Which statement correctly contrasts the design of PWRs and BWRs?

• A. PWRs use an indirect cycle with a liquid primary loop; BWRs are direct cycle with boiling in the core.
• B. PWRs boil in the core; BWRs use steam generators externally.
• C. PWRs have no shielding; BWRs require no shielding.
• D. PWRs use gas cooling; BWRs use liquid cooling.

Answer: (A)

The difference lies in how and where steam is produced from the reactor heat. In a pressurized-water reactor, the reactor heats a primary coolant loop that stays closed and pressurized; the heat is transferred across a steam generator to a separate secondary water loop, which then becomes steam to drive the turbine. The primary loop remains isolated from the turbine and the secondary loop, hence the term indirect cycle with a liquid primary loop.

In a boiling-water reactor, the coolant boils directly in the reactor core and the generated steam goes straight to the turbine, with no separate steam generator or secondary loop. This is a direct cycle.

So the statement that correctly contrasts the designs is the one describing an indirect primary loop with a separate steam generator for PWRs and a direct cycle with core boiling for BWRs. The other statements conflict with these fundamental distinctions: PWRs do not boil in the core, BWRs do not use an external steam generator, shielding and cooling methods described are not the defining differences between these two reactor types.