Energy is released in a nuclear reactor through process of nuclear fission where the nucleus in a fissionable substance split into smaller fragment nuclei. This event causes other fissionable nuclei to split producing a chain reaction of fission events.
When nucleus undergoes fission releases two or three neutrons referred to as fission neutrons at high speeds, energy as radiation and kinetic energy from fission neutron and the fragment nucleui. Fission neutron can cause further fission resulting in chain reaction. In a nuclear fission reaction on average exactly one fission neutron from each fission event goes on to produce further fission ensuring energy is released at a steady rate in the reactor.
Fuels in a fission reactor may contain fissionable isotopes:
Most reactors use enriched uranium which consisting mostly of non fissionable U-238 and 2-3% of of the uranium isotope U-235 which is fissionable. In comparison natural uranium contains 99% of U-238.
In nuclear reaction U-238 doesn't undergo fission but change into heavy nuclei which are fissionable including plutonium-239, which can be used in different kinds of nuclear reaction but in a uranium reactor.
Consists of uranium fuel rods spaced evenly in the reactor core. Core contains fuel rods, control rods and water at high temperatures. Fission neutrons are slowed down by atoms in water molecules. Necessary as fast neutrons don't cause further fission of U-235. Water acts as a moderator as slows down the fission neutrons.
Control rods absorb surplus neutrons, keeping the chain reaction under control. Depth of the rods in the core adjusted to maintain steady chain reaction, longer absorb more as more in core.
Water acts as a coolant, molecules gain kinetic energy from the neutrons and control rods, water is pumped around the core. Goes through sealed pipes to and from a heat exchanger outside the core. Water transfers energy for heating to the heat exchanger from the core.
Reactor core is made of thick steel in order to withstand high pressures and temperatures. Core is enclosed in by thick concrete walls. These absorb radiation that escapes through the walls of the steel vessel.
Stars release energy by fusing small nuclei such as hydrogen to form larger nuclei. A glass of water could provide same amount of energy as a tanker full of petrol however not possible to make fusion reactor on earth.
Two small nuclei release energy when they are fused together to form a single nucleus, this process is called nuclear fission. This releases energy only if the relative mass of the nucleus formed is no more than 55 similar to iron nucleus. Energy must be supplied to create bigger nuclei. Sun has 75% hydrogen and 25% helium core consists of plasma (bare nuclei with no electrons). These nuclei move about and fuse together when they collide and in the process release energy
Protons fuse together to form helium nucleus
When two protons, hydrogen, fuse they form a heavy hydrogen nucleus, other particles are created and emitted at the same time. Two more protons collide separately to form another heavy hydrogen nucleus. These two heavy hydrogen nuclei collide to form the helium nucleus. The energy released at each stage dissipates as kinetic energy of the product nucleus and other particles emitted.
Very difficult to make fusion source of energy, plasma of light nuclei must be heated to very high temperatures before the nuclei will fuse, as two nuclei approaching each other will repel each other due to their positive charges. If the nuclei are moving fast enough they will overcome this force of repulsion and fuse together.
In a fusion reactor:
Plasma is heated by passing a very large electric current through it
Plasma is contained by a magnetic field so it doesn't touch the reactor walls if it did it would go cold and fusion would halt
This has been a great problem a successful fusion reactor would release more energy than used to heat the plasma. At present scientists working on experimental fusion reactors are able to do so by fusing heavy hydrogen nuclei to form helium nuclei however this is only possible for a few minutes.
Fusion reactors future
In the future pratical fusion reactors could meet all our energy needs. The fuel for fusion reactors is readily available as heavy hydrogen is naturally present in sea water. Reaction product, helium, is a non radioactive, inert gas, so is harmless. Energy released could be used to generate electricity.
Fission reactors mostly use uranium, which can only be found in certain parts of the world, also they produce nuclear waste that has to be securely stored for many years. Although fission reactors have been in operation for over 50 years unlike fusion reactors which are still under development.
Radioactivity all around us
Able to use the Geiger counter that clicks even without a radioactive source near it as a result of background radiation, radioactive substances are found naturally all around us. Form cosmic rays, food and drink, medical, air travel, nuclear power, ground and buildings, natural radioactivity in the air and nuclear weapons testing.
Medical sources include x-rays as well as radioactive substances as x-rays have ionising effect. People who work in jobs involving the use of ionising radiation must wear radiation monitors to ensure they're not exposed to too much ionising radiation.
Background radiation in the air is mostly due to radon gas that seeps through the ground from radioactive substances in rocks deep underground. This gas emits alpha particles so it's a health hazard if breathed in. Able to seep into homes and other buildings in certain location. Where apparent steps must be taken to stop where there are people. For instance pipes under buildings can be installed and fitted to a suction pump to draw gas out of the ground before it seeps into the building.
In 1986 a nuclear reactor in Ukraine exploded, emergency workers and scientists struggled for days to contain the fire. Cloud of radioactive material from the fire drifted over many parts of Europe, Britain. More than 100,000 people were evacuated form Chernobyl and surrounding areas. Over 30 people died in the accident, many more have developed leukemia or cancer since then. It's the world's worst nuclear accident.
The early universe
Big band created universe, ain't nobody believe dat, about 13 billion years ago in such space, time and radiation were created. At first universe hot glowing ball of radiation and matter, as expanded temperature fell, now universe is cold and dark except for hot spots called stars.
Stars we see are all in the Milky Way galaxy, our home galaxy. Sun is one of billions of stars in the Milky Way galaxy. By using powerful telescopes able to see many more stars in the Milky Way galaxy, we can also see individual stars in other galaxies. Now know there are billion of galaxies in the universe, there is vast empty space between them, light from the furthest galaxies that we can see has taken billion of years to reach us.
Dark Age of the unive sprse
As universe expanded, became transparent as radiation passed through the empty space between its atoms, the background microwave radiation that causes the spots on an untuned television was released at such stage. Dark age of the universe had begun, for next few billion years, universe was completely dark, patchy, expanding cloud of hydrogen and helium. Then the stars and galaxies formed and lit up the universe.
Force of gravity,
Uncharged atoms don't repel each other, however they can attract each other. During the dark age of the universe, force of gravitational attraction was at work without any opposition from repulsive forces. As universe expanded 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 formed the clumps into galaxies and stars a few billion years after the big bang, the Dark Age came to the end, as the stars lit up the universe.
History of a star:
Birth of a star
Stars form from clouds of dust and gas. Particles in the clouds are pulled together by their own gravitational attraction, the clouds merge together, they become more and more concentrated to form a protostar (the name for a star to be). As the protostar becomes denser it gets hotter if it becomes hot enough the nuclei of hydrogen atoms and other light elements fuse together. Energy's 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.
History of a star:
Stars like the Sun radiate energy as a result of hydrogen fusion in the core. They are called main sequence stars as this is the main stage in their lifespan. This can maintain the energy output for millions of years until the star runs out of hydrogen 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. Star is stable as forces are balanced, force of gravity that makes a tar contract is balanced by the outward force of the radiation from it's core. These forces stay in balance until most of the hydrogen nuclei in the core have been fused together.
History of star:
End of star (Sun size or smaller)
When star runs out of hydrogen nuclei to fuse together, it reaches the end of its main sequence and swells out. Stars about the same size as the Sun or smaller swell out, cool down and turn red. Star is now a red giant at this stage helium and other light elements in its core fuse to form heavier elements. When there's no more light elements in its core, fusion stops and no more radiation is released, due to it's own gravity, the star collapses in one itself. As it collapses it heats up and turns from red to yellow to white. It becomes a white dwarf, this is a hot, dense white star much smaller in diameter than previously. Stars like the Sun then fade out, go cold and become black dwarfs.
History of a star:
End of star (Stars bigger than the Sun)
Such a star swells out to become a red super-giant 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 called a supernova. Such an event can outshine an entire for several weeks.
Life cycle of a sun
Remnants after a supernova occurs
Explosion compresses the core of the star into a neutron star, this is an extremely dense object composed only of neutrons. If the star is big enough, it becomes a black hole. The gravitational field of a black hole is so strong that nothing can escape from it, not even light, or any other form of electromagnetic radiation can escape.
Birthplace of light elements
Light elements are formed as a result of fusion in stars. Stars like the Sun fuse hydrogen nuclei into helium and similar small nuclei including carbon. As it becomes a red giant it fuses helium and other small nuclei into larger nuclei. Nuclei larger than iron cannot be formed by this process as too much energy is required.
Birthplace of heavy elements
Heavy elements are formed when a massive star collapses then explodes as a supernova. Enormous force of the collapse fuses small nuclei into larger than iron, the explosion scatter the star into space. The debris from a supernova contains all the known elements from the lightest to the heaviest. Eventually new stars form as gravity pulls the debris together. Planets form from the debris surrounding a new star, as a result such planets ill be composed of all the known elements too.
Formation of solar system
Life on planyets
Molecules of carbon based chemicals are present in space, life on earth most likely started with God, hey man your doing science, so stared most likely from chemicals reacting in lighting storms. Therefore were looking for any scientific evidence about life on other planets, either in our own Solar Systems or around other stars.
Space probes sent to Mars: These have tested the atmosphere, rocks and soil on Mars looking for microbes or chemicals indicating life was once present on Mars. Water is necessary for life. Astronomers now have strong evidence of the presence of underground water breaking through to the surface of Mars
Sear for extra-terrestrial intelligence: known as SETI, or just aliens, has gone one on for more than 40 years using radio telescopes. Signals from space would indicate the existence of living beings with technologies at least as advanced as our own. No signals have yet to be detected, duh duh duhhhh!
Heaviest known natural element is uranium which has a half life of 4500 million years. The presence of uranium in the Earth is evidence that the Solar system must have formed from the remnants of a supernova. Elements such as plutonium are heavier than uranium, scientists can make these elements by bombarding heavy like uranium with high speed neutrons. They would have been present in the debris which formed the Solar System, however elements heavier than uranium formed have long since decayed.