NUCLEAR
facts
How does a nuclear
reactor work?
SIMPLY PUT, A NUCLEAR REACTOR IS A DEVICE THAT PRODUCES HEAT. IN A NUCLEAR POWER STATION,
THE REACTOR PERFORMS THE SAME FUNCTION AS A BOILER IN A CONVENTIONAL COAL, GAS OR
OIL-FIRED STATION.
Whether from a conventional boiler All CANDU reactors heat water which
produces steam in the steam generator.
or a nuclear reactor, heat is required
This steam spins a large turbogenerator
to turn water into steam. This steam which produces electricity.
is needed to spin large turbines,
which in turn drive generators that
produce electricity. A major differ-
ence between a nuclear power sta-
tion and a conventional fossil-fuelled
station is that there is no release of
combustion products to the environ-
ment from a nuclear station.
All nuclear reactors operate on the same basic principle, although there are
different kinds of nuclear reactors in use throughout the world. A nuclear
reactor creates heat by splitting uranium atoms. This “fission” of uranium
atoms is called a “nuclear reaction.”
When the centre or nucleus of a uranium atom is struck by a neutron travel-
ing at the right speed, it splits into fragments that separate rapidly and generate
heat. It also gives off new neutrons. To sustain a continuous nuclear reaction,
the speed of these neutrons must be slowed down or moderated. Canadian
nuclear reactors use heavy water as the neutron moderator. Chemically, heavy
water is called deuterium oxide. Thus the Canadian reactor is named CANDU,
for CANada Deuterium Uranium.
Because of the heavy water moderator, CANDU reactors can use natural
uranium as the source of fuel to create the nuclear reaction.
Each of the fuel channels in a CANDU reac-
tor is filled end to end with fuel bundles con-
taining the uranium oxide nuclear fuel.
What is the core of a CANDU reactor?
The core of a CANDU reactor is contained in a large, horizontal, cylindrical
tank called a “calandria” which contains the heavy water moderator. Several
hundred fuel channels run from one end of the calandria to the other. Each
channel has two concentric tubes. The outer one, called the calandria tube,
forms the inside boundary of the calandria. The inner one, called the pressure
tube, holds the fuel and the pressurized heavy water coolant. The fuel, in the
form of bundles of rods containing uranium pellets, is inserted into the pres-
sure tubes by remotely operated fuelling machines, which can function while
the reactor is operating.
In a closed circuit, the heavy water
coolant is pumped through the tubes
containing the fuel bundles to pick
up heat generated from the nuclear
reaction, then to steam generators to
produce steam from ordinary water
and back to the reactor. The steam
is piped outside the reactor contain-
ment building to conventional tur-
bines and generators that produce
electricity. In this way, the nuclear
reactor is separate from the equip-
ment used to produce electricity.
How do Canada’s
3
nuclear reactors
compare?
5
Very well.
2 Most other reactors in the world use
light or ordinary water, or graphite
as a moderator. These are less effi-
cient moderators than heavy water,
and the fuel has to be enriched with
4
1 fissionable atoms of uranium.
Currently CANDU reactors use nat-
ural uranium, resulting in better
fuel efficiency and lower fuel cost.
See also the pages on nuclear power
at the Web site of Ontario Power
6 Generation Inc. www.opg.com
and
the Web site of Atomic Energy
7 of Canada Limited www.aecl.ca
1. Emergency shut 3. Adjuster rods 5. Inlet feeder pipes
down system 4. Fuel channels and 6. Moderator inlets
2. Calandria end fittings 7. Reactor vault
130 Albert Street, Suite 1610
Ottawa, Ontario K1P 5G4
Tel. (613) 237-4262
Fax (613) 237-0989
www.cna.ca
