Lecture 7: Materials & Fuel Flashcards
What are the three aims of understanding for materials used in nuclear reactor?
Understand the failure processes due to radiation.
Understand the corrosive nature of water.
Understand the corrosive nature of temperature.
Define displacement damage:
Damage as a result of nuclear interactions, typically scattering, which can cause lattice defects.
What are the two potential interactions between radiation and materials?
Shielding - properties of material are not degraded.
Damage - changes caused to material properties.
How does the intensity of a narrow beam of mono-energetic particles vary through a material?
Intensity decreases exponentially.
I=I_0 e^-nsigmax
Define the linear attenuation coefficient:
Describes the fraction of beam that is absorbed or scattered per unit thickness of material.
Define the half-value layer (HVL):
Thickness of material required to reduce the intensity of radiation by 50%.
What is the equation to calculate the half-value layer?
HVL = 0.693 / u
Define the attenuation length:
Reciprocal of linear attenuation coefficient.
The average distance travelled by a particle before it is absorbed or scattered.
What are the units of mass attenuation coefficient?
u / rho = cm2 / g
Which radiation type is most penetrating?
Gamma rays
What material properties are needed to absorb gamma rays?
High atomic numbers and high density.
Define radiation damage:
Disruption to an initially undamaged structure caused by high-energy radiation passing through it.
What is the effect of radiation damage?
Causes degradation and aging.
Restricts performance and defines service lifetime.
What is the effect on yield stress as a result of irradiation?
Increases
What is the effect on elongation as a result of irradiation?
Reduces
Do materials become more or less brittle as a result of irradiation?
More brittle
What happens to the dimensions of a material after radiation?
Swelling increases volume.
List eight defects induced by radiation damage:
Vacancies
Interstitials
Electronic
Voids
Bubbles
Dislocations
Loops
Precipitates
Define vacancies:
Missing atoms
Define interstitials:
Atoms at sites in the crystal lattice not usually occupied.
Define electronic defect:
Missing or trapped electrons
Define voids:
clusters of missing atoms
Define dislocations:
Line defects that start and end at surfaces or grain boundaries.
Define loops:
Line defects that loop back on themselves
Define precipitates:
second phase particles
Define bubbles:
clusters of gas atoms occupying voids
What are the four components of a nuclear fuel assembly?
Fissile substance (e.g. U235)
Host matrix (e.g. oxide or metal U238)
Metal cladding (to prevent fission gases escaping)
Additional fittings (to hold structures together and locate into reactor)
Is U238 fertile or fissile?
Fertile
How many fuel assemblies per core in a PWR?
193
How many pins per fuel assembly in a PWR?
200
Why are fuel pellets dished/concave at each end?
To accommodate the increased thermal expansion of the pellet interior relative to the edges.
Why are fuel pellet edges chamfered?
To ease the assembly of pellets into pins and prevent damage.
How is LLW disposed of?
Placed in metal containers and buried.
How is ILW disposed of?
Encapsulated in cement
How it HLW disposed of?
Immobilised in glass and stored underground
