Chapter 7: Fuel

Question 1

What happens in the fuel during operation?

Answer 1

Porosity changes, grain growth and recrystallization, extensive cracking, RIM structure (Pu rich), Swelling, precipitation and release of fission products, degraded thermal conductivity and mechanical and chemical interaction with the cladding due to swelling and fission product transport

Question 2

Does the fuel density increases or decreases with burnup?

Answer 2

Decreases

Question 3

Does the volume of the fuel decreases or increases with burnup?

Answer 3

Increases

Question 4

Why does swelling occur in the fuel?

Answer 4

Swelling produced by solid & Gaseous fission products along with radiation damage

Question 5

What effect does swelling have in the fuel cladding interaction

Answer 5

Improves heat transfer but introduces significant mechanical stress on cladding

Question 6

What does LHR stand for?

Answer 6

Linear Heat Rate (heat generation rate per unit length of fuel rod)

Question 7

What effect does Xenon gas release in gap has in thermal conductivity?

Answer 7

It reduces thermal conductivity

Question 8

What effect does iodide has in fuel cladding interaction?

Answer 8

Iodine is release and is capable of forming MI2 compounds which means that it picks up metal from the cladding therefore disintegrating the cladding.

Question 9

Where is the burnup higher? In the middle or the outside of the pellet?

Answer 9

Burnup is higher in the outside due to Pu buildup.

Question 10

Swelling improves thermal conductivity but fission product gas release decreases it. What is the overall result?

Answer 10

Decrease in thermal conductivity

Question 11

As burnup increases what happens to the temperature distribution inside the pellet?

Answer 11

Temperature is higher in the center.

Question 12

As burnup increases what happens to porosity?

Answer 12

Porosity increases from the center to the outer rim.

Question 13

Where does the cladding crack occurr when fuel crack is in contact with the cladding?

Answer 13

At the same location since the cladding is bound to the fuel or friction forces are high.

Question 14

What is the size of the gas bubbles at the interior of the pellet?

Answer 14

Larger in the interior

Question 15

What is the average kinetic energy of the light fission product?

Answer 15

100 MeV

Question 16

What is the average kinetic energy of a heavy fisison product?

Answer 16

70 MeV

Question 17

What does SHI stand for?

Answer 17

Swift Heavy Ion (SHI)

Question 18

What are the two main models for SHI?

Answer 18

Thermal spike model and Coulomb explosion model

Question 19

What does the thermal spike model consist of?

Answer 19

Excited electrons rapidly transfer energy to phonons. Very large energy heat deposition leading to localized melting and rapid cooling therefore resulting in high defect densities or amorphization.

Question 20

What does the couomb explosion model consist of?

Answer 20

Large positive space charge resulting from electronic excitation lead to stron atomic repulsion, atomic displacements and a cylidnrical shock wave.

Question 21

What is the final and only member of the mass 137 chain and why?

Answer 21

Cs-137 since its half life is long compared to irradiation times in reactor and the precursors are relatively short-lived by comparison

Question 22

What are the fived groups that describe the chemical states of the fission products?

Answer 22

Dissolved in UO2, Metallic inclusions, second oxide phase, alkali metals, rare gases.

Question 23

What are the three main gas behaviors of rare gases?

Answer 23

They have zero thermodynamic stability in UO2 therefore they remain atomically-dispersed in the fuel lattice, precipitate into small high-pressure bubbles inside the grain, and are released from the fuel by diffusion.

Question 24

What are the 5 main basic processes involing fission gas bubbles and dissolved gas atoms.

Answer 24

Nucleation, growth, resolution, coalescence and release