Nuclear Reactor Systems
This domain tests how major reactor systems work together: primary coolant, pressure control, heat removal, containment, reactivity control and emergency cooling.
Core rules
- A PWR has a high-pressure primary loop, usually around 15.5 MPa, with a pressurizer, steam generators and a separate secondary loop. The BWR boils in the core and sends steam directly to the turbine.
- The pressurizer controls primary pressure: heaters raise pressure by evaporating water; spray condenses steam and lowers pressure. Pressurizer level is an inventory indicator, not a direct measure of core inventory.
- Reactivity is controlled by control rods, soluble boron in PWRs, and void feedback in BWRs. The reactor protection system trips on parameters such as high power, high pressure, low flow or low DNBR.
- Emergency core cooling systems typically include accumulators, high-pressure and low-pressure injection, and recirculation from containment sumps.
- Decay heat removal after shutdown is a safety function independent of the main turbine and feedwater train; residual heat removal and auxiliary feedwater are key.
Worked example
A PWR is at full power when main feedwater is lost. Feedwater flow falls, steam generator level drops, primary temperature rises and coolant expands. The pressurizer level rises and pressure rises. The reactor trips on high pressurizer pressure or low DNBR. Turbine bypass or steam dump removes stored heat, auxiliary feedwater restores steam-generator inventory, and the pressurizer spray or heaters then hold pressure in band. The reasoning is: loss of heat sink raises primary temperature, which raises pressure; the trip removes fission heat, but decay heat still requires a heat sink. If auxiliary feedwater fails, the event escalates toward loss of secondary heat removal and possible core damage.
Traps
Do not treat pressurizer level as core inventory. Do not assume a reactor trip ends the heat problem; decay heat remains. Do not confuse ECCS actuation signals: low pressurizer pressure, low pressurizer level and high containment pressure are distinct. In BWRs, do not apply PWR boron-dilution logic blindly. Do not overlook single-failure assumptions when assessing system success.
Key points
- Pressurizer heaters and spray control PWR pressure.
- Pressurizer level indicates primary inventory, not core level.
- ECCS includes accumulators, HP injection, LP injection and recirculation.
- Auxiliary feedwater is a safety-related heat sink.
- BWRs boil in the core and use void feedback.
- Reactor trip does not remove decay heat.