Lecture 7: Materials & Fuel

Question 1

What are the three aims of understanding for materials used in nuclear reactor?

Answer 1

Understand the failure processes due to radiation.
Understand the corrosive nature of water.
Understand the corrosive nature of temperature.

Question 2

Define displacement damage:

Answer 2

Damage as a result of nuclear interactions, typically scattering, which can cause lattice defects.

Question 3

What are the two potential interactions between radiation and materials?

Answer 3

Shielding - properties of material are not degraded.
Damage - changes caused to material properties.

Question 4

How does the intensity of a narrow beam of mono-energetic particles vary through a material?

Answer 4

Intensity decreases exponentially.
I=I_0 e^-nsigmax

Question 5

Define the linear attenuation coefficient:

Answer 5

Describes the fraction of beam that is absorbed or scattered per unit thickness of material.

Question 6

Define the half-value layer (HVL):

Answer 6

Thickness of material required to reduce the intensity of radiation by 50%.

Question 7

What is the equation to calculate the half-value layer?

Answer 7

HVL = 0.693 / u

Question 8

Define the attenuation length:

Answer 8

Reciprocal of linear attenuation coefficient.
The average distance travelled by a particle before it is absorbed or scattered.

Question 9

What are the units of mass attenuation coefficient?

Answer 9

u / rho = cm2 / g

Question 10

Which radiation type is most penetrating?

Answer 10

Gamma rays

Question 11

What material properties are needed to absorb gamma rays?

Answer 11

High atomic numbers and high density.

Question 12

Define radiation damage:

Answer 12

Disruption to an initially undamaged structure caused by high-energy radiation passing through it.

Question 13

What is the effect of radiation damage?

Answer 13

Causes degradation and aging.
Restricts performance and defines service lifetime.

Question 14

What is the effect on yield stress as a result of irradiation?

Answer 14

Increases

Question 15

What is the effect on elongation as a result of irradiation?

Answer 15

Reduces

Question 16

Do materials become more or less brittle as a result of irradiation?

Answer 16

More brittle

Question 17

What happens to the dimensions of a material after radiation?

Answer 17

Swelling increases volume.

Question 18

List eight defects induced by radiation damage:

Answer 18

Vacancies
Interstitials
Electronic
Voids
Bubbles
Dislocations
Loops
Precipitates

Question 19

Define vacancies:

Answer 19

Missing atoms

Question 20

Define interstitials:

Answer 20

Atoms at sites in the crystal lattice not usually occupied.

Question 21

Define electronic defect:

Answer 21

Missing or trapped electrons

Question 22

Define voids:

Answer 22

clusters of missing atoms

Question 23

Define dislocations:

Answer 23

Line defects that start and end at surfaces or grain boundaries.

Question 24

Define loops:

Answer 24

Line defects that loop back on themselves

Question 25

Define precipitates:

Answer 25

second phase particles

Question 26

Define bubbles:

Answer 26

clusters of gas atoms occupying voids

Question 27

What are the four components of a nuclear fuel assembly?

Answer 27

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)

Question 28

Is U238 fertile or fissile?

Answer 28

Fertile

Question 29

How many fuel assemblies per core in a PWR?

Answer 29

193

Question 30

How many pins per fuel assembly in a PWR?

Answer 30

200

Question 31

Why are fuel pellets dished/concave at each end?

Answer 31

To accommodate the increased thermal expansion of the pellet interior relative to the edges.

Question 32

Why are fuel pellet edges chamfered?

Answer 32

To ease the assembly of pellets into pins and prevent damage.

Question 33

How is LLW disposed of?

Answer 33

Placed in metal containers and buried.

Question 34

How is ILW disposed of?

Answer 34

Encapsulated in cement

Question 35

How it HLW disposed of?

Answer 35

Immobilised in glass and stored underground