Subkriittisyys

Subkriittisyys refers to a state in nuclear reactor physics where the neutron population is decreasing. In a nuclear reactor, a chain reaction is sustained by neutrons causing fission events, which in turn release more neutrons. A reactor is considered critical when the rate of neutron production exactly matches the rate of neutron loss (due to absorption or leakage). When a reactor is subcritical, more neutrons are being lost than are being produced, leading to a decline in the fission rate.

This state is typically achieved by reducing the reactivity of the reactor core. This can be done by inserting control rods, which absorb neutrons, or by changing the concentration of neutron-absorbing materials in the coolant or moderator. The subcritical state is a fundamental aspect of reactor control and safety. It is the normal operating condition when a reactor is being shut down or is in a safe standby mode.

When a reactor is shut down, it is brought into a deeply subcritical state to ensure that the chain reaction ceases completely and the neutron flux drops to very low levels. While subcritical, the reactor can still generate heat from the decay of fission products, but this decay heat is significantly less than the heat produced during normal operation. Monitoring the neutron population is crucial to confirm that the reactor is indeed subcritical and remains so. The mathematical description of subcriticality involves neutron kinetics equations that relate neutron population changes to reactivity.