Physics

Nuclear fission is the splitting of an atomic nucleus

There are two fissionable isotopes in common use in nucleur reactors:

• Uranium-235
• Plutonium-239

Naturally occurring uranium is mostly uranium-238 which is non-fissionable. The uranium is enriched so that it contains 2-3% uranium-235.

The process:

• The isotope absorbs a neutron that is fired at it
• The nucleus splits into two smaller nuclei- 2 or 3 neutrons are also released as well as energy
• The energy can be used to produce electricty
• A chain reaction can occur when each fission reaction releases neutrons that go on to cause more fission reactions

Nuclear fusion is the process of forcing two light nuclei close enough together so they form a single larger nucleus.

The nuclei are made to collide at high speed and at very high temperatures to overcome the repellent forces between the nuclei 

The reaction is contained by a magnetic field because of the temperatures involved

Hydrogen isotopes used:

This is the process that causes energy to be released in stars

Background radiation sources:

• Radon gas is a major source of background radiation which seeps through the ground from radioactive substances in rocks - radon gas emits alpha particles so is harmful if breathed in
• Cosmic rays from outer space, food and drink, air travel and nuclear weapons testing
• Medical sources of radiation include X-rays as well as radioactive substances used in scans

Safety with radiation:

Uranium and plutonium are chemically removed from fuel rods in nuclear reactors so they can be used again

The remaining radioactive waste is stored in secure conditions for many years

People who work with nuclear reactors should;

• Keep as far as possible from sources of radiation
• Spend as little time exposed as possible
• shield themselves with materials such as concrete and lead

The universe was created by the Big Bang 13 billion years ago

• At first the universe was a hot glowing ball of radiation
• In the first few minutes, nuclei of the lightest elements formed
• As the universe expanded over millions of years, its temperature fell and uncharged atoms were formed

The formation of galaxies:

•  The universe was a dark patchy cloud of hydrogen and helium
• Eventually dust and gas were pulled together by gravitational attraction to form stars
• The resulting intense heat started off nuclear fusion reactions causing them to emit visible light and other radiation

Very large groups of stars are called galaxies. There are billions of stars in a galaxy and billions of galaxies in the universe. The universe is mostly empty space as there is vast distances between stars and galaxies

• Gravitational forces pull clouds of dust and gas together to form a protostar.
• The protostar becomes denser and the nuclei of hydrogen and other light elements start to fuse together- energy is released so the star becomes hotter and brighter
• Main sequence star: the star radiates energy because of the fusion of hydrogen at its core. The star is stable because the inward force of gravity is equal to the outward force of radiation
• Red giant/supergiant: Eventually, the star runs out of hydrogen nuclei, it swells then cools down and turns red

Star similar to the size of the sun:

• Helium and other elements fuse to form heavier elements
• Fusion stops and the star contracts to form a white dwarf
• Eventually no more light is emitted and the star becomes a black dwarf

A star much larger than the sun:

• The red supergiant continues to collapse
• Eventuallly the star explodes in a supernova- the outer layers are thrown out into space, the core is left as a neutron star
• If this is massive enough, it becomes a black hole

Chemical elements are formed by fusion processes in stars 

--> The nuclei of lighter elements fuse to form the nuclei of heavier elements (this process produces a large amount of energy)

Elements heavier than iron are only formed in the final stages of a large star.

These heavier elements requires the input of energy- all the elements get distributed through space in a supernova explosion

The presence of the heavier elements in the sun and inner planets is evidence that they were formed from debris scattered by a supernova

(The sun and the rest of the solar system were formed from the debris of a supernova)

Scientists can date rocks and fossils by measuring the amount (count rate) of radio isotope carbon-14 in the rock

Carbon-14 has a half life of 5700 years so the age of the rock can be calculated using this figure

Problems with carbon dating:

• Samples can be contaminated with other ancient materials 
• The count rate can be too small to measure (if the sample is older than 600,000 years)
• Nuclear bombs (since 1940) have added to background radiation

An example: The half-life of carbon-14 is 5700 years. If a fossil bone has a count of 25 and a normal piece of bone has a count of 200, how old is the fossil?

200/2 =100          100/2 =50          50/2 =25

The count has halved three times (three half lives) so 5700 x 3 = 17100 years old