Nuclear energy is produced by splitting uranium atoms in a reactor using which substance for control?

• A. Graphite
• B. Light water
• C. Heavy water
• D. Sand

Answer: (C)

In a fission reactor, keeping the chain reaction going requires slowing down the fast neutrons produced by fission so they’re more likely to cause another fission when they collide with uranium nuclei. A moderator does this slowing. Heavy water (deuterium oxide) is an excellent moderator because it slows neutrons effectively while absorbing very few of them, which helps maintain the reaction and allows the reactor to run efficiently with natural uranium fuel. Graphite can also slow neutrons, but heavy water has the advantage of lower neutron absorption, making it especially good for controlling the reaction in many designs. Light water can slow neutrons as well, but it absorbs more neutrons than heavy water, which changes how the reactor is operated. Sand isn’t used in this role. So, heavy water is the best choice among the options for moderating and aiding control of the nuclear reaction.