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.
