Fission and fusion

nuclear reactions, including… can be a source of energy

fission

fusion

radioactive decay

what is nuclear fission?

the process of splitting a nucleus.

describe the process of nuclear fission

• when a U-235 nucleus absorbs a slow moving neutron, it splits into two daughter nuclei and 2 or 3 neutrons.

• this process releases energy.

• the neutrons can then induce further fission events in a chain reaction by striking other U-235 nuclei.

how can a chain reaction of nuclear fission can be set up by the neutrons produced

• The three neutrons produced by the fission may hit other nuclei of uranium 235, causing the process to repeat.

• For a chain reaction to occur, there is a minimum mass of uranium 235 required.

• This is called the critical mass.

nuclear reactor: control rods

• made of boron

• absorb neutrons to ensure that only one fission neutron per event foes on to induce further fission (chain reactions.)

nuclear reactor: moderator

• usually water

• slows down neutrons by collisions

• so that they are moving slow enough to be absorbed by another U-235 nucleus.

• the water also acts as a coolant to prevent the system from overheating.

the reactor core

• thick steel vessel

• withstands high temperatures and pressure.

• absorbs some of the radiation.

• core kept in building with thick concrete reinforced walls (that acts as radiation shields to absorb any escaping radiation from the reactor core.)

nuclear fusion

• the process of fusing two nuclei to form a larger nucleus.

causes a…

• very small loss of mass and release of energy.

what is energy source for stars/ how does the sun release energy?

nuclear fusion

why doesn’t nuclear fusion occur at low temperatures and pressures

the electrostatic repulsion of protons is too great.