Nuclear reactions, fission and fusion

In a nuclear reaction, nuclei change into other nuclei. Like a chemical equation, it must balance: the total charge (Z) and the total number of nucleons (A) are the same on both sides.

The energy released is the Q value, from the mass that disappears:

Q = (mass before – mass after) × c²

• Q > 0: mass is lost, energy comes out: exothermic.
• Q < 0: energy must be supplied: endothermic.

Chemical reactions also change mass, but by about a billionth: far too little to measure. Nuclear reactions release a million times more energy per atom.

Fission: a heavy nucleus such as uranium-235 absorbs a slow neutron and splits into two middle-sized nuclei, plus 2 or 3 new neutrons and about 200 MeV. For example:

²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n + about 200 MeV

The new neutrons can split more uranium nuclei: a chain reaction. In a nuclear reactor it is kept steady: a moderator (water or graphite) slows the neutrons, and control rods (cadmium or boron) soak up extra neutrons. India runs many such reactors for electricity.

Fusion: light nuclei join to make a heavier one. Deuterium + tritium → helium-4 + a neutron + 17.6 MeV. Per kilogram of fuel, fusion gives even more energy than fission.

But nuclei are positively charged and repel each other. To get close enough for the nuclear force to act, they must overcome the Coulomb barrier, which needs temperatures of millions of degrees. That is why it is called thermonuclear fusion.

The Sun and other stars shine by fusion, turning hydrogen into helium in their hot cores. Controlled fusion on Earth is still being developed.