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25 Cards in this Set
- Front
- Back
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The process of reducing the energy of a neutron to the thermal region by elastic scattering is referred to as ___
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thermalization, slowing down, or moderation
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The material used for the purpose of thermalizing neutrons is called a _____
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moderator
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A good moderator reduces the speed of neutrons in a small number of collisions, but does not ___
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absorb them to any great extent
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Slowing the neutrons in as few collisions as possible is _____ in order to reduce the amount of neutron leakage from the core and also to reduce the number of resonance absorptions in non fuel materials
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desirable
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The ideal moderating material (moderator) should have the following nuclear properties.
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• large scattering cross section
• small absorption cross section • large energy loss per collision |
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______ is the average decrease per collision in the logarithm of the neutron energy.
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average logarithmic energy decrement
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The higher the moderating ratio, the_____ the material performs as a moderator
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more effectively
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_______ are those neutron-producing reactions that always occur in reactor materials
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Intrinsic neutron sources
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An ______ is an assembly placed in or near the reactor for the sole purpose of producing source neutrons
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installed neutron source
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List three examples of reactions that produce neutrons in intrinsic neutron sources.
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Spontaneous fission
Photoneutron reaction Alpha Neutron |
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List three examples of reactions that produce neutrons in installed neutron sources.
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Spontaneous fission
Photoneutron reaction Alpha Neutron |
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The _____ is the number of atoms of a given type per unit volume of the material
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atom density
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The probability of a particular reaction occurring between a neutron and a nucleus is called the _______
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microscopic cross section (σ)
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Some nuclei behave as if they were thousands of times bigger so ____ was determined to be the name for an area = 1 x 10-24 cm2.
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Barn
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Whether a neutron will interact with a certain volume of material depends not only on the microscopic cross section of the individual nuclei, but also on the number of nuclei within that volume.
Therefore, it is necessary to define another kind of cross section known as the _____ |
macroscopic cross section
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______ is the probability of a given reaction occurring per unit travel of the neutron
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macroscopic cross section
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The _____ represents the effective target area that a single nucleus presents to a bombarding particle.
Microscopic units are given in barns (or 1 x 10-24 cm2). |
microscopic cross section (σ)
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The ____ represents the effective target area that is presented by all of the nuclei contained in 1 cm3 of the material.
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macroscopic cross section (∑)
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This average distance traveled by a neutron before interaction is known as the ____ for that interaction and is represented by the symbol λ.
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mean free path
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Neutrons whose energies have been reduced to values in this region (< 1 eV) are designated as _____
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thermal neutrons
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Although the logarithmic energy decrement is a convenient measure of the ability of a material to slow neutrons, it does not measure all necessary properties of a moderator.
A better measure of the capabilities of a material is the _____. |
macroscopic slowing down power
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_____ is the product of the logarithmic energy decrement and the macroscopic cross section for scattering in the material
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Macroscopic slowing down power (MSDP)
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_____ is the ratio of the macroscopic slowing down power to the macroscopic cross section for absorption.
The most complete measure of the effectiveness of a moderator |
Moderating ratio
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The great majority (over 99.993%) of the neutrons produced in fission are released within about 10‑13 seconds of the actual fission event.
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Prompt Neutrons
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A small portion of fission neutrons are ____, which are produced some time after the fission process has taken place.
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delayed neutrons
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