NANTeL - National Academy for Nuclear Training e-Learning Nuclear Plant Systems Overview Questions and Answers 1 — Questions and Answers
Question 1: In a Pressurized Water Reactor (PWR), what is the primary function of the steam generator?
- To use steam to spin the turbine-generator.
- To transfer heat from the primary coolant loop to the secondary loop, creating steam. (Correct answer)
- To condense steam back into water after it passes through the turbine.
- To control the rate of the nuclear reaction by absorbing neutrons.
Correct answer: To transfer heat from the primary coolant loop to the secondary loop, creating steam.
The steam generator is a heat exchanger that transfers thermal energy from the hot, pressurized water in the primary (reactor coolant) system to the water in the secondary system. This causes the water in the secondary system to boil and produce steam, which then drives the turbine-generator.
Question 2: A control room operator observes a gradual, unintended decrease in reactor power in a Boiling Water Reactor (BWR). Which of the following system changes could be a direct cause?
- An increase in feedwater temperature.
- A decrease in the speed of the turbine.
- An increase in recirculation pump speed.
- A decrease in recirculation flow, causing more steam voids in the core. (Correct answer)
Correct answer: A decrease in recirculation flow, causing more steam voids in the core.
In a BWR, the water acts as both a coolant and a neutron moderator. Increasing the recirculation flow forces more liquid water through the core, reducing the number of steam voids. Fewer voids lead to better neutron moderation and an increase in reactor power. Conversely, a decrease in recirculation flow leads to more steam voids, which are less effective at moderating neutrons, thus causing reactor power to decrease.
Question 3: Which of the following best describes the primary role of the Emergency Core Cooling System (ECCS)?
- To provide power to the plant during a station blackout.
- To cool the spent fuel pool during normal operations.
- To provide cooling to the reactor core to prevent fuel damage during a loss-of-coolant accident (LOCA). (Correct answer)
- To filter radioactive particles from the containment atmosphere after an event.
Correct answer: To provide cooling to the reactor core to prevent fuel damage during a loss-of-coolant accident (LOCA).
The fundamental purpose of the ECCS is to provide emergency cooling to the reactor core in the event of a pipe break or other accident that results in a loss of normal coolant. This is crucial for removing decay heat and preventing the fuel from overheating and melting.
Question 4: What is the principal function of the containment building in a nuclear power plant?
- To house the main turbine and generator.
- To act as the primary operational control center for the plant.
- To serve as the final barrier to prevent the release of radioactive materials into the environment during an accident. (Correct answer)
- To store new and spent nuclear fuel.
Correct answer: To serve as the final barrier to prevent the release of radioactive materials into the environment during an accident.
The containment building is a massive, robust structure of steel and reinforced concrete designed to contain the energy and radioactive materials that could be released during a severe accident. It is the fourth and final barrier in a nuclear plant's defense-in-depth safety philosophy.
Question 5: In the context of a nuclear power plant's secondary system, what is the direct function of the main turbine?
- To pump water back to the steam generator.
- To condense steam into water.
- To convert the thermal energy of steam into mechanical rotational energy. (Correct answer)
- To generate the magnetic field for the electrical generator.
Correct answer: To convert the thermal energy of steam into mechanical rotational energy.
High-pressure steam produced in the steam generators (in a PWR) or the reactor vessel (in a BWR) is directed to the main turbine. The steam expands and pushes against the turbine blades, causing them to rotate at high speed. This rotation is the mechanical energy used to turn the electrical generator.
Question 6: During a plant tour, a new employee asks why the water in the primary loop of a Pressurized Water Reactor (PWR) doesn't boil, despite being at a very high temperature. What is the correct explanation?
- The water is a special type that has a much higher boiling point.
- The flow rate is too high for bubbles to form.
- The water is kept at a very high pressure by the pressurizer. (Correct answer)
- The control rods absorb the excess heat that would cause boiling.
Correct answer: The water is kept at a very high pressure by the pressurizer.
In a PWR, the primary coolant system is maintained at a very high pressure (e.g., around 2250 psi). This high pressure raises the boiling point of water significantly, allowing it to be heated to over 600°F (315°C) without turning into steam. This hot liquid then transfers its heat to the secondary loop in the steam generator.
NANTeL - National Academy for Nuclear Training e-Learning Nuclear Plant Systems Overview Questions and Answers 1
In a Pressurized Water Reactor (PWR), what is the primary function of the steam generator?