High Pressurizer Level: Consequences & Pressure

[oksuz_]

What would be the consequences if the pressurizer level is high? Does it cause high pressure because there is less space for steam?

Thank you in advance!

< Mentor Note -- Adding a link >
https://en.wikipedia.org/wiki/Pressurizer

 

[Astronuc]

What would be the consequences if the pressurizer level is high? Does it cause high pressure because there is less space for steam?

Thank you in advance!

< Mentor Note -- Adding a link >
https://en.wikipedia.org/wiki/Pressurizer

The main concern would if the pressurizer went hard, i.e., was completely filled with water. There are high level and low level limits to provide margin to a pressurizer full of water or empty and full of steam. Both extremes would be a concern.

One would have to investigate why the pressurizer level is high. That would mean there is more coolant inventory in the RCS than necessary assuming that the temperatures and pressures are in the normal range, or other malfunction. Water would move into the pressurizer if the reactor temperature increased and the coolant in the core expands. Higher level in the pressurizer means a greater head or static pressure, and the rise in water level would compress the steam. If steam pressure is too high, then a pressure relief value would open, reducing the pressure.

 

[Hiddencamper]

High level can reduce the effectiveness of sprays which can challenge pressure control.

High pressurizer level also causes a reactor trip in many designs. In the Westinghouse design, the high pressurizer level trip is a backup to the high pressure trip and also helps to prevent water relief through the pressurizer relief valves which are not explicitly designed for water.

Interestingly enough, low pressurizer level typically doesn't cause a reactor trip, but it does deenergize the pressurizer heaters which can lead to a low pressurizer pressure safety injection signal, which does trip the reactor.

 

[etudiant]

The main concern would if the pressurizer went hard, i.e., was completely filled with water. There are high level and low level limits to provide margin to a pressurizer full of water or empty and full of steam. Both extremes would be a concern.

One would have to investigate why the pressurizer level is high. That would mean there is more coolant inventory in the RCS than necessary assuming that the temperatures and pressures are in the normal range, or other malfunction. Water would move into the pressurizer if the reactor temperature increased and the coolant in the core expands. Higher level in the pressurizer means a greater head or static pressure, and the rise in water level would compress the steam. If steam pressure is too high, then a pressure relief value would open, reducing the pressure.

If memory serves, in the TMI incident the operators shut off the feedwater because of a mistaken belief that the pressurizer had gone solid.
So it is really important, but not easy, to keep track of the water level within the reactor.

 

[Hiddencamper]

If memory serves, in the TMI incident the operators shut off the feedwater because of a mistaken belief that the pressurizer had gone solid.
So it is really important, but not easy, to keep track of the water level within the reactor.

They shut off safety injection.

Feedwater goes to the steam generators. They had no feedwater at the start of the event because main feedwater tripped due to a maintenance error, and the auxiliary feedwater valves were tagged out for maintenance. This allowed the steam generators to boil dry, causing the primary coolant system pressure to rise, lifting the PORV (which stuck open). Operators did restore auxiliary feedwater (and injected too fast to a dry steam generator causing some tubes to rupture). When aux feed was restored, primary system temperature began dropping, and with the PORV stuck open pressure kept dropping.

Safety injection initiated early in the event due to setpoints being too close to the physical system response to a turbine trip, and operators shut down safety injection and blocked its restart in accordance with operating procedures. If safety injection kept running there would have been no accident, however the reactor ended up becoming saturated and the resulting level swell caused the reactor to displace water into the pressurizer giving false high level indications.

Today, safety injection cannot be shut down until the reactor is subcooled and pressure is stable or rising (indicating no leaks or stuck open valves), even if level is high or the reactor coolant system may be solid. Additionally the safety injection signals for the reactor back in the 70s had to see a coincident low pressurizer level with low pressurizer pressure (other signals are low steam headed pressure/rate and high containment pressure). It was known before TMI that during a stuck open porv event, the pressurizer would show false high level indications preventing that particular automatic injection signal from running (this happened in Europe first, a few years later at Davis Besse, then at TMI, operators managed to figure it out at the first two plants, and failed at TMI). The NRC knew about this and did not pursue regulatory changes to remove the low level signal from the pressurizer safety injection signal. Today, level is not used for safety injection logic, only pressure. This is acceptable because even in odd / slow scenarios, if water level is dropping, the pressurizer heaters will lock out / shut off and pressure will drop low enough to start safety injection before the core is uncovered.

PWR guys please help fact check me. I'm knowledgeable but never licensed on a pwr.