Nuclear Fission and Fusion 76-77 Flashcards
How can fission be induced?
Fission can be induced by making a neutron enter a 235U nucleus, causing it to become very unstable.
What is a thermal neutron?
Only low energy neutron is called a thermal neutron.
Why does fission limit the number of elements that exist?
In general, the larger the nucleus, the more unstable it will be - so large nuclei are more likely to spontaneously fission.
This means that spontaneous fission limits the number of nucleons that nucleus that a nucleus can contanain , other words it limits the number of positive elements.
Draw the structure of a nuclear reactor, label and explain the key components of a nuclear reactor.
Explain how a nuclear reactor harnesses energy.
- Coolant is sent around the reactor to remove heat produced in the fission - often the coolant is the same water that is being used in the reactor as a moderator. The heat from the reactor can then be used to make steam for powering electricity-generating turbines.
- Control rods control the chain reaction by limiting the number of neutrons in the reactor. They absorb neutrons so that the rate of fission is controlled. Control rods made from boron, can be inserted by varying amounts to control the reaction rate. Control rods allow a steady rate where one fission follows another.
What is critical mass?
The amount of “fuel” you need to do this is called the critical mass - any less than the critical mass (sub-critical mass) and the reaction will just peter out.
How does an atomic bomb work?
In an atomic bomb, many new fissions will follow each fission, causing a runaway reaction which could lead to an explosion.
Why are the waste products of nuclear fission unstable and radioactive?
The waste products have a larger proportion of neutrons than stable nuclei of a similar atomic number - this makes them unstable and radioactive.
What are the waste products from the nuclear fission dealt with?
The products can be used for practical applications such as tracers in medical diagnosis.
When a material is removed from the reactor, it is initially very hot, so is placed in cooling ponds until the temperature falls to a safe level.
The radioactive waste is then stored underground in sealed containers until its activity has fallen sufficiently.
What energy must two nuclei overcome to fuse?
Nuclei can only fuse if they have enough energy to overcome the electrostatic repulsive force between, and get close enough for the strong interaction to bind them.
How much kinetic energy is needed to cause fusion?
1MeV of kinetic energy.
What is plasma?
At temperatures of 107 K atoms don’t exist. The negatively charged electrons are stripped away leaving postively charged nuclei and free electrons.
How can the Sun continully allow fission to occur?
A lot of energy is released during nuclear fusion because the new, heavier nuclei have a much larger binding energy per nucleon. This helps to maintain the temperature for further fusion reactions to occur.
