- Pressurized Water Reactor
Pressurized Water
Reactors (also known as PWRs) keep water under pressure so that it heats, but
does not boil. This heated water is circulated through tubes in steam
generators, allowing the water in the steam generators to turn to steam, which
then turns the turbine generator. Water from the reactor and the water that is
turned into steam are in separate systems and do not mix.
1. The Fuel
The fuel used in nuclear generation is primarily uranium 235. It is
manufactured as small round fuel pellets. A single pellet is less than an inch
long, but produces the energy equivalent to a ton of coal. The pellets are
placed end-to-end into fuel rods that are 12 feet long. Over 200 of these rods
are grouped into what is known as a fuel assembly.
2. Reactor
The process of producing electricity begins when uranium atoms are split
(i.e., fission) by particles known as neutrons. Uranium 235 has a unique
quality that causes it to break apart when it collides with a neutron. Once an
atom of uranium 235 is split, neutrons from the uranium atom are free to
collide with other uranium 235 atoms. A chain reaction begins, producing heat.
This reaction is controlled in several ways, including by control rods which
absorb neutrons. Control rods are inserted among the fuel assembly rods
that hold the uranium pellets. When they are in place, they absorb the atomic
particles that would normally initiate the chain reaction. When they are
withdrawn from the fuel assembly, fission is allowed to occur.
3. Pressurizer
The heat produced in the reactor is transferred to the first of three
water systems: the primary coolant. The primary coolant is heated to over 600 degrees
Fahrenheit. In a pressurized water reactor, a pressurizer keeps the water under
pressure to prevent it from boiling.
4. Steam Generator
The hot, pressurized water passes through thousands of tubes in nearby
steam generators. These tubes are surrounded by another water system called the
secondary coolant. The heat from the primary coolant is transferred to the
secondary coolant, which then turns into steam.
The primary and secondary systems are closed systems. This means that
the water flowing through the reactor remains separate and does not mix with
the water from the other system or the lake.
5. Turbine
The steam is piped from the containment building into the turbine
building to push the giant blades of a turbine. The turbine is connected to an
electric generator by a rotating shaft. As the turbine blades begin to spin, a
magnet inside the generator also turns to produce electricity.
6. Condenser Coolant
After turning the turbines, the steam is cooled by passing it over tubes
carrying a third water system, called the condenser coolant or lake water. The
steam is cooled so it condenses back into water and is returned to the steam
generator to be used again and again.
7. Lake or Cooling Towers
At some nuclear stations, lake water flows through thousands of
condenser tubes to condense steam back to water. It is then discharged down a
long canal (for cooling) and eventually enters the main part of the lake. At
other plants, the condenser cooling water is circulated through cooling towers
to remove the extra heat it has gained. The water is pumped to the top of the
cooling towers and is allowed to pour down through the structure. At the same
time, a set of fans at the top of each tower pulls air up through the condenser
water. This lowers the temperature of the water. After it is cooled, the
condenser water flows back into the turbine building to begin its work of
condensing steam again.
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Boiling Water Reactor
In Boiling Water Reactors (also known as BWRs), the water heated by
fission actually boils and turns into steam to turn the turbine generator. In
both PWRs and BWRs, the steam is turned back into water and can be used again
in the process.
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