How does a nuclear reaction take place?

Answer 1

First nuclear reactions always involve the nucleus and except for K capture beta decay never involve any of the electrons around the nucleus.

There are seven different types of ordinary nuclear reactions:

1. fission, a massive nucleus splits into two lighter fragment nuclei (about 1/3 & 2/3 the mass of the original nucleus) and several free neutrons, fission can happen spontaneously in some isotopes (e.g. plutonium-240) but is usually triggered by the capture of a neutron, as fission always produces free neutrons it is possible to produce a neutron chain reaction to keep the process going
2. fusion, light nuclei join forming a heavier nucleus, this reaction can only happen under conditions of very high temperature and pressure (causing the nuclei to be fully ionized, traveling at high velocity, and pressed tightly together) it is very hard to to get started and keep going (except deep inside stars) Note that fusion is the only one of these seven nuclear reactions that is affected in any way by the temperature or pressure of the environment it happens in
3. alpha decay, a nucleus spontaneously ejects a helium nucleus (i.e. alpha particle)
4. beta- decay, a neutron in the nucleus spontaneously transforms into a proton and the nucleus ejects an electron and an electron antineutrino
5. beta+ decay, a proton in the nucleus spontaneously transforms into a neutron and the nucleus ejects an positron and an electron neutrino
6. K capture beta decay, a proton in the nucleus spontaneously transforms into a neutron and the nucleus captures an electron from the innermost (i.e. K) electron shell and ejects an electron neutrino
7. gamma decay, a nucleus in a metastable (i.e. excess energy) state spontaneously relaxes its proton and/or neutron shells to a lower energy state and ejects a gamma photon with an energy equal to the energy lost in the nucleus

However if you include other subatomic particles not present in ordinary matter (e.g. muons, antimatter particles, strange particles) a much wider and more confusing variety of nuclear reactions can happen that are beyond the scope of the original question. I will only mention one of these nuclear reactions: muon catalysed cold fusion. This is interesting because it permits the fusion nuclear reaction to happen at ordinary room temperature.

In muon catalyzed cold fusion the electrons around hydrogen nuclei are replaced with muons (particles identical to electrons in every way except that they have 200 times the mass), being much more massive than electrons their orbitals are much smaller. So much smaller that the nuclei can come close enough to each other at ordinary room temperature that the nuclei can fuse! The fusion energy release causes the product nucleus to lose its muons and become ionized. This process is called "muon catalyzed" because these free muons can now replace electrons around fresh hydrogen nuclei, repeating the nuclear reaction over and over without requiring any additional muons. The only problem with muon catalyzed cold fusion is that the muons required to begin this nuclear reaction are very expensive to produce.