The condition in which neutron production from fission exactly balances losses from absorption and leakage, so the neutron population holds steady and the effective multiplication factor equals 1. Below it the reactor is subcritical and power dies away; above it, supercritical and power rises. Reactor startups approach criticality deliberately, watching the count rate climb, while criticality safety exists to prevent it everywhere outside the core.
Estimated critical position is bank D at 120 steps, pull in 10-step increments and plot inverse count rate.
Critical is a routine operating state for a reactor, not an emergency; the word alarms outsiders far more than it does control-room staff.
Also heard as
- critical
Related terms
k-effective
The effective neutron multiplication factor: the ratio of neutrons in one generation to the previous one.
Reactivity
A measure of how far a reactor is from criticality, positive when power will rise and negative when it will fall.
Criticality safety
The discipline of preventing an accidental chain reaction wherever fissile material is handled outside a reactor.
Control rod
A movable rod or blade of neutron-absorbing material used to control reactor power and shut the reactor down.