What Is Criticality in a Nuclear Power Plant?

It's Not as Disastrous as it Sounds

Nuclear Power plant
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When the atom-splitting reactor of a nuclear power plant is operating normally, it is said to be “critical” or in a state of “criticality.”

Using the term “criticality” may seem counter-intuitive as a way to describe normalcy. In everyday parlance, 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 -- long, slender, zirconium metal tubes containing pellets of fissionable material -- 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.

Fission produces a great deal of energy in the form of very high heat and radiation. That’s why reactors are housed in structures sealed under thick metal-reinforced concrete domes. Power plants harness this energy and heat to produce steam to drive generators that produce electricity.

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 calibrate 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 down the control rods into the reactor core depending on whether more or less fission, neutron production and power are desired.

Should a malfunction occur, technicians can remotely plunge control rods into the reactor core to quickly soak up neutrons and shut down the nuclear reaction.

What Is Supercriticality?

At start-up, the nuclear 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.

Far from the worrisome state suggested by its name, criticality is a desirable and necessary state for a nuclear power plant producing a consistent and steady stream of energy.