Nuclear Power Plant Main Steam Systems

Main Steam System

Function

The Main Steam System used in any power plant has 1 major purpose:

Provide steam from the source (reactor, steam generator, or steam separator) to the turbine

The system may have several other functions

Provide the ability to prevent overpressurization of the steam source (if the source puts out more heat than the turbine can accept)
Provide the ability to prevent overcooling of the reactor coolant system (if the steam system draws off more heat than the source can provide)

Major Components

The major components in most Main Steam Systems are:

Steam Line from the Reactor (if a BWR) , Steam generator (if a PWR), Steam Separator (if a GCR) to the turbine.
Main Steam Isolation Valve - usually an air operated or motor operated valve used to isolate the steam source from the turbine.
Safety valves - large relief valves that will open if steam pressure gets too high (same purpose as the pop valve on your hot water heater).
Power Operated Relief Valves - large air or motor operated valves that usually lift at a setpoint lower than the safety valves - in order to keep the steam pressure from getting too high.
Non-return Valve - a large valve that prevents backward steam flow in the steam line.

System Diagram

The sketch below illustrates the main steam system flow path.

The orange color shows the flow path of the steam as it goes, in this case, from the steam generator to the high pressure turbine (then leaving in blue because there is water in the steam) then going to the moisture separator where the water is removed, then (in orange) to the low pressure turbine.

Graphic courtesy NEK