Energy from a detonation splits into three major channels: heat (thermal energy), the blast or shock wave, and prompt nuclear radiation. The heat component is the largest portion, causing intense heating and thermal radiation. The second largest portion is the blast/shock wave, which carries roughly a third of the energy and is responsible for the overpressure and mechanical damage to surroundings. A smaller fraction appears as prompt nuclear radiation. If one portion is about 35%, that corresponds to the blast/shock energy. Thermal energy, by contrast, would be the largest share, not the second.

Heavy water as both coolant and moderator is the defining feature of the CANDU design. Deuterium oxide (heavy water) moderates neutrons very effectively while absorbing far fewer neutrons than ordinary water, so the reactor can run on natural uranium without enrichment. This setup also allows online refueling, which is a practical advantage of CANDU reactors. The other options use different cooling/moderation schemes—light water as the moderator and coolant, or gas-cooled with graphite moderator—so they do not employ heavy water in both roles.

Heat is produced by fission in the fuel rods and must be carried away to keep the reactor from overheating. The component that does this by flowing between the rods is the coolant, which transfers the heat out of the core. In many reactors, water serves this role and can also act as a moderator, meaning it slows neutrons through collisions with its hydrogen atoms to help sustain the fission process. The other options don’t fit the description: a moderator slows neutrons but isn’t the flow that removes heat, while regulator and control rods are solid components inserted to absorb neutrons and control reactivity, not to carry away heat.

Delayed radiation after a fission event is governed by the decay of radioactive fission fragments produced at the moment the nucleus splits. These fragments are typically unstable, and each one decays with its own characteristic half-life, emitting radiation as it does. Because the total radiation is the sum of many different decays, the overall radiation level decreases with time as the shorter-lived isotopes decay faster and the longer-lived ones fade more slowly. The result is a decay in radiation over time, not a constant level and not an increasing one. This delayed emission is why safety planning accounts for post‑fission radiation that diminishes as cooling proceeds, sometimes showing multiple decay ‘phases’ because of the mix of isotopes involved.