Nuclear physics

What does Einstein’s mass-energy equation mean?

Mass and energy are equivelent

What are the two interpretations of Einstein’s mass-energy equation?

• mass is a form of energy

• energy has mass

Why is the rest mass of an electron different to an electron in a particle accelerator?

• The electron in the particle accelerator is moving so has more energy than an electron at rest

• Since ∆E = ∆mc², the mass of the electron in the particle accelerator must be greater than the rest mass of an electron

Explain why energy is released in alpha decay

• The total mass of the alpha particle and daughter nucleus are less than the parent nucleus. As such total mass decreases.

• Since the mass has changed, some energy must be released during the decay for mass-energy to be conserved

• Energy is released (not absorbed) as the mass decreases

Explain why energy is released in beta decay

• The total mass of the electron or positron and daughter nucleus are less than the parent nucleus. As such total mass decreases.

• Since the total mass has changed, some energy must be released during the decay for mass-energy to be conserved

• Energy is released (not absorbed) as the mass decreases

What is the value of an atomic mass unit?

1 u = 1.661 × 10^–27 kg

How would you calculate the binding energy from the mass defect?

Binding energy = mass defect × c²

Explain why the binding energy is different for different nuclei.

Different nuclei have different numbers of nucleons so have different bingding energies

Draw a sketch of binding energy per nucleon against nucleon number.

Where on a sketch of binding energy per nucleon against nucleon number do fusion and fission take place?

Which is the most stable nuclei?

iron-56

Which three elements don’t fit the curve?

Helium, oxygen and carbon

What is the composition of mined Uranium?

• 99.3% uranium-238

• 0.7% uranium-235

Why are thermal neutrons needed in a fission reactor?

• The fuel in fission reactors can split spontaneously without neutrons, but this is unlikely.

• Thermal neutrons are needed to indeuce fission: the neutrons are absorbed by the fuel, making it unstable and causing it to decay.

What is the decay equation for the induced fission of uranium-235 by a thermal neutron?

Draw a diagram to show an uncontrolled fission reaction

Sketch and label a fission reactor.

What is the function of the coolant in a fission reactor?

To remove the thermal energy produced from the fission reactions within the fissile fuel

What is the function of the moderator in a fission reactor?

To slow down the fast neutrons produced in fission reactions.

Why is a moderator needed in a fission reactor?

Induced fission of uranium-235 produces fast neutrons.

The chance of these fast neutrons being absorbed by uranium-235 nuclei is quite small, whereas thermal neutrons have a greater chance of producing induced fission.

As such a moderator is needed to slow down the fast neutrons and produce thermal neutrons

What is the function of the control rods in a fission reactor?

To absorb neutrons from fission reactions so that exactly one slow neutron survives per fission reaction

Why is enriched uranium used in the fuel rods?

What are the benefits of fission reactors?

• They don’t emmit CO₂

• A small quantity of fuel produces large amounts of energy

What are the environmental impacts of fission reactors?

• They produce hazardous and radioactive waste materials such as plutonium 239.

• These materials can have half lives which are tens of thousands of years

• These materials cannot enter our water or food suplies so must be buried underground for centuries

Why is it difficult to get fusion reactors to work?

• Extremely high temperatures are needed for fusion to occur. It is very hard to maintain such temperatures for long enough to sustain fusion.

• It is very hard to confine the extremely hot fuel in a reactor

Show the equations for proton-proton chain.

What is the nuclear symbol for a electron?

What is the nuclear symbol for a positron?