In nuclear power-plant cycles, which reactor type does not require a separate heat exchanger (steam generator) to supply steam to the power turbine, because steam is generated directly in the reactor vessel and sent to the turbine?

Introduction / Context:
A key difference among commercial nuclear power reactor types is how they produce and deliver steam to the turbine. Some designs generate steam in a secondary loop via a steam generator (a type of heat exchanger), whereas others make steam directly inside the reactor vessel. Understanding this distinction is essential for comparing system complexity, contamination control, and component count.

Given Data / Assumptions:

• We are comparing common thermal reactor types used for electricity generation.
• “Does not require a heat exchanger to supply steam” means turbine steam is produced in the reactor vessel itself and not in a separate steam generator.
• Options include molten-sodium-cooled, helium-cooled, boiling water, and pressurised water concepts.

Concept / Approach:
In a Boiling Water Reactor (BWR), feedwater enters the reactor vessel, where it is heated to saturation and allowed to boil within the core region. The resulting two-phase mixture is sent to internal steam separators and dryers; the dry steam is routed directly to the turbine. No separate steam generator is used. By contrast, Pressurised Water Reactors (PWRs) keep the primary water under high pressure to avoid boiling; heat is transferred to a secondary loop via a steam generator. Helium- and sodium-cooled reactors also transfer heat to a separate water/steam system to produce turbine steam.

Step-by-Step Solution:
Identify which design generates turbine steam inside the reactor vessel: the BWR.Note that PWRs necessarily use steam generators; sodium- and helium-cooled systems also need an intermediate steam-producing heat exchanger.Therefore, the reactor that does not require a separate steam generator is the Boiling Water Reactor.

Verification / Alternative check:
System schematics of BWRs show moisture separators/dryers inside the vessel and steam lines emerging directly to the turbine stop valves. PWR diagrams always include steam generators between the reactor coolant and the turbine cycle.

Why Other Options Are Wrong:
Molten sodium cooled: uses intermediate/steam generators for safety and chemistry reasons.Helium cooled: transfers heat to a separate steam or Brayton cycle unit.Pressurised water: requires steam generators by design.

Common Pitfalls:
Assuming “boiling” always implies a secondary loop; forgetting that “pressurised water” specifically avoids boiling in the primary loop and thus needs a steam generator.

Final Answer:
Boiling water