Topic 8: Power production 8.3 & 8.4 (A) Fossil fuels & Nuclear Energy Allen High School IB Physics SL Source: Chris Hamper Physics

The history As technology has advanced, it has become easier to extract fossil fuels like coal. This is a difficult and dangerous job; however coal produces more heat and energy than wood. The invention of James Watt’s steam engine changed the world in 1769 and still today. Steam engines turned a wheel and were powered by coal (twice the energy density than wood). This invention kicked off the Industrial Revolution and the ability to transport goods around the world. Coal cities boomed.

Coal to Oil (petroleum) Oil technology allows us to drill and pump crude oil (thick sticky substance). Oil has a higher energy density than coal, but before 1852, it was more difficult to utilize than coal. In 1852, Ignacy Lukasiewicz invented a method to refine crude oil to make kerosene (cleaner fuel with even higher energy density). It became possible to inject the fuel inside a piston of an engine (internal combustion), which revolutionized transport. Easy to transport, but hard to clean up oil spills.

Generation of electricity In 1831, Michael Faraday discovered that moving a wire in a magnetic field created current. But in 1866, Werner Siemens invented the dynamo, which brought electricity generation to the big scale. In 1884, Sir Charles Pearson invented the steam turbine, which was the final puzzle piece. Electricity was now the easiest way to transfer energy from one place to another.

Fossil fuel Power stations Coal-fired stations burns the coal, which boils the water, then produces steam and powers the turbine, which turns a generator and produces electricity. The steam leaving the turbine is cooled, causing it to condense and is returned to the boiler. Overall efficiency of coal-fired stations are 40%. Degraded energy is exhaust gas, waste heat, and also friction in the components of the turbine and generator.

Coal-fired power station

Fossil fuel power stations Oil-fired stations are the same as coal, but oil is cleaner and easier to transport. Gas-fired stations are more efficient because there are two stages of energy use. Burning gas is blasted through a turbine Heat produced can boil water & power turbine. Gas-fired can be 59% efficient, but if the “wasted” heat is utilized, it could be as high as 80%.

What can be more efficient and cleaner??

Nuclear power Fission: Big nucleus (U-236) splits into two smaller nuclei, resulting in a loss of mass (defect) and hence a release of energy. 23692U  14256Ba + 9236Kr + 10n The energy released from the ∆m is given to the fission fragments as KE. If one mole of Uranium split, then the energy released would be 16.5 x 10 12 J, which is a lot more energy than coal. The neutron released in the above reaction is an essential part of nuclear reactions.

Nuclear reactions A neutron is added a U-235 nucleus to produce U-236, which then splits in two. As a result there are too many neutrons and some are released. These neutrons can be captured by more U atoms and so on, leading to a chain reaction. The chain reaction depends on slow moving neutrons. Neutrons are slowed down by introducing some other nuclei between U atoms. Colliding with the other nuclei slow down the neutrons.

Chain reaction

Nuclear reactions A minimum amount of U is needed in order for these chain reactions to take place. This is called Critical Mass. Once U is extracted from the ground (as uranium ore) and processed, only U-235 (only 0.7%) is used as nuclear fuel. Uranium must be enriched (increasing the U-235 percentage). The fuel is then made into fuel rods (cylinders stacked together); the rods are then bundled together and many are placed in the reactor.

Nuclear reactions Is this an atomic bomb? NO, not all nuclear energy results in a bomb. An atomic bomb is an out-of-control reaction takes place. The total mass is above the critical mass. It depends on the initial mass. A nuclear weapon begins with 85% U-235, where normal nuclear reactions are less than 20%. How do we prevent an out-of-control reaction? Control the number of neutrons produced and the speed of the neutrons (we want them slowed down).

Nuclear power stations A nuclear reactor produces the heat and contains the fuel rods surrounded by the moderator. The control rods are raised/lowered to control the rate of reaction (they absorb the neutrons not needed). There is a pressured vessel, which has a gas circulating to pick up heat from fuel rods and transfer it to the heat exchanger. The water then turns to steam and turns a turbine and generates electricity. See picture on next slide.

Nuclear reactor

plutonium When U-238 absorbs a neutron, it turns into U-239, which then decays to produce beta radiation and Np-239, which then decays again by beta radiation to Pu-239, Plutonium. Plutonium also undergoes fission and can be used as a fuel or in the manufacture of nuclear weapons. The production of Pu from U can be extracted and used for subsequent energy production (or bombs).

Why does Good nuclear energy still get a bad rap? Chernobyl; Three-Mile Island; emphasis on “nuclear” as a weapon. As with all energy sources, there are cons/problems with nuclear. Radioactive waste from U extraction (“short-term”) & spent fuel rods (the latter leads to long-term storage). Nuclear meltdown: when reactions are not controlled and fuel rods melt together. Pressure vessel bursts, thus releasing radioactive material into the atmosphere. (Chernobyl, Ukraine, 1986).

Fission vs. fusion Fusion is the opposite of fission. Fusion involves fusing light nuclei to form a larger nuclei. Once again, the mass defect is converted to energy. Fusion happens on the Sun. There is less known and less experience with Fusion as compared to Fission. The difficulty with Fusion is maintaining and confining a high-temperature, high-density plasma necessary for a fusion reaction.

Fission vs. Fusion Going back to the Energy Density chart. Fuel Energy Density (MJ/kg) Fusion fuel 300,000,000 Uranium-235 90,000,000 Think of the power efficiency if we were able to use Fusion more often!