Boiling Water Reactor (BWR) :
This is the simplest type of water reactor. It has a steel pressure vessel surrounded by a concrete shield.
Fuel used is enriched uranium oxide, Ordinary water is used both as moderator and coolant. The steam is generated in the reactor itself.
Feedwater enters the reactor vessel at the bottom and takes the heat produced due to the fission of fuel and gets converted into steam.
This steam leaves the reactor at the top and after passing through the turbine and condenser returns to the reactor.
Uranium fuel elements are arranged in a particular lattice form inside the pressure vessel containing water.
A BWR assembly comprises 90-100 fuel rods and there are up to 750 assemblies in a core holding up to 140 tonnes of uranium.
The secondary control system involves restricting water flow through the core so that more steam in top part reduces moderation.
Most of the radioactivity in the water is very short-lived (mostly N-16, with a 7-second half-life), so the turbine hall can be entered soon after the reactor is shut down.
Advantages:
this reactor includes a small size pressure vessel, high steam pressure, simple construction and elimination of heat exchanger circuit resulting in a reduction in cost and gain in thermal efficiency.
Overall efficiency is about 33%.
Disadvantages:
1. In view of the direct cycle there is a danger of radioactive contamination of steam, therefore, more elaborate safety measures are to be provided for piping and turbine. These add to cost.
2. Because of the danger of small amounts of fissile materials passing through along with the coolant, more biological protection is required.
3. Wastage of steam results in lowering of thermal efficiency on the part-load operation.
4. It can not meer sudden increase in load.
The 420 MW power station, at Tarapur (India), consists of two enriched uranium reactors of the boiling water type.
These reactors were built with the help of the G.E.C. of the United States and became operational on 1.4.1969.
Pressurized Water Reactor (PWR):
A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of uranium.
The pressure vessel is of steel. Water under pressure is used both as a coolant and moderator.
The pressure vessel and the heat exchanger are surrounded by a concrete shield.
In this reactor, bulk boiling water is prevented as the water
is pressurized to about 150 atmospheres.
The hot water from the reactor flows to a heat exchanger (or steam generator) where its heat is transferred to the feed water to generate steam.
The secondary cooling operates at a low pressure. The primary coolant then flows from the heat exchanger to the primary circulating pump which pumps it back to the reactor.
The steam is condensed in the condenser and the condensate returns to heat exchanger forming a closed circuit.
The primary circuit of a pressurized water reactor (PWR) contains a 'pressurizer'. This is simply a pressure vessel with an electric heating coil at the bottom and a water spray at the top.
The top of the vessel is filled with steam at primary circuit pressure. When the primary circuit pressure decreases, the heating coil gets energized and boils the water to form steam resulting in an increase in steam content in the vessel.
This results in an increase in pressure of the primary circuit. In case the steam pressure of the primary circuit becomes too high cold water is sprayed into the steam in the pressurizer.
The steam is condensed and therefore primary circuit pressure is reduced.
The steam generated is of rather poor quality, temperature around 250°C and pressure 42 kg/cmſ.
Advantages:
1. compactness
2. the possibility of breeding plutonium
3. isolation of radioactive materials from the main steam system
4. cheap light water can be used as coolant-cum-moderator (v) high power density,
5. The reactor responds to supply more power when the load increases. The positive power demand coefficient makes this
almost automatic.
Disadvantages:
1. Use of high-pressure water system. So a strong pressure vessel is required which results in high cost.
2. Formation of low temperature (250° C) steam.
3. Use of expensive cladding material for prevention of corrosion.
4. High losses from the heat exchanger.
5. High power consumption by auxiliaries.
6. In comparison to other types, the requirement of more elaborate safety devices.
7. These reactors cannot be refueled while operating and for recharging the reactor is to be shut down for a couple of months. Also, there is difficulty in fuel element design and fabrication circuits.
8. The thermodynamic efficiency of this plant is low (about 20%) due to low pressure in the secondary.
The reactors installed at Rajasthan Atomic Power Station, Madras Atomic Power Station, and Narora Atomic Power are of pressurized water reactor type. However, the Project
uses heavy water as a coolant and moderator. Such reactors are known as pressurized heavy water reactors (PHWR).