TLO 5 Neutron Flux, Microscopic & Macroscopic Cross-sections Flashcards
(21 cards)
ELO 1 Explain atomic density
The number of atoms of a given type for unit volume of the material.
N = [p(6.022E23)]/M
ELO 1 Explain Neutron Flux
The number of neutrons passing through the unit area per unit time.
The total path length covered by all neutrons in one cm³ during 1 second.
Flux = neutron density × neutron velocity
ELO 1 Explain thermal neutron flux and fast neutron Flux.
The number of thermal neutrons crossing a unit area in a given amount of time period.
The number of fast neutrons crossing a unit area in a given amount of time.
ELO 1 Explain Microscopic Cross-Section and what factors make up the total microscopic cross-section of a nucleus.
Microscopic cross-section is the probability of a particular reaction occurring between a neutron and a nucleus. It depends on the characteristics of the nucleus, not the size. Barn is the unit of measurement.
Total microscopic cross-section is the sum of scattering cross-section and absorption cross-section.
ELO 1 Explain Microscopic Cross-Section for Scattering and the two components which make it up.
The probability of a neutron scattering.
Microscopic scattering cross-section is the sum of elastic scattering cross-section and inelastic scattering cross-section.
ELO 1 Explain Microscopic Cross-Section for Absorption and the two components which make it up.
The probability of a neutron being absorbed.
Microscopic absorption cross-section is the sum of fission cross-section and capture cross-section.
ELO 1 Explain the ‘Barn’ unit of measurement.
The unit of measurement for microscopic cross-sections.
1 barn = 10^-24 cm²
ELO 1 Explain Macroscopic Cross-Section
The probability of a given reaction occurring per unit travel of the neutron. The unit of measurement is 1/cm or cm^-1
Macroscopic cross-section = atomic density × microscopic cross-section
ELO 1 Explain the difference between microscopic and macroscopic cross-sections.
Microscopic cross-section represents the effective target area that a single nucleus presents to an incident neutron. The units are barns or cm².
Macroscopic cross-section represents the total effective target area of the nuclei contained in one cm³ of the material. The units are 1/cm or cm^-1
ELO 1 Explain Mean Free Path
The average distance of travel by a neutron before interaction. Represented by the symbol (lambda).
Mean Free Path is the inverse of macroscopic cross-section.
Mean Free Path = 1/ Macroscopic cross-section.
Units are in cm.
ELO 2 Define the fast neutron class
Fission neutrons are born as fast neutrons. Energy levels >1 MeV.
ELO 2 Define the intermediate neutron class
Neutrons with energy levels between 1eV and 1MeV
ELO 2 Define the slow neutron class
Neutrons with energy levels less than 1eV
ELO 2 Define a thermal neutron
Neutrons with an energy level of 0.025 eV at 68ﾟF. Velocity & energy increase with temperature.
These are in the slow neutron class.
ELO 3 Describe how absorption and scattering cross-sections vary with neutron energies in the 1 eV region.
In 1 eV region, scattering cross-section is relatively consistent. The absorption cross-section is steadily decreasing in the 1/v region and begins resonance peaking at ~1eV.
ELO 3 Describe how absorption and scattering cross-sections vary with neutron energies in the resonance absorption region.
In the resonance absorption region, absorption cross-sections have sharp resonance peaks. Scattering cross-sections have resonance peaks, but they are much smaller than absorption peaks.
ELO 3 At low neutron energies (<1 eV) the absorption cross-section for a material is ____________ proportional to the neutron velocity.
Inversely
ELO 4 Describe the macroscopic cross-section and mean free path at various temperatures.
Macroscopic cross-section is inversely proportional to temperature.
Mean Free Path is inverse to macroscopic cross-section, therefore Mean Free Path is directly proportional to temperature.
ELO 5 The neutron flux that varies from the top to bottom of the core is called the _______ flux and the neutron flux that varies from across the top is _______ flux.
Axial
Radial
ELO 6 Describe how changes in neutron flux and macroscopic cross-section affect reaction rates.
Reaction rate (R) = macroscopic cross-section × neutron flux.
Therefore, raising either flux or macroscopic cross-section will raise reaction rates, and vice-versa.
Note: macroscopic cross-section decreases with fuel burnup so operators have to increase flux to maintain reaction rates.
ELO 7 Describe the relationship between neutron flux and reactor power.
Rx power and neutron flux are directly proportional if reactor volume and macroscopic cross-section are constant.
As stated earlier, if macroscopic cross-section decreases with fuel burnup, then neutron flux must be increased to maintain power constant
