Chapter 7 - Fuel Behaviour Flashcards
What is the natural abundance of U235?
0.711%
What are the 6 steps in the LWR fabrication?
- Uranium Mine or In-situ Leach
- Uranium Mill or Processing Plant
- Conversion
- Enrichment
- Fuel Production
- NPP
Name the evolution of the fuel microstructure with temperature
- minor densification <1100C
- intergranular porosity 1100 - 1300C
- Equiaxed Grain growth 1300 - 1600C
- Columnar Grain growth 1600 C
- Central void >1600C
Does the density decrease or increase with burnup? What is the percentage decrease of density at 100 GWd/tU
It decreases with burnup. around 15%.
Does the pellet volume decrease or increase with burnup? What is the percentage change at 80 MWd/kgU?
It increases due to swelling. 5% volume change
What causes the fuel to swell?
solid and gaseous fission products and radiation damage
What are the effects of swelling when the clad gap is closed?
improve heat transfer due to direct contact but introduces significant mechanical stress on cladding.
What is the fission gas release percentage on a rod at an average burnup of at 100 MWd/kgU?
25%
Why is cracking related to fission product release rate?
The cracking increases the surface area of the fuel which increases the rate at which fission products can leave the fuel.
Why does the fuel crack?
Thermal stresses, and swelling causing mechanical stresses.
What is fuel cladding chemical interaciton?
Fuel-cladding chemical interaction (FCCI) is a phenomenon that occurs at the fuel-cladding interface during the irradiation of U-Zr and U-Pu-Zr metallic nuclear fuel and stainless steel cladding. The inter-diffusion zone that develops places both the fuel and cladding at risk through the reduction in cladding strength and the formation of a (U,Pu)/Fe eutectic in the fuel.
Why is a void form in the UO2?
The nuclear fuel can swell during use, this is because of effects such as fission gas formation in the fuel and the damage which occurs to the lattice of the solid. The fission gases accumulate in the void that forms in the center of a fuel pellet as burnup increases. As the void forms, the once-cylindrical pellet degrades into pieces. The swelling of the fuel pellet can cause pellet-cladding interaction when it thermally expands to the inside of the cladding tubing. The swollen fuel pellet imposes mechanical stresses upon the cladding.
Fuel cladding chemical interaction what is the element distribution describe.
The cladding diffuses into the reaction layer/oxide layer/bonding layer. The presence of uranium in the fuel is evident but also plutonium increases in the outer rim of the fuel pellet.
What are three significant enrichments elements in the rim structure?
Cs, Mo, and Te
How are cladding elements transproted to the fuel. Describe what reaction makes cladding elemnts to fuel mass transport?
Volatile compounds like Ioidine which lead to mass transport. Iodine gas + metal -> MI compound
The mass transport reaction is form in colder or hotter areas?
colder areas because of heat of formation is negative
Describe the volatile compound mass transport process
I + M means MI solid compounds forming in near clad areas where the temperature is colder. As it diffuses the MI desintegrates on the hot fuel side leaving metal in the fuel. The gas can migrate again to pick up more metal from the cladding leading to cladding desintegration
What is the rim structure? What is the effect on fuel performance?
Pu build up at the pellet periphery consisting on a dense small sub grains due to recrystallization or polygonization and accumulation of pores.
Higher burnup, two to three times more.
Does burnup differ through the relative radius in fuel?
It varies especially in the outer rim where burnup can be more than twice that of the center.
Why does thermal conductivity decreases with burnup? What are other effects?
due to fission product, gas porosity and radiation damage buildup which impacts the temperature distribution which in turn controls fission product migration and release, grain growth and swelling.
Why are cracks in fuel?
Due to thermal stresses induced by large radial changes in temperature
Does temperature differ through the relative radius in fuel? Why?
The temperature is lower at the rim in the high burnup structure. This lower temperature and decrease thermal conductivity is due to the rim structure making it a less dense porous area.
Does plutonium concentration differ through the relative radius in fuel? Why?
More Pu is found at the periphery, around 3-4% more
Does Xe concentration differ through the relative radius in fuel? Why?
More Xe is found at the periphery.