Once Through Steam Generator

Once Through Steam Generator (OTSG)

Once through steam generators are a type of heat exchanger used in pressurised water reactors (PWR) for boiling water into steam. Typically, there are two to four OTSGs per reactor. Each OTSG may be up to 21m in height and weigh up to 800 tons. Each OTSG is connected to a primary and secondary loop. The primary loop contains coolant heated by the reactor. The secondary loop contains the water and steam that is distributed to the steam turbine.

Primary Inlet (hot leg)

Primary coolant from the reactor enters the top of the OTSG. Due to the relatively high temperature of the coolant compared to its temperature at other points in the same system, the heated coolant from the reactor is referred to as 'hot'. The part of the system containing the hot primary coolant is referred to as the ‘hot leg’.  The primary coolant system does not boil despite being at a higher temperature, this is because the system pressure is higher.

Primary Outlet (cold leg)

After passing through the tubes, primary coolant exits the OTSG through the lower primary outlet. Due to its reduced temperature compared to when it entered the OTSG, it is now referred to as the ‘cold leg’. After leaving the OTSG, the coolant returns to the reactor where it is reheated.

Steam Outlet

Secondary coolant is heated as it passes through the shell-side of the OTSG. As heat is transferred to the secondary coolant, it begins to boil. Superheated steam leaves the OTSG and is delivered to the steam turbine(s).


Tubes are connected to the upper and lower tubesheets. When considering all tubes, they are referred to as a ‘tube bundle’. Primary coolant from the hot leg enters the top of the tubes and travels downwards through the tubes towards the lower tubesheet. Primary coolant is the tube-side fluid (flows inside the tubes) whilst secondary coolant is the shell-side fluid (flows outside of the tubes). Each tube acts as a heat exchanger allowing heat to pass from the primary coolant system to the secondary coolant system, but without them coming into direct contact with each other.

Upper Tubesheet

Tubesheets are used to evenly distribute tubes within the OTSG and to lend structural support to the tubes. In an OTSG, the tubes are welded to the tubesheets, although rolling and expanding tubes into tubesheets is more common.

Lower Tubesheet

Tubesheets are used to evenly distribute tubes within the OTSG and to lend structural support to the tubes. In an OTSG, the tubes are welded to the tubesheets.

Lower Primary Manway

Access to the lower bowl can be obtained through a manway.

Feedwater Inlet

Secondary coolant is fed into the OTSG through a feedring. Secondary coolant enters the OTSG as water and leaves as steam.


The downcomer is the annular space between the shroud and OTSG shell. Secondary coolant enters through the feedring, flows downwards via the downcomer, then enters the space surrounding the tubes and begins to flow upwards through the OTSG.


The outer casing of the OTSG is referred to as the ‘shell’.

Emergency Feedring Inlet

During transient conditions such as a loss of main feedwater (MFW), degraded power conditions, or a main steam line break, feedwater will be supplied to the steam generator via the emergency feedwater (EFW) system.

Encyclopedia - saVRee