Week 7: Nuclear fission (part 1) Flashcards
What does it mean if energy produced from fission is prompt?
The energy appears at the moment of fission.
What does it mean if energy produced by fission is delayed?
The energy only appears after some time has passed.
Why is there still a production of energy after a reactor has been shut down?
The decay of the fission products continues, producing 6-8% of the energy the reactor was at before shut down.
The production of this energy will decrease over time, dropping off as the fission fragments decay to stable nuclides.
What are delayed neutrons?
They are neutrons produced in the beta decay of fission products.
They only contribute to 0.7% of the neutrons in a U-235 fuelled reactor.
Why are the light fission products radioactive/unstable?
Heavy nuclides contain large numbers of neutrons due to the balance between the repulsive Coulomb force and attractive strong force.
When fission occurs the much lighter nuclei produced inherit these neutrons and become much more neutron-rich than what is needed for stability.
How do most fission products decay?
The high numbers of neutrons means that most fission products will beta decay.
They release energy in the form of electrons, gamma rays and anti-neutrinos.
What is the neutron multiplication factor?
k is the average number of neutrons from a fission reaction that will cause another fission.
What happens to the neutrons from a fission reaction that do not go on to cause another fission?
Either absorbed or leak from the reactor.
What is the formula for the multiplication factor, k?
k = Neutrons in one generation / Neutrons in the previous generation.
What does it mean if k=1?
Critical
The neutron population is steady and the power is steady,
What does it mean if k < 1?
Sub-critical
The neutron population is decreasing and the power is decreasing.
What does it mean if k > 1?
Super-critical
The neutron population is increasing and the power is increasing.
What is critical mass?
The mass at which criticality is achieved under the specified conditions.
What are two properties possessed by materials used in chain reactions?
Fissile and fissionable.
What does fissile mean? Give three examples of fissile materials.
Nuclides can be induced to fission with thermal neutrons.
U-235
Pu-239
U-233
What does fissionable mean? Give three examples of fissionable materials.
Nuclides can be induced to fission with fast neutrons.
U-238
Th-232
Pu-240
Describe a thermal neutron reactor design.
A typical thermal reactor design has uranium fuel spread through a neutron moderator.
Fast neutrons from one fission are quickly moderated and diffuse back to the fuel to produce the next fission.
What are the components of a thermal reactor?
Fuel
Moderator
Cladding
Coolant
Control rods
Structure
Pressure vessel
Give an example of a material used for fuel in a thermal reactor.
Uranium dioxide (UO2) or uranium metal, formed into rods.
Give an example of a material used for a moderator in a thermal reactor.
Light or heavy water (liquid) or graphite (solid blocks).
Give an example of a material used for cladding in a thermal reactor.
Zirconium alloy (Zircaloy) or stainless steel.
Give an example of a material used for coolant in a thermal reactor.
Light water or CO2.
Give an example of a material used for control rods in a thermal reactor.
Boron carbide (B4C), silver-indium-cadmium (AIC)
Give an example of a material used for the structure in a thermal reactor.
Steel
