Lecture 3: Basic Reactor Physics Flashcards
How is neutron flux calculated?
Neutron number density x Neutron speed
How does neutron current differ from neutron flux?
Neutron current takes the direction of neutrons into account (+ or - matters)
Define source rate?
The rate of production of neutrons per unit volume
Why is moderation so important?
Probability of successful fission of fuel depends on the neutron energy
What is the average lifetime of a neutron?
10^-3 seconds
What is average number of neutrons produced per reaction?
2.59
What is the typical uranium contents of fuel?
96% U238
4% U235
Define criticality:
neutrons in = # neutrons out
Steady state. Reactivity = 0.
How is the rate of neutron number density calculated?
= Production rate - Absorption rate - Rate of flow of neutrons away from location
Define the diffusion approximation
Neutrons tend to drift from regions of high flux to low flux at a rate proportional to the flux gradient.
Where is the power distribution typically maximum?
At centre of vessel
How is the spatial distribution of thermal power and neutron flux related?
Lots of neutrons
Lots of fission
Lots of fragments colliding with lattice
Location of greatest energy generation
Define the multiplication factor?
Ratio of:
Rate of neutron production this generation
Rate of neutron loss this generation
What symbol is used to define multiplication factor?
k
What are the three multiplication factor states?
k < 1: Subcritical (ramp-down)
k = 1: Critical (steady-state)
k > 1: Supercritical (ramp-up)
What is k_infinity?
A material property to describe the multiplication factor if a reactor were infinitely large, therefore no leakage.
How to calculate k_infinity?
Ratio of:
Rate of neutron production this generation
Rate of neutron absorption this generation
What is a prompt neutron?
Neutrons produced instantaneously at the time of fission
What are delayed neutrons?
Neutrons emitted after a delay as part of the decay process for fission fragments
What are the approximate timings for a prompt and delayed neutron?
Prompt: 10^-4 s
Delayed: 0.1 s
Name two prompt feedback mechanisms:
Resonance absorption and doppler broadening
Fuel expansion
Name a delayed feedback mechanism:
Moderator heating
What is the importance of reactivity feedback mechanisms?
Required when neutron production is too high and must be reduced.
How does resonance absorption and doppler broadening effectively slow neutron production?
Increase the temperature of U238, thus improving effectiveness of resonances in capturing neutrons.
How does fuel expansion effectively slow neutron production?
Expanding the fuel reduces the number density reducing fission probability
How does moderator heating effectively slow neutron production?
As moderator temp is increased, its number density increases, reducing its ability to moderate so doesn’t slow down neutrons as well and reduces fission.
