What is Uranium 238 – Definition

²³⁸U is a fissionable isotope, but is not fissile isotope. ²³⁸U is not capable of undergoing fission reaction after absorbing thermal neutron, on the other hand ²³⁸U can be fissioned by fast neutron with energy higher than >1MeV. ²³⁸U does not meet also alternative requirement to fissile materials. ²³⁸U is not capable of sustaining a nuclear fission chain reaction, because too many of neutrons produced by fission of ²³⁸U have lower energies than original neutron.

²³⁸U belongs also to the group of fertile isotopes. Radiative capture of a neutron leads to the formation of fissile ²³⁹Pu. This is the way how ²³⁸U contributes to the operation of nuclear reactors and production of electricity through this plutonium. For example, at a burnup of 40GWd/tU, about 40% of the total energy released comes from bred plutonium. This corresponds to a breeding ratio for this fuel burnup of about 0.4 to 0.5. This effect extends the cycle length for such fuels to sometimes nearly twice what it would be otherwise.

See also: Comparison of cross-sections

Plutonium 239 breeding

Neutron capture may also be used to create fissile ²³⁹Pu from ²³⁸U, which is the dominant constituent of naturally occurring uranium (99.28%). Absorption of a neutron in the ²³⁸U nucleus yields ²³⁹U. The half-life of ²³⁹U is approximately 23.5 minutes. ²³⁹U decays (negative beta decay) to ²³⁹Np (neptunium), whose half-life is 2.36 days. ²³⁹Np decays (negative beta decay)  to ²³⁹Pu.

Doppler broadening - inherent safety feature of uranium 238

The Doppler broadening of resonances is very important phanomenon, which improves reactor stability. The prompt temperature coefficient of most thermal reactors is negative, owing to the nuclear Doppler effect (fuel temperature coefficient – FTC). The broadening of uranium 238 resonances constitute fast acting negative feedback effect that is essential to ensure safe operation of all reactors.

In power reactors this effect acts as follows:

The reactor power increase (heating of a fuel) causes broadening of ²³⁸U resonances. This  results in a higher probability of neutron absorbtion, thus causes negative reactivity insertion (reduction of reactor power).

In the thorium fuel cycle ²³²Th is analogous to ²³⁸U in the U-Pu fuel cycle, because it is the fertile component. Some of the radiative captures in ²³²Th are resonance events, so that the Doppler broadening role of ²³⁸U is similar.