“CRITICALITY” AND SAFETY IN A NUCLEAR POWER PLANT

When the fission reactor of a nuclear power plant is operating normally it is said to be “critical” or in a state of “criticality. The word often describes situations with potential for disaster. Nevertheless, in the context of nuclear power, “criticality” indicates that a reactor is operating safely. Criticality is a balanced state.

Nuclear reactors use uranium fuel rods to create energy through fission. Fission is the process of splitting the nuclei of uranium atoms to release neutrons that in turn split more atoms, releasing more neutrons. Criticality means that a reactor is controlling a sustained fission chain reaction where each fission event releases a sufficient number of neutrons to maintain an ongoing series of reactions. In the balanced state of criticality, fuel rods inside a nuclear reactor are producing and losing a constant number of neutrons, and the nuclear energy system is stable. In a nuclear reactor, the neutron population at any instant is a function of the rate of neutron production (due to fission processes) and the rate of neutron losses (via non-fission absorption mechanisms and leakage from the system). When a reactor’s neutron population remains steady from one generation to the next (creating as many new neutrons as are lost), the fission chain reaction is self-sustaining and the reactor’s condition is referred to as “critical”. When the reactor’s neutron production exceeds losses, characterized by increasing power level, it is considered “supercritical”, and; when losses dominate, it is considered “subcritical” and exhibits decreasing power.

Controlling criticality: When a reactor is starting up, the number of neutrons is increased slowly in a controlled manner. Neutron-absorbing control rods in the reactor core are used to control neutron production. The control rods are made from neutron-absorbing elements such as cadmium, boron, or hafnium. The deeper the rods are lowered into the reactor core, the more neutrons the rods absorb and the less fission occurs. Technicians pull up or lower the control rods into the reactor core depending on whether more or less fission, neutron production, and power are desired. If a malfunction occurs, technicians can remotely plunge control rods into the reactor core to quickly soak up neutrons and shut down the nuclear reaction.

What is “super criticality”?

At start-up, the reactor is briefly put into a state that produces more neutrons than are lost. This condition is called “supercritical” state, which allows the neutron population to increase and more power to be produced. When the desired power production is reached, adjustments are made to place the reactor into critical state that sustains neutron balance and power production. At times, such as for maintenance shutdown or refueling, reactors are placed in a “subcritical” state so that neutron and power production decrease.