If keff equals 1, the reactor is:

If keff equals 1, the reactor is:

• A. subcritical
• B. critical
• C. supercritical
• D. critical at all times

Answer: (B)

Effective multiplication factor keff tells you if the neutron population is steady, increasing, or decreasing. When keff equals 1, the number of neutrons from one generation that cause fission in the next generation is exactly the same, so the chain reaction remains self-sustaining at a constant level. This is the definition of critical. If keff is less than 1, the neutron population and reactor power would diminish over time (subcritical). If keff is greater than 1, the neutron population and power would rise (supercritical). The idea of being “critical at all times” isn’t how keff is described; keeping keff at 1 is the operating goal, but any disturbance can push it away from 1, whereas the fundamental behavior at keff = 1 is a constant neutron population.

Effective multiplication factor keff tells you if the neutron population is steady, increasing, or decreasing. When keff equals 1, the number of neutrons from one generation that cause fission in the next generation is exactly the same, so the chain reaction remains self-sustaining at a constant level. This is the definition of critical. If keff is less than 1, the neutron population and reactor power would diminish over time (subcritical). If keff is greater than 1, the neutron population and power would rise (supercritical). The idea of being “critical at all times” isn’t how keff is described; keeping keff at 1 is the operating goal, but any disturbance can push it away from 1, whereas the fundamental behavior at keff = 1 is a constant neutron population.