Physics Unit 7 - Energy from the Nucleus

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  • Physics Unit 7 Energy from the nucleus
    • Nuclear Fission
      • Chain reactions
        • Energy is released in a nuclear reactor as a result of nuclear fission. In this process, the nucleus of an atom of a fissionable substance splits into two smaller 'fragment' nuclei. This event can causeother fissionable nuclei to split. This then produces a chain reaction of fission events.
      • Fission neutrons
        • When a nucleus undergoes fission, it releases....
          • Two or three netrons (known as 'fission' neutrons) at high speeds.
          • Energy, in the form of radiation, plus kinetic energy of the fission neutrons and the fragment nuclei.
        • The fission neutrons may cause further fission resulting in a chain reaction. In a nuclear fission reactor, on average, exactly one fission neutron from each fission evet goes on to produce further fission. This ensures energy is released at a steady rate in the reactor.
      • Fissionable Isotopes
        • The fuel in a reactor must contain fissionable isotopes.
        • Most reactors at the present time are designed to used 'enriched uranium' as the fuel. This consists mostly of the non- fissionable uranium isotope U-238 and 2-3% of the uranium isotope U-235 which is fissionable. In comparison, natural uraniums is more than 99% U-238.
        • The U-238 nuclei in a nuclear reactor do not undergo fiission but they change into other heavy nuclei., including Pu-239. This isotope is fissionable. It can be used in a different type of reactor but not in a uranium reactor.
      • Inside a nuclear reactor
        • The reactor core contains the fuel rods, control rods andwater at high pressure.The fission neutrons are slowed downby collisions with the atoms in the water molecules. This is necessary as fast neutrons don't cause further fission of U-235. We say water acts as a moderater because it slows down the fission neutrons.
        • Control rods in the core absorb surplus neutrons. This keeps the chain reaction under control. The depth of the rods in the core are adjusted to maintain a steady chain reaction.
        • The water acts as a coolant. It's molecules gain kinetic energy from the neutrons and the fuel rods. The water is pumped through the core. The water transfers energy for heating to the exchanger from the core.
        • The reactor core is made fo thick steel to withstand the very high temperature and pressure in the core. The core is enclosed by thick concrete walls. These absorb radiation that escapes through the walls of the steel vessel.
    • Nuclear Fusion
      • Imagine if we could get energy from water. Stars release energy as a result of fusing small nuclei such as hydrogen to form larger nuclei. Water contains lots of hydrogen atoms. A glass of water could prpvde the same amount of energy as a tank of full petrol. But only if we could make a fusion reactor here on earth.
      • Fusion Reactions
        • Two small nuceli release energy when they are fused together to form a single larger nucleus. This process is called nuclear fusion. It releases energy only if the relative atomic mass of the nucleus is no more than 55 (about the same as an Fe nucleus). Energy must be supplied to create bigger nuclei.
        • The Sun is about 75% H and 25% He. The core is so hot that it consists of a plasma of bare nuclei with not electrons. the nuclei move about and fuse together whent they collide. When the fuse, they release energy.
        • (1) When 2 protons (i.e H nuclei) fuse, they form a 'heavy hydrogen' nucleus H(^2). Other particles are created and emitted at the same time.
        • (2) Two more protons collide sepearately with H(^2) nuclei and turn them into heavier nuclei.
        • (3) Two heavier nuclei collide to for the    He (^4) nucleus.
        • (4) The energy released at each stage is carried away as kinetic energy of the product nucleus and other particles emitted.
      • Fusion reactors
        • There are enormous techincal difficulties with making fusion a useful source of energy. The plasma of light nuclei will fuse. This is because 2 nuclei approaching each other will repel each other due to their positive charges. If the nuclei are moving fast enough they overcome the force of repulsion and fuse together.
        • In a fusion reactor the plasma is heated by passing a very large electric current through it.
        • The plasma is also contained by a magnetic field so it doesn't touch the reactor walls. If it did, it wuld go cold and fusion would stop.
        • Scientists have been working on these problems since the 1950s. A successful fusion reaction would release more energy than it uses to heat the plasma. At the present time, scientists working on experimental fusion reactors are able to do this by fusing heavy H nuclei to form He nuclei - but only for a few minutes.
      • A promising future
        • Practical fusion reactors could meet all of our energy needs.
        • The fuel for fusion reactors is readily available as heavy H and is naturally present in sea water.
        • The reaction product, He, is a non- radioactive inert gas, so is harmless.
        • The energy released could by used to generate electricity.
        • In comaprison, fission reactors only us U which is found in certain parts of the country.
    • Nulear Issues
      • Radioactivity all around us
        • When we use a Geiger counter, it clickes even without a radioactive source near it. This is due to background radiation. Radioactive substances are found naturally all around us.
        • The radiation from radioactive substances are hazadous, as it ionizes everything it passes through.
        • Medical sources include X-rays as well as radioactive substances, as X-rays have an ionising effect.People that work in jobs that use ionising radiation have to wear personal radiation monitors to make sure they are not exposed to too much ionising radiation
        • Background radiation is mainly due to radon gas that seeps through the ground from radioactive in rocks deep underground. Radon gas emits alpha particles so it is a health hazad if breathed in because it can ionise your cells. It can also seepint homes and other certain locations.
      • Chernobyl
        • In 1986, a nuclear reactor in Ukraine exploded. Emergency workers and scientists stuggled for days to contain the fire.  cloud of radioactive material drifted over many parts of Europe, including Britain. More than 100,000 people were evacuated from Chernobyl and surrounding areas.Over 30 people died in the accident. Many more have developed leukaemia or cancer since then. It was and still is the world's worst nuclear accident.
      • Radioactive Risks.
        • The effect on living cells of radiation from radioactive substances dpeends on:
          • The type and the amount of radiation recieved. (The dose).
            • Whether the source pf the radiation is inside or outside the body.
              • How long the living cells are exposed to radiation.
                • The larger the dose fo radiation someone gets, the greater the risk of cancer.
                  • The smaller the does, the less risk - but it is never 0. So there is a low level of risk in all of us due t background radiation.
                  • Workers who are at risk of ionising radiation can cut down their risk by:
                    • Keeping as faw away as possible from the radiation, using special handling tools with long handles.
                    • Spending as little time as possible in 'at-risk' areas.
                    • Shielding themselves from the radiation form standing behind thick concrete barriers and/or thick lead plates.
    • The Early Universe
      • The big bang that created the universe was about 13,000,000,000 (13billion) years ago. Space, time and radiation were created in the big bang. At first the universe was a hot glowing ball of radiation and matter. As it expanded, it's temperature fell. Now the universe is cold and dark expect for the hot stars scattered around.
      • The stars we see in the night sky are all in the Milky Way galaxy. Using powerful telescopes we can see many more stars in the Milky Way galaxy. We can also see individual stars in other galaxies.
      • We now know there are billions of galaxies in the universe. There is vast empty space between them. Light from the furthest galaxies we can see has taken billions of years to reach us.
      • The Dark Age of the Universe
        • As the universe expanded, it became transparent as radiation passed through empty space between its atoms.The background microwave radiation that causes the spots on an untuned television was released at this stage. The Dark Age of the universe had begun!
        • For the next few billion years, the universe was a completely dark, patchy expanding clound of H and He. Then the stars and galaxies formed and lit up the universe!
      • The Force of grvity takes over
        • Uncharged atoms don't repel each other. But they do attract each other. During the Dark Age of the universe, the gravitational attraction was at work without any opposition from reulive forces.
          • As the universe continues to expand it became more patchy as the denser parts attracted nearby matter. Gravity pulled more matter into the denser parts and turned them into gigantic clumps.
            • Eventually, the force of gravity turned the clumps into galaxies and stars. A few billion years after the Big Bang, the Dark Age came to an end, as the stars lit up the universe.
    • The Life History of Stars
      • The birth of a star
        • Stars form out of cloud and dust.
          • The particles in the clound are pulled together by their own gravitational attraction. The clouds merge together. They become more and more concentrated to become a protostar, the name of a star to be.
            • As a protostar becomes denser, it gets hotter. It become hot enough the nuclei of H and other light elements fuse together. Energy is released in this fusion so the core gets hotter and brighter and starts to shine. A star is born!
              • Objects may form that are too small to become stars. Such objects may be attracted by a protostar to become planets.
      • Shining stars
        • Stars like the sun radiate energy because of H fusion in the core. They are claled main sequence stars because this is the main stage in the life of a star. It can maintain its energy output for millions of years until the star runs out of H nuclei to fuse together.
          • Energy released in the core keeps the core hot so the process of fusion continues. Radiation flows out steadily from the core in all directions.
            • The star is stable because the forces within it are balanced. The force of gracity that makes a star contract are balanced by the outward force of radiation from its core. These forces stay in balance until most of the H nuclei in the core as been fused together.
      • The end of a star
        • When a star runs out of H nuclei to fuse together, it reaches the end of its main sequence stage and it swells out.
          • Stars about the same as the sun or samller swell out, cool down and turn red.
            • The star is now a red giant. At this stage He at other light elements in it's core fuse to form heavier elements.
            • When there are no more light elements in it's core, fusion stops and no more radiation is released. Due to it's own gravity, the star collapses in on itself. As it collpases, it heats up and goes from red to yellow to white. It becomes a white dwarf. This is a hot, dense white star hte is much smaller in diameter than it was. Stars like the Sun then fade out, go cold andbecome black dwarfs.
              • Stars much bigger than the sun end their lives much more dramatically.
                • Such a star swells off to become a red supergiant which then collapses.
                  • In the collapse, the matter surrounding the stars core compresses the core more and more. Then the compression suddenly reverses in a cataclysmic explosion known as a supernova. Such an event can outshine an entire galaxy for weeks.
      • What remains after a supernova occurs.
        • The explosion compresses thecore of the star into a neutron star. This is an extremely dense object composed only of neurtons. If the star is massive enough, it becomes a black hole instead. The gravitational field of a black hole is so strong that nothing can escape from it. Not even light any other form of electromagnetic radiation can escape.
    • How the chemical elements formed
      • The birth place of chemical elements
        • Light elements are formed as a result of fusion in stars.
          • Stars like the Sun fuse H nuclei (i.e. protons) into He and similar small nuclei, including C. When it becomes a red giant, it fuses He and other small nuclei into large nuclei.
            • Nuclei larger than Fe cannot be formed by this process because too much energy is needed.
              • Heavy elements are formed when a massive star collapses then explodes as a supernova.
                • The enormous force of the collapse fuses small nuclei into nuclei larger than Fe. The explosion scatters the star into spaces.
                  • The debris from the supernova contains all the known elements from the lightest to the heaviest. Eventually, new stars form  as gravity pulls the debris together.
                    • Plants form from debris surounding a new star. As a result, such planets will be composed of all the known elements too.
      • Planet Earth
        • The heaviest know natural element is U. It has a half life of 4500 million years. The presence of U in the Earth is evidence that the Solar System must have formed from the remnants of a supernova.
        • Elements such as Pu are heavier than U. Scientists can make these elements by bombarding heavy elemnts like U with high-speed neutrons. They would have been present in the debris which formed the Solar System. Elements heavier than U formed then have long sicne decayed.

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