Environmental Engineering Reference
In-Depth Information
Reactors have several common components:
•
Fuel:
Uranium is the basic fuel used in all reactors. The uranium is enriched and processed into uranium
oxide (UO
2
). The UO
2
is formed into ceramic pellets and loaded into long tubes, usually zirconium al-
loy. When grouped together in a bundle, the tubes form a fuel assembly that is located in the core of the
reactor.
•
Moderator:
It is usually water, occasionally heavy water, and rarely graphite. The moderator will slow
down the release of neutrons in the core.
•
Control rods:
The rods are made of cadmium, hafnium, or boron. They are inserted and withdrawn from
the core to control the rate of fission by absorbing neutrons.
•
Coolant:
The coolant is a liquid or gas that is circulated through the core to transmit heat away.
•
Containment:
Modern reactors have a containment structure that encapsulates the core. It is designed to
prevent external intrusion and to protect everything surrounding it from the effects of radiation caused
from a malfunction inside. It is usually constructed of steel and at least three feet of steel reinforced con-
crete.
•
Core:
The core may contain up to 75,000 fuel rods. The core is where fuel assemblies are located and
where nuclear fission takes place.
Nuclear reactor types
Nuclear reactors have several common features in construction and operation, and are grouped into four gener-
ations. Each generation is grouped by age, and within each generation there are several reactor types.
•
Generation I
reactors were designed and built during the 1950s and 1960s. Very few of these early
designs are still running. Generation I reactors are only expected to last about 40 years. The safety
devices are primitive by today's standards and some do not have containment domes. For example, a
Graphite-Moderated Reactor is a generation I type of reactor that uses water for cooling and steam and
uses graphite as the moderator. Used in the former Soviet Union (USSR), these reactors were very un-
stable and are no longer being used. (see Chernobyl case study).
•
Generation II
reactors are in use today and are known as Pressurized Water Reactors, Boiling Water Re-
actors, and Heavy Water Reactors. Three types of generation II reactors are:
•
Pressurized water reactors
are the most common type of reactors in use today. They use ordinary
water as both a coolant and a moderator. They have a primary and secondary circuit. The primary cir-
cuit supplies water under high pressure to cool the core. A separate secondary circuit supplies steam
to drive a turbine generator, creating electricity. The core holds 150 to 250 fuel assemblies, each con-
taining 200 to 300 fuel rods for a total of 30,000 to 75,000 fuel rods. A single fuel assembly is ap-
proximately 13 feet high and weighs about 1,450 pounds.
•
Boiling water reactors
use ordinary water as both a coolant and a moderator. Using a single circuit,
water is supplied under low pressure to cool the reactor. The lower pressures allows the water to boil
and is designed to permit 12 percent to 15 percent of the water in the top portion of the reactor to con-
vert to steam, which is used to drive the turbine generator. The core holds 750 fuel assemblies, each
containing 90 to 100 fuel rods, for a total of 67,500 to 75,000 fuel rods. A single fuel assembly is ap-
proximately 14.5 feet high and weighs about 704 pounds.
•
Heavy water reactors
use uranium (U-235) as fuel and requires a more efficient moderator, heavy
water. Heavy water is made up of deuterium, which is an isotope of hydrogen that contains an extra
neutron, making it heavier than water. It has two circuits. The primary circuit supplies heavy water to
