Nuclear Energy

Coolant: A liquid or gas circulating through the core so as to transfer the heat from it. In light water
reactors, the moderator which is water also serves as the primary coolant. Except in Boiling Water
Reactors (BWRs), this primary coolant passes through another heat exchanger, to convert another
loop of water into steam. This steam drives the turbine. The advantage of this design is that the
radioactive water/steam (that is, the primary coolant) does not come into contact with the turbine.
Pressure vessel: Usually a robust steel vessel containing the reactor core and moderator/coolant.
Steam generator (not in BWR) or Secondary heat exchanger: Here, the primary coolant bringing
heat from the reactor is used to convert another loop of water into steam to drive the turbine.
Containment: The structure around the reactor core which is designed to protect it from outside
intrusion and to protect those outside from the effects of radiation in case of any malfunction inside.
It is typically a metre-thick concrete and steel structure.
Refuelling the reactors: In most reactors, the refuelling is done at intervals of 1-2 years, when a
quarter to one-third of the fuel assemblies are replaced with fresh ones. For this, the reactor needs to
be shut down. But in the CANDU type reactors, which have pressure tubes (rather than a pressure
vessel enclosing the reactor core) and can be refuelled under load by disconnecting individual
pressure tubes, the reactor need not be shut down.
Types of Nuclear Power Reactors
At a basic level, reactors may be classified into two classes: Light Water Reactors (LWRs)
and Heavy Water Reactors (HWRs). LWRs are largely of two types, Pressurised Water Reactors
(PWRs) and Boiling Water Reactors (BWRs); each come in multiple variations.. Heavy Water
Reactors can be of different types, one of the most well known being the CANDU (acronym for
“CANada Deuterium Uranium”) reactors developed by Canada which are a type of Pressurised
Heavy Water Reactors (PHWRs).
LWRs are the most widespread type of reactors in operation today. Of the 437 reactors in
operation at the end of 2009, 357 were LWRs, of which 265 were PWRs and 92 BWRs. Apart from
these, the other reactor types in operation were: 45 pressurized heavy-water reactors (PHWRs), 18
gas-cooled, graphite-moderated reactors, (GCRs), 15 light-water-cooled, graphite-moderated
reactors (LWGR), and 2 fast breeder reactors (FBRs). liii
Below, we discuss the most well-known type of nuclear power reactor – the PWR, the
reactor design of most of India's reactors - the PHWR or CANDU Nuclear Power Reactor, and also
the Fast Breeder Reactor, which India has been trying to build for many decades and of which there
are only two reactors in operation today, a prototype unit in Japan and the BN-600 reactor Russia
(which is technically not a fast breeder – See Chapter 9).
(i) Pressurised Water Reactor
This is the most common type, with 265 in use for power generation and several hundred
more employed for naval propulsion. PWRs use ordinary water as both coolant and moderator.
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