Fast neutron meaning

In nuclear regulatory practice, a fast neutron is a high‑energy neutron that has not been slowed (moderated) by materials such as water or graphite. It contrasts with a thermal neutron, whose energy corresponds to normal ambient heat. Fast neutrons typically have energies above about 0.1 MeV. This is a descriptive scientific term rather than a statutory definition. UK and Irish radiation protection and nuclear safety frameworks regulate ionising radiation generally, but technical standards, regulator guidance (e.g. ONR in the UK) and safety cases for nuclear installations, fuel cycle facilities and transport packages routinely distinguish fast from thermal neutron spectra because energy affects criticality analysis, shielding design, dose assessment and material activation. Legally relevant features include: greater penetration and different dose conversion factors; the need for hydrogenous shielding or moderators; and implications for reactor type (including fast reactors), storage and handling of fissile material, and decommissioning planning. Usage and practical meaning are consistent across England and Wales, Scotland, Northern Ireland and Ireland, with jurisdiction‑specific guidance issued by the competent authority in each country.