POWER August 2010 - 48

NUCLEAR POWER
Inside the CANDU Reactor
The heart of each Bruce unit is the CANada
Deuterium Uranium (CANDU) pressurized
heavy water reactor. Developed
in Canada, the first CANDU reactor entered
service in 1962. The last CANDU
reactors to enter service were RPAS-3
and -4 and Kaiga-1 and -2 in India in
2000, followed by China's Qinshan Unit
3-1 in 2002 and 3-2 in 2003. According
to the CANDU Owners Group, there are
34 operating CANDU reactors in Canada
and abroad, not including the two Bruce
units currently being refurbished.
The CANDU design is quite different
from U.S. light-water designs. Instead of
enriching U-235 to 5% and using regular
or " light water " as a moderator to slow
down the neutrons for fission to occur
and as a coolant, CANDU reactors are fueled
with natural uranium (uranium with
an isotopic composition found in nature,
containing 99.3% U-238, 0.7% U-235,
and a trace of U-234) and use deuterium
(D2O) as a moderator and coolant.
Deuterium (heavy water) is an isotope
of hydrogen that contains an extra neutron
not found in hydrogen. Deuterium
occurs naturally in all bodies of water (1
part per 7,000 in the Great Lakes).
CANDU Reactor Core. Within a CANDU
reactor core, the calandria (a large metal
drum with flat end shields) is filled with
the heavy water moderator and pierced
by an array of horizontal tubes approximately
6.3 meters in length called " calandria
tubes " (Figure 2). Each calandria
tube, in turn, contains an inner " pressure
tube, " in which natural uranium
fuel bundles-the source of the heat
that creates steam-are laid end-toend.
The pressure tubes and calandria
tubes are made of a zirconium alloy that
absorbs fewer neutrons than other metals.
Collectively, the pressure and calandria
tube systems are referred to as " fuel
channels. " The calandria contains 480
fuel channels.
CANDU Fuel. Fuel for Bruce Nuclear
Generating Station is in the form of
compressed and sintered natural uranium
dioxide pellets that are sheathed and
sealed in Zircaloy-4 tubes. Thirty-seven
tubes (elements) are assembled between
two end plates to form one fuel bundle.
48
2. Down to the core. This photo shows
the core of the CANDU reactor, including
the calandria and shield tank assembly, during
a fuel channel replacement. Courtesy:
Bruce Power
3. Automated refueling. Specialty
robotics automate the task of feeding fuel
assemblies into one of the Bruce A reactors.
Courtesy: Bruce Power
4. Double your safety. The CANDU reactor design has two independent fast shutdown
systems that are physically separate and have their own power supplies and monitoring
equipment. Courtesy: Bruce Power
Shutdown system No.1
Shutdown system No.2
Helium
tank
Poison
tanks
Reactor
Shutdown
rods
Injection
nozzles
Absorber
elements
Moderator
Each of the 480 fuel channels contains
12 bundles, and each bundle weighs
22.5 kg. A small number of depleted uranium
fuel bundles are inserted at specific
locations to fine-tune reactivity in
the core. Fresh fuel bundles are received
at a fuel-receiving area, enclosed in
www.powermag.com
protective palletized crates containing
36 bundles. Fuel storage areas have the
capacity to store approximately 23,000
bundles.
CANDU Refueling. The CANDU system
employs a unique, online, full-power refueling
system (Figure 3). Two identical
POWER | August 2010
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POWER August 2010

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