Boiling Water Reactor (BWR) basics: what working fluid serves as the reactor coolant in the core?

Introduction / Context:
In a Boiling Water Reactor (BWR), the reactor coolant removes fission heat from fuel rods and also acts as the working fluid for the turbine. Understanding the state of the coolant in the core and how the cycle is arranged is a basic reactor-engineering concept.

Given Data / Assumptions:

• The BWR uses light water for both moderation and cooling.
• Core operation allows nucleate boiling within fuel channels.
• Steam is separated from liquid before the turbine.

Concept / Approach:
By design, the coolant entering the core is liquid water. As it flows upward, controlled boiling occurs, generating a two-phase mixture. Steam is then separated and dried; the resulting dry steam drives the turbine directly. The term “coolant” refers to the fluid introduced to remove heat from the core—liquid water—not the downstream phase after separation.

Step-by-Step Solution:
1) Identify the coolant at core inlet: subcooled or saturated liquid water.2) Recognise that within the core, a mixture forms due to boiling.3) After separators/dryers, steam goes to the turbine while liquid recirculates.

Verification / Alternative check:
System schematics show recirculation pumps handling liquid water; steam lines branch only after moisture separators in the reactor vessel.

Why Other Options Are Wrong:
Hydrogen is not used as a BWR coolant.Steam alone is not injected as coolant; it is produced in situ.“Mixture” describes a state inside the core, but the coolant as a working fluid is water.

Common Pitfalls:
Equating “coolant” with whatever exits the core; forgetting the functional definition tied to what is pumped into the core to remove heat.

Final Answer:
Water