Control and Reaction States
3 min read
A fission chain reaction can develop in uranium under suitable conditions. If the neutron population is not appropriately controlled, the chain reaction may either die out or increase. Under certain conditions, such as those created in a nuclear weapon, it can increase extremely rapidly.
In nuclear reactors, the chain reaction must be carefully controlled so that the reactor can be started up, operated at the required power level, adjusted in power and shut down safely. The rate of fission depends on the neutron population in the reactor core. The chain reaction can therefore be controlled by adjusting neutron absorption, for example by inserting or withdrawing control rods containing neutron-absorbing materials. According to the behaviour of the neutron population, a reactor can be in one of three states: subcritical, critical or supercritical.
Video: Subcritical state of a fission chain reaction.
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Subcritical
In the subcritical state (k_eff < 1), each generation of fissions produces fewer neutrons that cause further fissions than the preceding generation. The neutron population therefore decreases and a self-sustaining chain reaction cannot be maintained. A reactor becomes subcritical during shutdown, for example when neutron-absorbing control rods are inserted sufficiently far into the core.
Video: Critical state of the nuclear reaction.
Free Downloads › Animations / Critical state of the nuclear reaction
Critical
In the critical state (k_eff = 1), each generation of fissions produces, on average, enough neutrons to cause the same number of fission in the next generation. The neutron population and fission rate therefore remain constant. This is the normal state of a nuclear reactor operating at a steady power level.
Video: Supercritical state of a fission chain reaction.
Free Downloads › Animations /Supercritical state of a fission chain reaction
Supercritical
In the supercritical state (k_eff > 1), each generation of fissions produces more neutrons that cause further fissions than the preceding generation. The neutron population and reactor power therefore increase. A controlled slightly supercritical state is used during reactor startup and when increasing the reactor power level. In normal reactor operation, power changes are kept slow enough to remain controllable, largely thanks to the small fraction of delayed neutrons emitted by fissions products.
About two billion years ago, natural self-sustaining fission chain reactions occurred in uranium deposits at Oklo in present-day Gabon. Groundwater acted as a neutron moderator, and changes in the amount of water in the deposits probably caused the natural reactors to operate intermittently.
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