What is a pressurizer?
A pressurizer is a large cylindrical tank installed within a pressurized water reactor coolant system. Its main purpose is to control the primary system coolant pressure, although it is also used for water level detection, and as a pressure relief device.
Nuclear Pressurizer Parts
How does a pressurizer work?
Primary coolant may be heated by the reactor, or, by electrical heaters at the base of the pressurizer. When the primary coolant temperature increases, its volume increases. The increase in volume also occurs within the pressurizer, because it is connected to the primary coolant system via a surge line. Thermal expansion of the primary coolant, causes the steam at the top of the pressurizer to be compressed, and the system pressure to increase.
The system pressure can be reduced by spraying relatively cool water into the pressurizer via a spray nozzle. This causes the steam within the pressurizer to condense, thus reducing its volume, and lowering the primary coolant system pressure.
Pressurizers are safety critical items within a PWR power plant, because they maintain the primary coolant in a liquid state, thus reducing the likelihood of the reactor core overheating.
Dynamic and Static Pressurizers
Pressure in the primary circuit of a pressurized water reactor (PWR) is maintained by a pressurizer. Pressurizers may be either static or dynamic; this 3D model represents a dynamic pressurizer. A dynamic pressurizer serves the following functions.
- Maintain a system's pressure above its saturation point.
- Provides a means of controlling system fluid expansion and contraction.
- Provides a means of controlling a system's pressure.
- Provides a means of removing dissolved gasses from the system by venting the vapour space of the pressurizer.
What are the main parts of a nuclear pressurizer?
Vessel
The pressurizer provides a point in the reactor system where liquid and vapour can be maintained in equilibrium under saturated conditions, for control purposes. Although designs differ from facility to facility, a typical pressurizer is designed for a maximum of about 2,500 psi (≈172 bar) and 680°F (≈360°C). The hemispherical top and bottom pressurizer heads are usually constructed of carbon steel, with austenitic stainless-steel cladding on all surfaces exposed to the reactor water system.
Vent
A vent is used to remove dissolved gasses from the system; a deaerator serves a similar function in a boiler feedwater circuit. Although many dissolved gases are vented from the pressurizer, the removal of oxygen and carbon dioxide are of the most importance. Oxygen makes the water corrosive, whilst carbon dioxide lowers the PH of the water, making the water more acidic and thus more corrosive.
Spray Nozzle
Pressure within the pressurizer is reduced using a spray nozzle. A spray nozzle is a device located in the top of the pressurizer; it is used to atomise incoming water from the cold leg. Spraying cold water into the pressurizer reduces the water temperature within the pressurize and causes some steam to condense, this results in a system pressure reduction.
Surge Diffuser
A surge diffuser is installed at the base of the pressurizer. Water discharged from the pressurizer passes through the surge line and into the primary circuit.
Electric Heater Bundle
Electric heaters are used to heat the water within the pressurizer. When the water reaches its saturation temperature, it begins to boil. Boiling water fills the void above the normal operating water level, creating a saturated environment of water and steam. The water temperature determines the amount of pressure developed in the steam space, the longer the heaters are engaged, the hotter the environment becomes. The hotter the environment, the greater the amount of pressure.
Level Sensing Nozzle
Level sensors are fitted at various points on the pressurizer vessel. Real-time monitoring of the pressurizer water level ensures personnel are immediately alerted should the pressurizer water level decrease. On this 3D model, the level sensor would be mounted at this location, and the same location lower down on the pressurizer shell.
Relief Valve
A pressurizer controls the cooling water system pressure and acts as a pressure relief device for the system. A pilot operated relief valve (PORV) is installed at the top of the pressurizer. If the system pressure increases above a given setpoint, the PORV opens and steam is discharged to a relief tank. A secondary relief device (often a rupture disc) is provided as an additional safety measure.
Normal Operating Water Level (NOWL)
The pressurizer is located higher above other reactor cooling water pressure vessels. It is the only vessel not completely flooded with cooling water. As the pressurizer has a normal operating water level, it can be measured to provide an early warning should the reactor coolant level decrease.
Additional Resources
https://en.wikipedia.org/wiki/Pressurizer
https://www.nuclear-power.net/pressurizer
https://www.sciencedirect.com/topics/engineering/pressuriser