Induced fission requires absorption of a thermal (slow) neutron

Nuclear Energy & Binding Energy — AQA A-Level Physics

Key Definition

Induced fission — Fission that occurs when a stable nucleus absorbs a slow-moving (thermal) neutron and becomes unstable, splitting almost immediately into smaller nuclei.

Key Definition

Thermal neutron — A neutron in thermal equilibrium with its surroundings, typically with kinetic energy of about 0.04 eV and speed around 2700 m s⁻¹ at 300 K.

U-235 absorbs a thermal neutron to become U-236, which is highly unstable and splits almost immediately.
Only slow neutrons can be absorbed. Fast neutrons simply bounce off the uranium nucleus.
The kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. of a thermal neutron is approximately (3/2)kT, where k is the Boltzmann constantThe gas constant per molecule. Relates the average kinetic energyThe capacity to do work. Measured in joules (J).The energy an object possesses due to its motion. of gas molecules to temperature. k = 1.38 x 10⁻²³ J K⁻¹. and T is the temperature.

E = \frac{3}{2}kT

$E$: average kinetic energyThe energy an object possesses due to its motion. of a thermal neutron (J)
$k$: Boltzmann constantThe gas constant per molecule. Relates the average kinetic energy of gas molecules to temperature. k = 1.38 x 10⁻²³ J K⁻¹. = 1.38 × 10⁻²³ J K⁻¹
$T$: thermodynamic temperature (K)