Nuclear Power Plant Types: Advantages and Disadvantages
TYPES OF REACTOR:
Figure-1 :Typical Diagram
of a MAGNOX Reactor
Figure 1 shows,
MAGNOX- Magnesium Alloy Graphite Moderated Gas
Cooled Uranium Metal Reactor
For magnox reactors, uranium ore
is chemically processed to yield uranium metal that is formed into fuel rods
which are inserted into cans made of an alloy consisting mostly of magnesium with small
amounts of aluminium and other metals. Heat generated due to fission is
extracted by CO2 gas that is circulated from the reactor core to the
steam generators. Graphite which is the moderator is, in the form of
rectangular blocks, placed in a steel pressure vessel. Fuel rods are placed
inside the graphite blocks such that coolant can flow through the annular space
between the fuel rod and the moderator. Control rods made of Boron are inserted
from the top. The whole assembly is housed inside a concrete containment
vessel. The steam generator is placed outside the containment. Gas circulator, turbo
alternator and the feed pump are also shown. Stand pipes, which are used for
fuel loading and unloading, are inserted from the top. Fueling machine can operate
from the top when the reactor is on power.
Advantages:
- Natural
uranium is used as fuel. So no enrichment is necessary.
- Fuel loading
and unloading including small repair is possible when the reactor is power. Such feature allows high level of availability.
- Track record of safety is excellent.
Heavy water
is water in which the hydrogen is replaced by its heavier isotope, deuterium, 2H.
The term light
water designates water containing the protium isotope (contains no neutrons)
Figure-2 : Typical Diagram of a Pressurized Water Reactor (PWR)
Figure-2 shows a Pressurized Water Reactor (PWR). The
containment structure includes Reactor Pressure Vessel, Steam Generator,
Pressurizer and the Primary Pump. Steam turbine, generator and condenser are situated
just like any other conventional thermal power station. It may be seen that the
control rods are inserted from the top. Fuels which are slightly enriched ( 3%
to 4%) uranium dioxide are in the form
of fuel bundles canned in Zircalloy. Light water is moderator as well as
coolant. High pressure is maintained inside the reactor vessel so that water
does not boil in the core. This is known as primary circuit. Steam is generated
at a separate steam generator which is known as secondary circuit. Incidentally,
Three Mile Island Reactor where accident occurred in 1979 was of this type.
Advantages:
- Water from the reactor and the water in the steam
generator that is turned into steam never mix. In this way, most of the
radioactivity stays in the reactor area.
-
In case of any accident radioactive
fall out is contained inside the containment, it cannot spread to surrounding
area. It was amply proved in case of Three Mile Island Accident in 1979.
Disadvantages:
- Loading and
unloading cannot be done when the reactor is on power. Reactor has to be shut
down and then the same is affected.
- Since enriched fuel is
used, enrichment services are necessary.
Figure-4 Figure 3: Typical Diagram of a Boiling Water
Reactor (BWR)
Figure-3 shows a Boiling Water Reactor (BWR). Unlike
the PWR it does not have a separate steam generator or in other words, the
Primary and the Secondary circuit is same. Water is boiled inside the pressure
vessel. Hence, steam generated is radioactive. Fuel elements, canning material,
moderator and coolant are exactly same as those of PWR. The distinguishing
feature of BWR is that the control rods are, on control point of view, inserted
from the bottom. Loading and unloading is exactly similar to PWR.
Advantages:
- Radioactive
fall out from the pressure vessel is contained in the containment.
Disadvantages:
- Loading and
unloading cannot be done when the reactor is on power. Reactor has to be shut
down and then the same is affected.
- Turbine
hall is also radioactive zone.
- Since
enriched fuel is used, enrichment services are necessary.
CANDU Reactor: which
is heavy water moderated and cooled reactor. The acronym "CANDU", a registered
trademark of Atomic Energy of Canada Limited,
stands for "CANada Deuterium Uranium". This is a reference to its
deuterium-oxide (heavy water) moderator
and its use of uranium
fuel (originally, natural uranium). All current power reactors in Canada are of the
CANDU type. Canada
markets this power reactor abroad .The reactors are used in nuclear power plants to produce nuclear
power from nuclear fuel.
The primary loop is in yellow and orange, the secondary in blue and red.
The cool heavy water in the calandria can be seen in pink, along with
partially-inserted shutoff rods.
The CANDU reactor is conceptually
similar to most light water reactors, although it differs in
the details.
