How does nuclear fission release energy?

Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei while releasing energy. The energy comes from the forces that hold the nucleus together. The fragments are more tightly bound per particle than the original heavy nucleus, and the difference is released, mostly as heat and radiation. [1] [2]

A physics textbook explains this through binding energy per nucleon. Energy is released if the products of a nuclear reaction have greater binding energy per nucleon than the parent nuclei. That value is about 7.6 MeV per nucleon for the heaviest nuclei and about 8.6 MeV for nuclei of mass number around 120. When a heavy nucleus splits roughly in half, the products have less mass per nucleon, so mass is destroyed and energy is released. The textbook estimates about 1 MeV per nucleon, or roughly 240 MeV per fission. [2]

The World Nuclear Association gives an average of about 200 MeV per fission of uranium-235, or 3.2 x 10^-11 joule. It contrasts this with about 4 eV per carbon atom burned in fossil fuels. The textbook's own worked example of 171 MeV is lower than its 240 MeV estimate, because real fission does not split the nucleus into two equal parts and produces neutrons. [3] [2]

In a typical reactor, a uranium-235 nucleus absorbs a neutron and becomes an unstable uranium-236 nucleus. The textbook describes the nucleus as behaving like a liquid drop. It deforms and narrows in the middle, and electric repulsion between the two ends then pushes it apart into two fragments and a few neutrons. The break-up is probabilistic, with several hundred possible combinations of fragments, and typically 2 or 3 neutrons are emitted, averaging 2.45. [4] [2] [3]

About 85% of the released energy starts as kinetic energy of the fission fragments. In solid fuel they travel only a microscopic distance, so that energy becomes heat. The rest comes from gamma rays and the kinetic energy of neutrons. [3]

The emitted neutrons can strike other uranium nuclei and split them, producing more neutrons. A self-sustaining chain reaction is possible when more than one neutron is produced per fission on average. A small share of neutrons are delayed, 0.66% for uranium-235, and this is the crucial factor that lets a reactor be held precisely critical. Control rods absorb neutrons and can be raised or lowered to set the rate of fission. [2] [3] [4]

A power plant uses the reactor to contain and control the chain reaction. The heat warms a coolant, typically water, to make steam. The steam spins turbines that drive an electric generator, in much the same way that heat from coal, gas or oil is used. [1] [5] [6]