radioactivity only depends on and is proportional to number of particle: \(A \propto N\), so ratio of two radioactivity does not change over time

fission conservation: nucleons are conserved, including the incoming neutron: \[ ^{235}_{92}\mathrm{U} + ^{1}_{0}\mathrm{n} \rightarrow ^{140}_{54}\mathrm{Xe} + ^{94}_{38}\mathrm{Sr} + x\,^{1}_{0}\mathrm{n} \]

fusion condition: 1. enough energy (high temperature) to overcome repulsive force 2. density: get close enough for strong nuclear force’s cause attraction 3. confine time: the high temperature high density environment should be stable

nuclear reactor related

critical mass to secure chain reaction

fission is used in nuclear reactor in nuclear electrc plant, therefore the waste is mainly fission induced products

fuel rods contains uranium, and is surrounded by moderator material (slows neutron down). Fuel rods are removed when the concentration of fission products is too high.

Control rod absorbs neutrons to slow down the reaction. do not confuse with moderator

Stars related

all stars use fusion

main sequence star: fuse Hydrogen, generate helium red giant: fuse helium final: fuse iron

on maintaining radius: fusion produces photons that creates thermal pressure inside the star, which prevent collapse from inward gravitational force

observe