Power Plant Engineering

10.16 LIGHT WATER REACTORS (LWR) AND HEAVY WATER
REACTORS (HWR)
Light water reactors use ordinary water (technically known as light water) as coolant and mod-
erator. They are simpler and cheaper. But they require enriched uranium as their fuel. Natural uranium
contains 0.6% of fissionable isotope U
235
and 99.3% of fertile Lj
23
and to use natural uranium in such

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POWER PLANT ENGINEERING
reactors it is to be enriched to about 3%, U
235
and for this uranium enrichment plant is needed which
requires huge investment and high operational expenditure. Heavy water reactors use heavy water as
their coolant and moderator. They have the advantage of using natural uranium as their fuel. Such
reactors have some operation problem too. Heavy water preparation plants require sufficient investment
and leakage of heavy water must be avoided as heavy water is very costly. Heavy water required in
primary circuits must be 99% pure and this requires purification plants heavy water should not absorb
moisture as by absorbing moisture it gets degraded. In order to have sufficient quantity of heavy water
required for nuclear power plants, the work is fast progressing in our country on four heavy water plant.
These plants are situated at Kotah (100 tonnes per year), Baroda (67.2 tonnes), Tuticorin (71.3 tonnes)
and Talcher (67.2 tonnes per year). These plants will give our country an installed heavy water produc-
tion capacity of about 300 tonnes per year.
10.16.1 Importance of Heavy Water
The nuclear power plants of Kota in Rajasthan, Kalpakkam in Tamil Nadu and Narora in U.P. use
heavy water as coolant and moderator. All these projects have CANDU reactors using natural uranium
as fuel and heavy water as moderator. After this enriched uranium natural water reactor at Tarapur, the
CANDU reactors are the second generation of reactors in India's nuclear power programme. The CANDU
reactor will produce plutonium which will be the core fuel for fast breeder reactor. In fact in breeder
reactor heavy water is used as moderator.
A CANDU reactor of 200 mW capacity requires about 220 tonnes of heavy water in the initial
stages and about 18 to 24 tonnes each year subsequently. Therefore, about one thousand tonnes of heavy
water will be required to start the different nuclear power stations using heavy water. The total capacity
of different heavy water plants will be about 300 tonnes per year if all the heavy water plant under
construction start production. It is expected that heavy water from domestic production will be available
from Madras and Narora atomic power plants. The management of the heavy water system is a highly
complicated affair and requires utmost caution. Heavy water is present in ordinary water in the ratio 1 :
6000. One of the methods of obtaining heavy water is electrolysis of ordinary water.