Nuclear Fission Flashcards
Prompt Energy
Energy released at the moment of fission (fragments, fission neutrons and γ rays)
Delayed Energy
Energy released some time after the initial fission event (β decay products and γ rays from further decays)
Decay Heat
Heat energy released even after a reactor has been shut down, producing 6-8% of the energy at peak operation due to delayed reactions.
Delayed Neutrons
Neutrons produced in delayed β- reactions.
What is a fission product yield curve?
A graph showing the probability of a decay product being produced, for the whole spectrum of possible products.
Why are fission products neutron rich?
Heavy nuclei have a large number of neutrons, so when fission products are produced they have a very large number of neutrons for their atomic number, and subsequently β decay.
Neutron multiplication factor?
k = neutrons in one generation / neutrons in previous generation. The average number of neutrons that go on to produce more fission reactions
At what value of k is a reaction critical?
k = 1. The reaction is continuing at a constant rate.
Critical mass
The mass at which criticality is achieved under specified conditions.
Fissile nuclides?
Nuclides which can be induced to fission with thermal neutrons. Energy released from neutron absorption is sufficient to overcome the activation energy for fission
Fissionable nuclides?
Nuclides that can be induced to fission by neutrons of high enough energy
Fertile nuclides?
Nuclides which can be used to produce fissile material by adding neutrons.
Typical components of a thermal neutron fission reactor (7 things - BIG CARD)
Fuel (uranium dioxide or metal, formed into rods)
Moderator (Light or heavy water, or graphite)
Cladding (Zirconium alloy or stainless steel)
Coolant (Light water or CO2)
Control Rods (Boron Carbide or Silver-Indium-Carbide)
Structure (Steel)
Pressure Vessel (Steel or concrete)
Neutron flux?
Product of neutron density and neutron speed. The number of neutrons crossing an area per unit time.
Reaction rate density?
The product of macroscopic cross section and neutron flux.
Homogenous vs Heterogenous systems?
Homogenous systems have fuel and moderator mixed together so that both materials experience the same neutron flux.
The 4 factor formula? (BIG CARD)
k = ηεpf η = fast neutrons produced from thermal fissions/thermal neutrons absorbed in fuel. ε = fast neutrons from all fissions/fast neutrons from thermal fissions. p = neutrons that reach thermal energies/fast neutrons that start to slow down. f = thermal neutrons absorbed in fuel/thermal neutrons absorbed in reactor.
Leakage neutrons?
In a finite reactor, neutrons that ‘leak’ out the reactor without interacting, characterised by non-leakage probabilities for fast and thermal neutrons.
Reactivity?
ρ = (k-1)/k. An alternate way of describing neutron multiplication, measured in $.
What does a reactivity of 1$ correspond to?
Prompt criticality - the power is increasing extremely fast. ρ = 0 for criticality.
Neutron lifetime?
The sum of the three timescales of a neutron’s life - fission, slowing down, and diffusion.
Reactor period?
The time it takes for the power or neutron population to increase/decrease by a factor of e.
What happens at prompt criticality?
The chain reaction is no longer reliant on the effects of delayed neutrons to slow it down, and can proceed solely on prompt neutrons.
Negative feedback in reactors?
An increase in temperature causes a decrease in reactivity (due to the thermal expansion of the water moderator, which decreases the moderating ability in the reactor).
This decreases the power which then decreases the temperature again (repeats). Quantified by coefficient of reactivity α_T.
Are water-cooled reactors over or under moderated?
Under moderated to achieve the negative feedback for safety.
How does negative feedback occur in solid reactors?
Thermal neutrons have higher energies, resulting in lower fission cross sections and making fissions so less likely. Also Doppler broadening of resonances increases the effect further.
Reactor poisons?
Nuclei that are strong neutron absorbers which do not contain any fissile materials. (eg Xe-135)
Burnable poisons?
Have a negative reactivity effect in the early reactor life but get used up over time to counter the effect of other poisons.
3 key ways radioactive waste is generated?
Activation products, fission products and higher actinides
Why did the Chornobyl reactor have a positive void coefficient?
It used water as a coolant and graphite as a moderator, meaning that when the water boils the graphite is still present and the reactivity increases due to less coolant.
Also hydrogen in water will absorb neutrons, hence boiling of the water reduced the effect of this
What were some of the causes of the Chornobyl accident?
- Positive void coefficient
- Lack of containment structure
- Bad control rod design (overflowing water)
- Non-compliance with safety standards
- Inadequate safety culture during construction
- Poor decisions made during test run
Why will 239Pu build up in a fission reactor?
238U can absorb neutrons, forming 239U. 239U then decays through two beta minus decays (past 239Np) to 239Pu. Note that 239Pu is fissile.
What is the thermal utilisation factor?
The fraction of neutrons that get absorbed in the fuel. Denoted in the four-factor formula as f.
f = fuel absorption cross-section / total reactor absorption cross-section
What is the resonance escape probability?
The proportion of neutrons that survive moderation from fast energies to thermal energies. Commonly referred to as p.
p = Neutrons that reach thermal energies/Fast neutrons that start to slow down
What is the fast fission fraction?
Commonly referred to as ε.
ε = Fast neutrons produced from all fissions/Fast neutrons produced from thermal neutrons
What is the thermal reproduction factor?
The number of neutrons produced per thermal neutron absorbed in the fuel. Commonly referred to as η.
If η < 1, a sustainable chain reaction is not possible.
η = Fast neutrons produced from thermal fission/Thermal neutrons absorbed in fuel
