The Arrangement of PWR
A pressurized water reactor (PWR) is a type of power plant reactor consisting of two basic circuits having light water as the working fluid. In one of the circuits water is heated to a high temperature and kept at high pressure as well, so that it does not get converted into a gaseous state. This superheated water is used as a coolant and a moderator for the nuclear reactor core hence the name PWR or pressurized water reactor.
The secondary circuit consists of water at high pressure in the gaseous state i.e. steam which is used to run the turbine-alternator arrangement. The point of interaction between these two circuits is the heat exchanger or the boiler wherein heat from the superheated high pressure water converts the water in the secondary circuit to steam.
Advantages of PWR
- Much fewer control rods are required in a PWR. In fact for a typical 1000 MW plant just around 5 dozen control rods are sufficient.
- Since the two circuits are independent of each other, it makes it very easy for the maintenance staff to inspect the components of the secondary circuit without having to shut down the power plant entirely.
- A PWR has got a high power density and this, combined with the fact that enriched Uranium is used as fuel instead of normal Uranium, leads to the construction of very compact core size for a given power output.
- One feature which makes a PWR reactor very suitable for practical applications is its positive demand coefficient which serves to increase the output as a direct proportion to demand of power.
- The water used in the primary circuit is different from that used in the secondary circuit and there is no intermixing between the two, except for heat transfer which takes place in the boiler or heat exchanger. This means that the water used in the turbine side is free from radioactive steam hence the piping on that side is not required to be clad with special shielding materials.
Drawbacks of PWR
- The primary circuit consists of high temperature, high pressure water which accelerates corrosion. This means that the vessel should be constructed of very strong material such as stainless steel which adds to construction costs of PWR.
- PWR fuel charging requires the plant to be shut down and this certainly requires a long time period of the order of at least a couple of months.
- The pressure in the secondary circuit is relatively quite low as compared to the primary circuit hence the thermodynamic efficiency of PWR reactors is quite low of the order of 20
One important point to note here is that despite the changing loads the pressure in the primary circuit needs to be maintained at a constant value. This is achieved by installing a device known as pressure equalizer in the primary circuit. It basically consists of a dome shaped structure which has heating coils which are used to increase or decrease pressure as and when required depending on varied load conditions.