Which reactor uses heavy water (D2O) as coolant and moderator and does not require enriched uranium but is difficult and expensive?

Which reactor uses heavy water (D2O) as coolant and moderator and does not require enriched uranium but is difficult and expensive?

• A. Light Water Reactors
• B. Heavy Water Reactors
• C. Magnox Reactor
• D. FNR Reactor

Answer: (B)

The main idea is neutron economy: how well the reactor medium slows and preserves neutrons to sustain fission. Heavy water (D2O) is an excellent moderator because deuterium absorbs far fewer neutrons than ordinary hydrogen, while still slowing fast neutrons effectively. That means a reactor can keep the chain reaction going with natural uranium, which contains only about 0.7% fissile U-235, without needing to enrich the fuel. Using heavy water as both the coolant and the moderator provides a very efficient neutron economy, so you can load natural uranium and rely on the neutral slow neutrons to continue fission. However, producing and maintaining large amounts of high-purity heavy water is costly, and the plant design must handle the heavy water system, which makes this approach expensive and complex. By contrast, other choices involve moderators or coolants that either absorb more neutrons or use fast neutrons with different fuel requirements, so they generally don’t run on natural uranium and aren’t built around heavy water.

The main idea is neutron economy: how well the reactor medium slows and preserves neutrons to sustain fission. Heavy water (D2O) is an excellent moderator because deuterium absorbs far fewer neutrons than ordinary hydrogen, while still slowing fast neutrons effectively. That means a reactor can keep the chain reaction going with natural uranium, which contains only about 0.7% fissile U-235, without needing to enrich the fuel.

Using heavy water as both the coolant and the moderator provides a very efficient neutron economy, so you can load natural uranium and rely on the neutral slow neutrons to continue fission. However, producing and maintaining large amounts of high-purity heavy water is costly, and the plant design must handle the heavy water system, which makes this approach expensive and complex.

By contrast, other choices involve moderators or coolants that either absorb more neutrons or use fast neutrons with different fuel requirements, so they generally don’t run on natural uranium and aren’t built around heavy water.