PWR and BWR Nuclear Reactors

Two types of nuclear reactors most widely used are

1. Pressurised Water Reactor (PWR) Power Plant.

2. Boiling Water Reactor (BWR) Power Plant.

1. PWR Power Plant :

Figure shows the details of this power plant.

The coolant in the primary circuit gets heated by absorbing the heat energy liberated in the reactor core due to nuclear fission. The hot liquid flows to the heat exchanger via a `pressurisizer'. `Pressurisers located in the primary circuit of the plant is a vessel with a heater at its bottom and water spray at its top. The top of this vessel is filled with steam at primary circuit pressure. If primary circuit pressure drops, then the heater is operated which generates steam and increases the steam content in the vessel. This results in increase of pressure. On the other hand, if primary circuit pressure becomes too high, then the cold water is sprayed into the steam through the sprayer. This condenses the steam and reduces the primary circuit pressure.

In the heat exchanger the hot liquid of primary circuit loses its heat to feed water of secondary circuit. Feed water evaporates into steam and passes into the turbine. Turbine being coupled to the alternator helps generation of electric power. Exhaust steam from the turbine is condensed in the steam condenser and the condensate is recirculated as feed water. This is called secondary circulation. The coolant leaving the heat exchanger is pumped back to the reactor as shown.

Advantages of PWR :

• it makes used of single fluid coolant moderator.

light water used in reactor is cheap and available in plenty.

• more compact.

• high power dentsity.

• fission products remain contained in the reactor itself and are not circulated.

• steam supplied to turbine is free from contamination.

• cooling system is simple.

• suitable for naval propulsion units.

Disadvantages :

• high primary loop pressure requires strong, costly, leak proof vessel.

• low pressure and temperature in secondary loop result in poor thermodynamic efficiency.

• steam produced requires superheating.

• use of water under pressure at high temperature creates corrosion problem

• preparation of fuel element is expensive.

• fuel suffers radiation damage.

• reactor is to be shut down for recharging.

2. BWR Power Plant :

Figure is the diagram of nuclear power plant employing Boiling Water Reactor. In this plant, steam is generated in the reactor itself. The feed water acts both as coolant and a moderator. Heat energy liberated during nuclear fission in the reactor evaporates the feed water into steam. The steam leaves the reactor and enters a turbine through a throttle valve, It does work in the turbine and exhausts into a condenser. Here it is cooled and the condensate formed is pumped back to the reactor. This type of plant is relatively simpler than PWR plant since the feed pressuriser and heat exchanger are eliminated. Steam leaving the reactor may be radio-active. Hence the piping and turbine should be shielded.

Advantages of BWR :

• there is only one working loop.

• pressure inside the vessel being not very high thicker vessel is not required.

• metal temperature is low for a given output.

• thermal efficiency is about 30% as compared to 20 to 22% in PWR.

• capable of meeting fluctuating load requirements.

• heat exchangers, pumps and auxiliary equipment are reduced or eliminated resulting is higher thermal efficiency.

• preferred in 100 — 1000 MW plants.

• various cycles such as natural circulation single cycle, forced circulation single or dual cycle can be adopted.

Disadvantages :

• limited power density i.e., 3 to 5% by mass can be converted to steam per pass through boiler.

• turbines require shielding.

• wastage of steam occurs in case of part load operations.