The fractional departure from criticality, defined as (k minus 1) divided by k, and expressed in pcm (one hundred-thousandth), percent delta-k over k, or dollars, where one dollar equals the delayed neutron fraction, about 0.005 to 0.007 in a light water reactor. Rods, boron, temperature, xenon and fuel burnup all add or remove reactivity. Adding a dollar or more makes a reactor prompt critical, the condition every reactivity control rule is built to prevent.
Dilution adds about 10 pcm per minute, so stop at 100 and let the temperature settle.
Operators treat every reactivity change as a deliberate, briefed manipulation with a peer check; an unexpected one is an event in itself.
Also heard as
- rho
Related terms
k-effective
The effective neutron multiplication factor: the ratio of neutrons in one generation to the previous one.
Control rod
A movable rod or blade of neutron-absorbing material used to control reactor power and shut the reactor down.
Void coefficient
The change in reactivity caused by steam bubbles or voids forming in the coolant.
Xenon poisoning
The loss of reactivity caused by xenon-135, a fission product that absorbs neutrons with enormous efficiency.
Poison
Any material that absorbs neutrons strongly, whether added deliberately for control or produced by fission.