About Criticality in a Nuclear Reactor:

• 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.
  • 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.
• 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. 
• This is the normal state of nuclear power generation.
• Fuel rods inside a nuclear reactor are producing and losing a constant number of neutrons, and the nuclear energy system is stable. 
• Nuclear power technicians have procedures in place, some of them automated, in case a situation arises in which more or fewer neutrons are produced and lost.
• 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 the 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 the critical state that sustains neutron balance and power production.