from energy needed for heating, confinement, cooling and conversion in the whole system.