P2.7 - Energy from the Nucleus

P2.7.1 - Nuclear Fission

Nuclear Fission

• Occurs when a stable isotope is struck by a neutron.
• -- the isotope absorbs the neutron, becomes unstable and then splits apart, releasing large amounts of energy.
• Isotopes that undergo fission include uranium-235 and plutonium-239
• --can be used in nuclear reactors and in nuclear weapons
• The fission of 1kg of uranium-235 releases more energy than burning 2 million kilograms of coal
• --less carbon dioxide released to the atmosphere.

How is uranium used in nuclear reactors?

• Two major types - uranium 235 and uranium 238.
• -only 235 can undergo fission.
• --we have to put the two together so it can actually be used.

Chain reactions

• Fission results in a chain reaction 
• --each time a nucleus splits it releases more neutrons, which can go and hit more uranium causing more fission reactions and so on.
• --it releases a lot of energy very quickly.
• If it is uncontrolled, heat can build up very quickly and must be controlled to maintain a steady input.

How are neutrons controlled?

• For nuclear fission to start in a reactor, a uranium 235 atom must absorb a low speed neutron (high speed neutrons are not readily absorbed by uranium nuclei)
• High speed neutrons are released during fission
• the reactors graphite core slows down the released neutrons so the chain reaction can keep going.
• Control rods made of boron -> absorb excess neutrons to prevent chain reactions getting out of control. 

How is nuclear waste stored?

• spent fuel rods are sent to a reprocessing plant to recover any usable uranium and plutonium
• many left over isotopes are useless and cannot be released into the environment -- some are put…