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At elevated temperatures and constant stress or load, many materials continue to deform at a slow rate
Creep
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The amount of energy required to displace an atom from its crystal lattice in a metal
Displacement energy
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Occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock‑ons)
Displacement spike
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The measure of the ease with which a material can be worked and made into desirable shapes and forms
Fabricability
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A neutron that has a kinetic energy greater than 0.1 MeV
Fast neutrons
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The tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength.
Fatigue failure
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Ability to withstand peripheral expansion without failure or the capacity of the material to be manufactured into the final required shape.
Formability
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A neutron that has a kinetic energy between 0.1 MeV and 1 eV
Intermediate neutron
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A permanently displaced atom that has lost its energy and is occupying a position other than its normal crystal lattice site
Interstitial
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If a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for most metals) in a collision with a radiation particle (usually a fast neutron), the nucleus will be displaced from its equilibrium position in the crystal lattice. The target nucleus (or recoiling atom) that is displaced is called a ________ nucleus or just a __________.
knocked-on, knock-on
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Describes how a metal reacts to mechanical deformation by removing chips, with respect to the amount of metal effectively removed and the surface finish attainable
Machinability
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The temperature at which a given metal changes from ductile to brittle fracture.
Nil-Ductility Transition (NDT) temperature
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A neutron that has a kinetic energy less than 1 eV
Slow neutrons
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_______ of a material refers to its mechanical and chemical inertness under the conditions to which it will be subjected
Stability
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The irradiation-induced volume increase that has been observed in UO2 and mixed-oxide fuels.
Swelling
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A neutron that is in equilibrium with its surroundings
Thermal neutrons
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Occurs when radiation deposits energy in the form of a knock-on, which in turn, transfers its excess energy to the surrounding atoms in the form of vibrational energy (heat).
Thermal spike
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When a metal atom is ejected from its crystal lattice the vacated site is called a _______
Vacancy
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When a metal is strained beyond the yield point. An increasing stress is required to produce additional plastic
deformation and the metal apparently becomes stronger and more difficult to deform
Work hardening (also known as strain hardening)
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DESCRIBE the considerations commonly used when selecting material for use in a reactor plant
Availability, cost, fabricability, formability, machinability, mechanical strength, ductility, stability, heat transfer
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Identify the importance of a material property and its application in a reactor plant
Preventing the release of radioactive fission products is a major concern in the design, construction, and operation of a nuclear plant. Mechanical strength plays an important role.
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State how the following types of radiation interact with metals
Alpha
Beta
Gamma
Neutrons
- Alpha - Easily stopped by metals
- Beta - Also stopped by metals
- Gamma - Interact with lead and would transfer energy as heat
- Neutrons - Iron in the form of carbon steel or stainless steel absorb a considerable proportion of the energy of fast neutrons. (ineleastic scattering occurs) Slow neutrons interact with water.
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Describe the requirements of a material used to shield against the following
Alpha
Beta
Gamma
Neutron
- Alpha - A thin piece of paper stops alpha
- Beta - A sheet of aluminum foil or plexiglass
- Gamma - Most difficult to shield. Most shielding is with concrete and lead
- Neutron - At low energies neutrons can be shielded by water (<1MeV) At high energies (>1MeV) Iron in the form of carbon steel or stainless steel are commonly used.
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State the measures taken to counteract or minimize the effects of the following
Fatigue failure
Work hardening
Creep
- Fatigue failure - For a pressurizer, the load variations are fairly low, but the cycle frequency is high; therefore, steel of high fatigue strength and of high ultimate tensile strength is desirable. The reactor pressure vessel and piping, by contrast, are subjected to large load variations but the cycle frequency is low; therefore, high ductility is the main requirement for the steel.
- Work hardening - reduces ductility, which increases the chances of brittle failure
- Creep - staying below the creep limit
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State the effect a large number of displacement spikes has on the properties of a metal
A 1MeV neutron may affect approximately 5,000 atoms, making up one of these spikes. The presence of many displacement spikes will change the properties of the material being irradiated
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Describe how the emission of radiation can cause dislocation of the atom emitting the radiation
A dislocation loop is formed when the collapse of adjacent atomic planes takes place
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State the two effects on a crystalline structure resulting from the capture of a neutron
- 1) Introductions of an impurity atom due to the transmutation of the absorbing nucleus
- 2) Atomic displacement caused by recoil atoms or Knock-ons
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State how thermal neutrons can produce atomic displacements
Directly, but they can do so indirectly as the result of radioactive capture and other neutron reactions or elestic scattering
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