Types of Nuclear Reactions
There are many different interactions between atoms and the elementary atomic particles. In the nuclear power industry, the key interaction is the encounter of a neutron and an atomic nucleus and the subsequent fission of the nucleus. However, even such an encounter could take any one of a number of different courses.
Video: No interaction
If a neutron travels near a nucleus but at a distance larger than 2 × 10−13 cm, it is not affected by the nuclear force and no interaction results.
Video: Elastic scattering
An incident neutron can scatter from a nucleus in accordance with the laws of conservation of energy and momentum and then continue to propagate in a different direction. The nucleus does not react to this collision in any way (it is not excited). This is referred to as elastic scattering. We can visualize this interaction as the collision of a ping-pong ball (the neutron) with a bowling ball (the nucleus). The collision of a uranium 235U nucleus and a fast neutron with energy exceeding 0.1 MeV often results in elastic scattering. Collisions with hydrogen atoms are frequently used to slow neutrons down.
The likelihood that a neutron in a certain condition (kinetic energy) shall be, under given conditions, absorbed by an atomic nucleus is referred to as the effective neutron absorption cross-section.
Video: Inelastic scattering
Alternatively, after its collision with a nucleus, the neutron might transfer part of its kinetic energy. The neutron is slowed down, the nucleus is excited by this excess energy and must release it by the emission of a photon or possibly by another change. If the amount of transferred energy is large enough, the nucleus might disintegrate.
Video: Radiative capture
During radiative capture, an incident neutron enters the target nucleus forming a compound nucleus and at the same time transferring all its energy to the nucleus. The nucleus is excited by this additional energy and must release it by emitting a photon, or possibly by another type of change. In the nuclear energy industry, boron or cadmium is often used to capture neutrons in order to control the number of free neutrons. These elements have a high propensity to absorb neutrons.
Fission occurs when a neutron enters a nucleus that then starts to vibrate to such an extent that the nuclear forces cannot hold it together anymore. The nucleus starts to elongate, a cleavage develops, and it finally splits into two similarly sized fragments. This is the usual outcome of a collision between a uranium 235U nucleus and a slow neutron having energy of about 0.02 eV. The two fragments fly away at a speed of 10,000 km/sec and they gradually slow down by colliding with other atoms. At the same time, two to three fast neutrons are emitted from the cleavage area. Plutonium or thorium can disintegrate in a similar way.
An atom of 238U often reacts by radiative capture, emitting a couple of beta particles, and thus transmuting to plutonium 239Pu.