E.4 Fission

fission

splitting of large unstable nuclei to form smaller ones. this can be spontaneous or encouraged such as a nuclear reactor which is called induced fission

the binding energy of the products is greater than the bidning energy of the reactants; mass of products is less than mass of reactants. the difference in mass is released as energy

small to big nuclei release energy by increasing in size

big to small nuclei release enrgy bu decreasing in size

fusion

combining of two smaller nuclei to create one larger one. this can only happen if the nuclei have enough energy to overcome electrostatic repulsion between their protons, so that they can be close enough to be attracted through strong force

how is energy released in a fusion/fission reaction

when binding energy of products is greater than binding energy for reactant

small nuclei- release energy by increasing in size

large nuclei - release energy by decreasing in size

example of the use of fission

the energy released in nuclear power plants comes from nuclear fission.

as large unstable nuclei break into smaller fragments and release energy, this energy is used to heat water and the steam spins the turbine generator and produces power

enrichement

The amount of 235uranium must be increased to 3% in natural uranium, usually 0.7%, remainder is 238 uranium. This heavier uranium absorbs neutrons and won’t undergo fission, whereas 235 absorbs and undergoes.

thus uranium must be enriched to be used as fuel

chain reaction

splitting of 235U requires energy to overcome strong force. this is done by adding a neutron which reduces stability and the nucleus splits in two. Then there are two many neutrons and thus from the two daughters neutrons are also released. But these released neutrons can by accidentally be absorbed by the other, and cause a chain reaction

neutrons are only absorbed if they travel slowly, otherwise they will pass straight through nucleus

moderation

neutrons emitted through fission usually a lot of kinetic energy, thus must be moderated or slowed down for chain reaction to occur

done by introducing a small nucleus between the neutron and 235 U.

this is often water, and neutrons collide with it to exchange energy and slow the neutron down (near-elastic collision)

critical mass

the minimum mass required for a chain reaction to occur.

• if it is too small before the neutrons have traveled far enough for their speed to slow down, they will have left the reacting piece or uranium

Nuclear power station contains and set up

Nuclear power reactors use heat produced during atomic fission to boil water and produce pressurized steam. The steam is routed through the reactor steam system to spin large turbines blades that drive magnetic generators to produce electricity.

control rods

the rate of reaction is limited as the fuel holds a lot of 238U, which absorbs neutrons. this can not slow down reaction, instead control rods made out of boron between fuel-rods prevents the neutrons from causing further fissions.

controls rate of reaction

meltdown

if not controlled properly, fuel rods can melt, so fuel can not be removed and pressure vessels may burst, sending radioactive material everywhere.

• caused by malfunctioning cooling system or leak in pressure vessel

shielding

through containment building, no dangerous material can leak

low-level waste

extraction of uranium from the ground, enriching fuel, or old reactors, are low-level waste. can not be recycled as some parts are still radioactive

high level waste

fuel rods have large half-lives, don’t know how to be stored, for now under water to cool off and then sealed in steel cylinders

nuclear fission

During nuclear fission, a neutron collides with a uranium atom and splits it, releasing a large amount of energy in the form of heat and radiation.

energy transfers in a nuclear reactor

1. nuclear energy in processed fuel, some heat loss

2. thermal energy in steam, heat loss

3. mechanical energy in turbine, unused fuel

4. electricity