Which statement correctly contrasts PWR and BWR regarding the primary cooling loop and neutron moderation?

Which statement correctly contrasts PWR and BWR regarding the primary cooling loop and neutron moderation?

• A. PWR uses graphite moderator; BWR uses heavy water.
• B. PWR uses a closed primary coolant loop moderated by water, with heat transferred to a separate secondary loop; BWR uses water as both coolant and moderator, producing steam directly in the reactor vessel.
• C. PWR uses steam directly in the reactor vessel; BWR uses a separate secondary loop.
• D. PWR and BWR have identical cooling loops.

Answer: (B)

The main idea is how cooling and neutron moderation differ between PWRs and BWRs. In a PWR, the core is cooled by water kept under high pressure in a closed primary loop, so it doesn’t boil. The heat from that primary loop is transferred to a separate secondary loop via a steam generator, and the steam in that secondary loop drives the turbines. The neutron moderator in the core is the same water in the primary loop. In a BWR, the water circulating through the core is allowed to boil, producing steam directly in the reactor vessel; there is no separate steam generator or secondary loop, and the water in the core acts as both coolant and moderator. This contrasting setup is what the statement describes.

The main idea is how cooling and neutron moderation differ between PWRs and BWRs. In a PWR, the core is cooled by water kept under high pressure in a closed primary loop, so it doesn’t boil. The heat from that primary loop is transferred to a separate secondary loop via a steam generator, and the steam in that secondary loop drives the turbines. The neutron moderator in the core is the same water in the primary loop. In a BWR, the water circulating through the core is allowed to boil, producing steam directly in the reactor vessel; there is no separate steam generator or secondary loop, and the water in the core acts as both coolant and moderator. This contrasting setup is what the statement describes.