MVP ARCHIVE HISTORY｜Energy : Energy History SPECIAL 10 : Cooling System
Cooling System
In a nuclear reactor, a large amount of heat is generated by nuclear fission.
While this heat is used for power generation, it must be continuously removed to keep the fuel and reactor equipment at a safe temperature.
It receives heat from the reactor, sends it to the power generation equipment, and finally releases it outside the power plant.
This series of roles is performed by the Cooling System.
Continuing to carry heat
Nuclear fission heat is first transferred from the fuel pellets to the cladding, and then to the coolant flowing outside it. In light water reactors, water is mainly used as the coolant.
Water receives heat while flowing between the fuel assemblies and carries it out of the reactor core.
Even if the reactor output is stable, if the flow that carries the heat is lost, the fuel temperature will rise, so cooling is a basic condition for operating a nuclear reactor.
Cooling of Pressurized Water Reactors
In a Pressurized Water Reactor (PWR), the water passing through the reactor is kept at high pressure to prevent it from boiling inside the reactor core.
The high-temperature primary coolant is sent to the steam generator, where it transfers heat to the secondary side water through heat transfer tubes, and the primary coolant that has lost its heat is returned to the reactor by a pump.
On the secondary side, water turns into steam, turns the turbine, and then returns to water in the condenser; two circulation systems that separate the reactor side and the turbine side carry the heat in stages.
Cooling of Boiling Water Reactors
In a Boiling Water Reactor (BWR), the water flowing through the reactor core boils directly inside the reactor pressure vessel, and the generated steam is sent directly to the turbine after moisture is removed.
The steam that has passed through the turbine is returned to water in the condenser and sent back to the reactor by a feedwater pump.
Although the equipment configuration differs between PWRs and BWRs, the basic role of circulating coolant and continuously removing heat from the reactor core is common to both.
Heat remaining after the reactor is shut down
Control rods are inserted to stop the nuclear fission chain reaction, but the heat generation of the reactor core does not immediately become zero because the nuclear fission products accumulated inside the fuel continue to undergo radioactive decay, generating decay heat .
Although the decay heat immediately after shutdown is smaller than the output during operation, it is large enough to potentially damage the fuel, and while decay heat decreases over time, it is necessary to continue cooling for a long period.
Emergency Core Cooling System
If coolant is lost due to pipe breakage or other reasons, the normal cooling system alone may not be able to sufficiently cool the reactor core.
For this reason, nuclear reactors are equipped with an Emergency Core Cooling System (ECCS).
The ECCS is a facility designed to inject water into the reactor core during abnormal conditions, removing heat while keeping the fuel covered with water.
Multiple methods are combined, such as injection systems that operate under high pressure, low-pressure injection systems that deliver large volumes of water after pressure has dropped, and equipment that automatically supplies accumulated water.
Systems are multiplexed and independent so that cooling functions can be maintained even if one piece of equipment becomes unavailable.
Power and Cooling
Electric power is required to operate the pumps and valves that circulate the coolant.
If the power plant is disconnected from the external power grid, emergency diesel generators and other sources supply power to safety equipment.
(Some reactors are equipped with batteries or passive cooling functions that do not require electricity.)
Even if the reactor can be shut down, decay heat may not be sufficiently removed if power cannot be supplied to the cooling equipment.
Therefore, securing a power supply is a critical element for maintaining the cooling system.
Cooling is performed in multiple stages
Cooling at a nuclear power plant is not performed by a single pipe or pump alone.
Heat moves through multiple stages: from the fuel to the primary coolant, from the primary coolant to the steam system, and from the steam to the condenser cooling water.
In addition to equipment that supports normal operation, cooling equipment for after shutdown, emergency water injection equipment, and equipment that ultimately releases heat into the sea or atmosphere are combined.
Each system shares a role while working toward the single goal of removing heat from the reactor core.
Archive Point
Cooling System is a group of facilities designed to extract heat generated in the reactor from the reactor core, pass it through power generation equipment, and ultimately transport it outside the power plant.
During normal operation, it circulates coolant to utilize heat for power generation, and after reactor shutdown, it removes decay heat produced by fission products.
In the event of an abnormality, the Emergency Core Cooling System supplies water to the reactor core to suppress the rise in fuel temperature.
Nuclear reactor safety is maintained not only by stopping the chain reaction, but also by continuously transporting the heat that remains even after shutdown to the outside without interruption.
