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Reactors

Nuclear energy is naturally stored in the “nuclei” (cores) of certain heavy elements. It can be released, in the form of heat. The heat can be converted to mechanical energy, which can be used to make electricity or other consumer products, such as synthetic fuels.

The nuclear energy released by “fissioning” (splitting) an atom’s nucleus is about 1,000,000 times greater than the chemical energy released by combustion, which comes from the electrons shared by two or more atoms.

How do Nuclear Power Plants Work?
Electricity is produced by using the heat from fission to create mechanical energy, which turns an electric generator. In most cases the heat is used to make steam, which turns a turbine, that then turns the generator.

Power Reactors
The two types of light water nuclear reactors operating in the United States are the pressurized water reactor (PWR) and the boiling water reactor (BWR).  The heat for the BWR is produced in the same way as for a PWR.  However, the water is allowed to boil and the steam proceeds directly to the turbine.  Once the steam passes through the turbine, it is condensed back to water just like in the PWR.  This water is then pumped back to the reactor to be heated again, continuing the process.

In the United States, the Nuclear Regulatory Commission must certify the designs for proposed nuclear power plants before a construction and operation license can be issued.  While momentum is building in the U.S. – two new plants Vogtle 3 & 4 are currently being built in Georgia – other countries have already demonstrated the success of various plant technologies.  Generation IV (or Gen IV) designs are nuclear reactor designs still in the conceptual phase and represent the future of nuclear power.


 

Pressurized Water Reactors (PWRs)

When you pour hot cocoa into a mug, you may notice that the mug soon becomes warm, perhaps even too hot to hold.  This is because heat will always flow from a hot material into a cooler one.  This scientific law helps us understand how to move the heat energy from inside a reactor to a place where it can be changed into electrical energy.  Because of the heat produced by the fission reaction, water that is circulated through the core becomes extremely hot.  Generally, when water reaches 100 Celsius (212 Fahrenheit), it boils and turns into a gas called steam.  Gases take up more space than liquids.  But inside a reactor, there is only a limited amount of space and the water cannot turn into steam.  As a result, it can be heated to 315 Celsius (600 Fahrenheit) while still remaining a liquid.  We say that the water is under pressure.  Because the water in the core is under enough pressure to remain a liquid, the reactor is called a pressurized water reactor or PWR for short.

PWR’s have three separate systems of pipes, or loops for moving heat.  Water in these loops never mixes together.  However, heat energy from one-loop moves to another.  In the first loop pressurized water is pumped through the reactor and then through extremely strong pipes that lead to several steam-generators.

First Loop

Inside the steam-generators, water in the first loop flows through hundreds of tubes.  Water from the second loop flows around these tubes.  The first loop carries water that is 315 Celsius (600 Fahrenheit).  Because heat flows away from heated surfaces toward cooler surfaces, the heat from the first loop, it turns to steam.  This is because water in the second loop is under less pressure.

PWR

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Second Loop
The second loop caries the steam to the turbine.  A turbine is basically a pinwheel with many blades that are spun by steam.  At power plants, turbines are attached to generators, which change the mechanical energy of the spinning turbine into electrical energy.  A generator works by rapidly spinning a coil of wire inside a magnetic field.  This produces electricity.

After turning the turbine, the steam in the second loop has lost most of its heat energy.  It is cooled and turned back into water so that it can be used again in the second loop.  This operation takes place in the condenser, which is located under the turbine.  In the condenser, the second loop transfers some of its heat to the third loop.  Again, heat is transferred from a heated substance to a cooler one.  A glass of ice water in the summer is a model of how a condenser works.  If you pour ice water into a glass and leave it on a table for a while, you will find that the glass seems to be sweating.  Beads of water form on the outside of the glass.  We know water cannot pass through the glass.  The drops of water have come from moisture in the air.  Heat energy from the warm summer air has moved to the cold glass.  Just as water turns into steam when it is heated, water vapor condenses back into water when it loses heat energy.

Third Loop
In the power plant, a third loop contains cooling water.  Steam in the second loop is cooled in the condenser when it transfers some of its heat to water in the third loop.  The purpose of the third loop is to remove heat from the steam in the second loop.  It is important to remember that the water from one loop never mixes from another loop.  Only the heat is transferred.  When the cooling water in the third loop has passed through the condenser, it has absorbed heat from the second loop.  This heat has to be removed.


 

Boiling Water Reactors (BWRs)

ANS_bwr

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In a typical U.S.  boiling water reactor water inside a thick steel reactor vessel passes over a reactor core constructed of long cylindrical fuel rods. Inside each fuel rod is a stack of ceramic fuel pellets that contain fissile elements.

The energy of the fission reaction inside these rods is transferred to the water as heat, evaporating the water and producing steam.

The amount of energy produced is controlled — and can be shut off entirely — by small changes in the position of control rods inside the core that absorb the neutrons which fission the fuel.

The high temperature, high-pressure steam rapidly expands between turbine blades, causing the turbine to turn and rotate an electric generator. This produces electricity.

The steam exits the turbine and then passes through a condenser, where it is cooled by cold water from a cooling tower or another source, such as a lake. When cooled, the steam condenses to liquid water and is returned to the reactor core to begin the process again.

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