Pressurized Water Reactor (PWR) – Advantages and Disadvantages
Pressurized Water Reactor (PWR):
It is a thermal reactor, using enriched uranium oxide, clad in zircalloy as fuel. 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 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 increase in steam content in the vessel.
This results in the 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/cm2.
Advantages Of Pressurized Water Reactor :
(ii) Possibility of breeding plutonium,
(iii) Isolation of radioactive materials from the main steam system,
(iv) Cheap light water can be used as coolant-cum-moderator,
(v) High power density, and
(vi) The reactor responds to supply more power when the load increases. The positive power demand coefficient makes this almost automatic.
However, it suffers from the following drawbacks:
(i) Use of high pressure water system. So a strong pressure vessel is required which results in high cost.
(ii) Formation of low temperature (250° C) steam.
(iii) Use of expensive cladding material for prevention of corrosion.
(iv) High losses from heat exchanger.
(v) High power consumption by auxiliaries.
(vi) In comparison to other types, requirement of more elaborate safety devices.
(vii) These reactors cannot be re-fuelled 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.
(viii) The thermodynamic efficiency of this plant is low (about 20%) due to low pressure in the secondary circuit.
The reactors installed at Rajasthan Atomic Power Station, Madras Atomic Power Station and Narora Atomic Power Project are of pressurized water reactor type. However they use heavy water as coolant and moderator. Such reactors are known as pressurized heavy water reactors (PHWR).
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