Fission and Fusion
Fission and fusion are special types of transmutation reactions. Fission occurs when a large nucleus is split into two smaller nuclei of approximately equal mass. This is typically accomplished by hitting the nucleus with a neutron which destabilizes the nucleus, causing the nucleus to fall apart. When this occurs, fission reactions release large amounts of energy. A large nucleus requires more energy to hold together than two smaller nuclei, so that extra energy not needed is released. To the right is an animation of fission. Below is the most common version of a fission reaction:
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Fission Nuclear Reactors
Fission reactions occur inside nuclear reactors under controlled conditions. When a nuclear reaction is controlled, the energy released from the fission can be used to generate electricity. In a nuclear reactor, a controlled chain reaction occurs. A neutron is fired at some uranium-235 nuclei. When the neutron hits the nucleus, it splits it into two smaller nuclei. Along with the two smaller nuclei, neutrons are expelled. Each neutron expelled by the fission reaction goes on to hit another nucleus and then another to produce a continuous chain reaction. An animation is to the left, and depiction of this chain reaction is below. |
A controlled fission reaction in a power plant is fairly simple to understand. The picture below should help to allow you to understand. A description is below:
In a fission reactor, the uranium-235 is allowed to undergo a chain reaction inside the containment building. Several safety features are in place to prevent the chain reaction from going too fast. The first is the control rods. Control rods are made of a material that absorbs neutrons. When the control rods are put into the reactor vessel (where the fission reaction occurs), the control rods absorb some of the neutrons given off by the fission reaction, thereby slowing the reaction down.
The heat from this reaction is used to heat some sort of fluid (usually a molten salt material) that is pumped through a pipe. The pipe flow into a steam generator where it cools as it heats the water to steam in the generator. The cooled fluid is then pumped back into the reactor vessel to be reheated.
The heat from this reaction is used to heat some sort of fluid (usually a molten salt material) that is pumped through a pipe. The pipe flow into a steam generator where it cools as it heats the water to steam in the generator. The cooled fluid is then pumped back into the reactor vessel to be reheated.
The heat in the pipe turns water into steam in the steam generator. This pressurized steam flows past a turbine which turns a generator. As the generator turns, it rotates an electric coil in a magnetic field, which produces electricity.. The steam will cool down as it passes over pipes that contain cold water, and the steam turns back into water. The water is then pumped back into the steam generator to be reheated.
As the steam is then condensed back into water by running the closed steam pipe through cold water. This cold water warms as the steam cools. The cooling water is now too warm to be dumped back into the nearby reservoir, so it must be cooled in a cooling tower. Cooling towers look like this: |
The water vapor escaping the tower has never been in direct contact with the nuclear material, and is therefore not harmful. Each of the three loops in the reactor, the steam generator/turbine and the cooling water are not in contact with the previous, and each pipe is lined with lead to prevent contamination of nuclear material. The safeguards at a nuclear power plant make it safer to live next to that a coal fired power plants.
Fusion
Another type of transmutation reaction is fusion. fusion occurs when two small nuclei join to form a large nucleus and large amounts of energy. Fusion reactions produce about 1000 times more energy per gram than its fission counterpart. An example of a naturally occuring fusion reaction is below. This reaction occurs in the sun.
Fusion
Another type of transmutation reaction is fusion. fusion occurs when two small nuclei join to form a large nucleus and large amounts of energy. Fusion reactions produce about 1000 times more energy per gram than its fission counterpart. An example of a naturally occuring fusion reaction is below. This reaction occurs in the sun.
Fusion reactions occur only at very high temperatures and/or pressures. Scientists are currently working on duplicating this reaction on Earth. The difficulty experienced is that the two hydrogen atoms repel one another and bounce off unless going fast enough or pushed close together with lots of pressure.
With current technology, the amount of energy needed to create a fusion reaction is equal to the amount that comes out of the reaction. This makes it not feasible as an energy source. |