Basic Components of PWR (part 5)

Reactor Coolant Pumps 

Main function:

Each contains a vertical, signal- stage, shaft pump designed to pump large volumes of coolant at high pressures and high temperatures.

An impeller attached to the bottom of the rotor shaft pumps reactor coolant. The coolant is drawn up through the bottom of the casing, up through the impeller, and discharged through the diffuser and an exit nozzle in the side of the casing. The impeller and casing represent proven conventional designs. The diffuser converts velocity head from the impeller to pressure head, and the circular casing collects the flow and discharged it to the single discharge nozzle. All parts of the pump in contact with the reactor coolant are of stainless steel, except for the bearing and certain special components.

Steam Generator

Function:

Generating the steam outside the PWR, steam generators are heat exchangers with pressurized water on the hot side (hot leg).

In large PWR, using of 4 steam generators are necessary to produce steam at about 293 C and, 6 MPA with an over all efficiency in the range of 32-33%.

Steam generator consists of three integral sections:

1.    An Evaporator section consists of: U- tube heat exchanger where heat from the reactor is transferred through the tube walls to convert pure secondary- side feed water into steam. The welding of the tubes must ensure to be with zero leakage across the tube joints. An emergency feed water connection is provided at the top of the bundle for feed water addition in the event of black out, or, other conditions where main feed water pumps are not available.

2.    A moisture Separation Section: A moisture separator recirculates the flow on the hot leg side of the tubes, the recirculated water flows through the space under preheated section, which provided to isolate the tube sheet from the colder feed water.

3.    The steam Drum: It has two bolted and gas kited access openings for inspection and maintenance.

Turbine          

It is a simple machine that consists of a series of bladed wheels fixed to an axial, which rotates at high speed as steam at high temperature and pressure strikes the turbine blades.

The inlet to the turbine is always dray steam and its temperature would be reduced and some of the steam is condensed.

 Types of Turbines:

1.    High Pressure Turbines: it is a double flow element with an impulse control stage followed by reaction blading in each end of the element. The steam enters the high-pressure element through two steam chests, one, located on each side of the high-pressure casing through 4 inlet pipes ( 2 in the base and 2 in the cover) and the other through the cross under piping to the moisture separator-reheaters. The rotors in high-pressure turbine are machined from an alloy steel forging.

2.    Low pressure Turbines: It is a double flow element employing reaction blading. Steam enters at the center of the bald path; flows through the blading to an exhaust opening at each end, and then down word to the condenser. Rotors in low pressure turbines are consisting of a series of alloy steel discs shrunk on a shaft and keyed in position are also machined from alloy still forgings. There is an intermediate pressure turbine that feed to it the steam with intermediate pressure.