Nuclear Steam Power Plants
1. Pressurized Water Reactor (PWR)
Pressurized water reactors (PWRs) are nuclear power reactors that use ordinary
water under high pressure as coolant and neutron moderator. The primary coolant loop is kept
under high pressure to prevent the water from boiling. This puts strong requirements on the
piping and pressure vessel and hence increases construction costs.
PWRs are one of the most common types of reactors and are widely used all over the world.
More than 230 of them are in use to generate electric power.
PWR has two coolant loops, so the water in the secondary loop is not contaminated by
Ordinary water is used as primary coolant in a PWR and flows through the reactor at
a temperature of roughly 315°C (600°F). The water remains liquid despite the high
temperature due to the high pressure in the primary coolant loop (usually around 152 bar
[2200 psig]). The primary coolant loop is used to heat water in a secondary circuit that
becomes saturated steam (in most designs 62 bar [900 psi], 276°C [530°F]) for use in
the steam turbine.
In a Secondary Cooling System (which include the Main Steam System and the
Condensate-Feedwater Systems), cooler water is pumped from the Feedwater System and passes
on the outside of those steam generator tubes, is heated and converted to steam. The steam
then passes through the a Main Steam Line to the Turbine, which is connected to and turns
the Generator. The steam from the Turbine condenses in a Condenser. The condensed water is
then pumped by Condensate Pumps through Low Pressure Feedwater Heaters, then to the Feedwater
Pumps, then to High Pressure Feedwater Heaters, then to the Steam Generators. The diagram above
simplifies the process by showing the steam turbine, condenser, pumps, feedwater heaters, the
steam generator, moisture separator and Reheater.
2. Boiling Water Reactor (BWR)
The BWR typically allows bulk boiling of the water and is characterized by two-phase fluid
flow (water and steam) in the upper part of the reactor core.
The operating temperature of the reactor is approximately 300°C (570°F) producing steam at
a pressure of about 70 bar (1000 psi). Current BWR reactors have electrical outputs of 570
to 1300 MWe.
The circulated water eventually is heated enough to convert to steam. Steam separators in the
upper part of the reactor remove water from the steam. The steam then passes through the Main
Steam Lines to the Turbine. The steam typically goes first to a smaller High Pressure (HP)
Turbine, then passes to Moisture Separators, then to the 2 or 3 larger Low Pressure (LP) Turbines.
There are 3 low pressure turbines, as is common for 1000 MWe plant. The turbines are
connected to each other and to the Generator by a long shaft.
The steam, after passing through the turbines, then condenses in the Condenser, which is
at a vacuum and is cooled by ocean, sea, lake, or river water. The condensed steam then is
pumped to Low Pressure Feedwater Heaters. The water then passes to the Feedwater Pumps which in
turn, pump the water to the reactor and start the cycle all over again.
Nuclear power plants generally cannot reheat process steam due to safety requirements for
isolation from the reactor core. This limits their thermodynamic efficiency to the order
TPT can help you in following questions:
- Optimization of thermodynamic design of FW Heaters and Deaerator
- Optimization of thermodynamic cycles improves of the efficiency of power plants
- Failure analysis & reliability/availability for improvement of existing heat exchangers