Which features characterize a CANDU (PHWR) reactor?

Which features characterize a CANDU (PHWR) reactor?

• A. Uses enriched uranium fuel, light water as moderator and coolant.
• B. Uses natural uranium fuel, heavy water as both moderator and coolant, and allows online refueling.
• C. Uses graphite moderator and once-through fuel.
• D. Operates at high reactor outlet temperatures with gas coolant.

Answer: (B)

CANDU reactors are defined by using natural uranium fuel, heavy water serving as both moderator and coolant, and the ability to refuel online. Natural uranium means the fuel does not need enrichment, which is possible because heavy water moderates neutrons very efficiently while absorbing relatively few of them. That strong neutron economy lets a reactor run with the low fissile content of natural uranium. With heavy water as both moderator and coolant, heat can be removed effectively without compromising the reactor’s neutron economy. The design also uses a network of separate fuel channels within a heavy-water moderator, enabling on-line refueling—fuel can be added or replaced without shutting the plant down, which boosts availability and capacity factor. Compared to other options, enriched uranium with light water is typical of conventional light-water reactors and does not reflect the PHWR approach. A graphite-moderated, once-through-fuel design is a different reactor class, not a CANDU. And a gas-cooled reactor with high outlet temperatures uses a gas coolant, which again does not describe the CANDU layout.

CANDU reactors are defined by using natural uranium fuel, heavy water serving as both moderator and coolant, and the ability to refuel online. Natural uranium means the fuel does not need enrichment, which is possible because heavy water moderates neutrons very efficiently while absorbing relatively few of them. That strong neutron economy lets a reactor run with the low fissile content of natural uranium. With heavy water as both moderator and coolant, heat can be removed effectively without compromising the reactor’s neutron economy.

The design also uses a network of separate fuel channels within a heavy-water moderator, enabling on-line refueling—fuel can be added or replaced without shutting the plant down, which boosts availability and capacity factor.

Compared to other options, enriched uranium with light water is typical of conventional light-water reactors and does not reflect the PHWR approach. A graphite-moderated, once-through-fuel design is a different reactor class, not a CANDU. And a gas-cooled reactor with high outlet temperatures uses a gas coolant, which again does not describe the CANDU layout.