Delayed neutrons

A small fraction of the neutrons from fission that are not released at the instant of fission but a fraction of a second to about a minute later, as certain fission fragments decay radioactively. For uranium-235 this fraction, written β (beta), is about 0.64% of all fission neutrons. It is markedly smaller for plutonium-239, about 0.2%, and for uranium-233, about 0.26%, so a core fuelled mainly with either has less margin to work with. Without delayed neutrons, the neutron population in a thermal reactor would change on a timescale of about a ten-thousandth of a second, and far faster in a fast reactor, too fast for control rods or operators to follow. Because reactors are run so that they need the delayed fraction to stay critical, the effective time between neutron generations stretches to roughly a tenth of a second, which is what makes a reactor controllable at all. A reactor that could sustain its chain reaction on prompt neutrons alone is called prompt critical, a state normal operation avoids by a wide margin.

Vocabulary id

concept:delayed-neutrons

Scheme

Concepts

Back to the glossary.