Nuclear Physics

4 Types of Nuclear Radiation §

Alpha Particles (Radiation) §

• Made up of two protons and two neutrons.
• Represented as ${}_2^{4}He$
• Doesn’t have any electrons, so overall charge of 2+.
• Relatively large, so it can’t penetrate very far into other materials.
• Absorbed by a single sheet of paper.
• Strongly ionising - easily knocking out electrons it collides with.

Beta Particles §

• Charge of -1
• Basically no mass.
• Neutron decays into a proton and an electron, where the electron is emitted at high speed.
• Moderately ionising.
• Absorbed by aluminium.

Gamma Rays §

• Aren’t particles at all.
• Waves of electromagnetic radiation.
• Moves at speed of light.
• No mass.
• Often emitted after alpha or beta radiation.
• Weakly ionising.
• High penetration requiring concrete/lead.

Neutrons §

• If too many neutrons occurs, causing it to become unstable, a neutron can be emitted to increase stability.

Half Life §

• Activity: overall rate of decay.
• Measured in becquerels, which is the decays per second (Bq)
• Half Life: time taken for a the number of radioactive nuclei in a sample to halve.
• $n=\frac{t}{t_{1/2}}$
  • t = the time where nuclear energy has occurred.
  • n = the number of half lives elapsed.
  • $t_{1/2}$ = half-life of the nuclei.
• $N=N_0{\left(\frac{1}{2}\right)}^n$
  • N = number of radioactive nuclei remaining.
  • $N_0$ = initial number of radioactive nuclei.
  • n = number of half lives elapsed.

Binding Energy §

• E = mc^2 gives the universal relationship between energy and mass.
• if a piece of material of mass 5.0 kg could be completely converted into energy, how much energy would be released?
• mass defect = (mass of nucleons) - (mass of nucleus)
• binding energy is defined as the energy required to separate all of the nucleons in a nucleus.
• binding energy (J) = mass defect (kg) x c
• binding energy (MeV) = mass defect (amu) _ 931.3_

Fusion and Fission §

Nuclear Fission §

• the energy released in nuclear power plants come from nuclear fissions.
• large, unstable nuclei break into smaller fragments, releasing energy as they do so. ${}^{235}U+neutron\to fission.fragment+neutrons+energy$

Nuclear fusion. §

• energy emitted by a star comes from nuclear fusion.
• the core temperature has to be extremely high for this to happen.
• combination of lighter isotopes of hydrogen to form helium, and the release of energy.
• ${}_1^{2}H{+}_1^{1}H{\to }_2^{3}He+energy$

binding energy §

• how does both fission and fusion release energy?

  energy is released in a nuclear reaction when the binding energy of the products is greater than the binding of the reactants.

• mass of the products is less than the mass of the reactants. the difference in mass is released as energy.

nuclear fission (again?) §

• 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.
• unlike natural radioactive decay, fission is not a natural event.
• used in both nuclear reactors and weapons.

uranium §

• there exists two main isotopes of uranium - 235, 238
• 238 is the major isotope, but it odes not undergo fusion.
• 0.7% of naturally occurring uranium is uranium-235, and 238 must be enriched to become 235 (?)

nuclear reactors §

• fuel rods
  • ?
• moderator
  • a material which slows down …
• control rods
  • these are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually in the primary cooling system)
  • stop a nuclear fission chain reaction.
• coolant
  • a liquid or gas circulating through the core transferring the heat from it. in light water reactors the moderator functions as coolant.
• steam generator
  • part of the cooling system where the heat from the reactor is used to make steam for the turbine.

simply put:

1. reactor produces heat.
2. hot coolant from core heats water to produce steam.
3. steam powers turbine generator.
4. steam cooled and condensed by cooling tower.

safety of nuclear reactors §

• nuclear reactors produce huge amounts of energy, much of which is in the form of gamma rays.
• gamma rays are highly energetic photon of electromagnetic radiation.
• if they come into contact with the body they can cause cellular mutations, which can lead to cancer.

Nuclear Fusion Benefits §

• abundant fuels - deuterium can be extracted from water and tritium is made from lithium, which is readily available.
• small amounts of fuel - 10g of deuterium and 15 grams of tritium could produce enough energy for the lifetime of an average person in an industrialised country
• clean - no greenhouse or other polluting gases are made.
• safe - no need to keep chain reactions under control
• less radioactive waste - the products of nuclear fusion are not radioactive, although the reactor walls will absorb neutrons and become radioactive.
• no weapons material produced- the products are not suitable for making nuclear weapons