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Electricity : Types of Plants : Pressurized Water Reactor |
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. 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. |
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