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.
Does porosity differ through the relative radius in fuel? Why?
Yes, the outer rim structure is 10 to 20 percente more porous.
What is the thermal conductivity a function of?
the thermal diffusivity, the density, and the heat capacity
What is the thermal diffusivity?
is the thermal conductivity divided by density and specific heat capacity at constant pressure.[1] It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI derived unit of m²/s.
Does the thermal diffusivity increases or decreases with temperature?
It decreases.
Does the thermal diffusivity decrease or increase with irradiation?
It decreases
Is it possible to recover thermal diffusivity?
Yes, by post-irradiation annealing
What is the thermal conductivity?
The thermal conductivity of a material is a measure of its ability to conduct heat
How can we measure thermal diffusivity?
is used to measure thermal diffusivity of a variety of different materials. An energy pulse heats one side of a plane-parallel sample and the resulting time dependent temperature rise on the backside due to the energy input is detected. The higher the thermal diffusivity of the sample, the faster the energy reaches the backside
Why does the cladding crack at the same location the fuel cracks?
the cladding is bound to the fuel of friction forces are high and the tension stress onthe cladding at the crackleads to cladding cracking
What are the four main aspects of irradiated nuclear fuel performance?
- Formation of distribution of fission gas bubbles
- Swelling and fission gas release causing cracking and formation of high burnup RIM structure
- Thermal conductivity degradation
- Mechanical and chemical interaction of the pellet-cladding.
What is the typical kinetic energy of the fission fragments?
Light FP = 100 MeV
Heavy FP = 70 MeV
How are ions slowed down in a material
electronic stopping which is dissipated as heat
At what burnup do you start to get pellet cladding interaction?
35 MWd/kgU
At 50 GWd/tU what is the thicknes of reaction layer?
around 10 micrometer
What group yield do rare earths belong to? Any other elements in this group yield?
dissolved in UO2, Mo and Zr also
What group yield do noble metals belong to? In what form?
metallic inclusions, appear elementally with Mo
What group yield do barium belong to? In what form?
second oxide phase, as BaZrO3
What group yield do cesium belong to? In what form?
alkali metals, in the form of Cs2O and Cs2UO4
What group yield do xenon and krypton belong to?
rare gases
How many group yields are sufficient for describing the physical and chemical behavior of the FP in the fuel?
45
Where does oxygen come from in the fuel that can form oxigen compounds with other elements?
From the fission of U. It releases two atoms of Oxygen
What are the five group yields/
- Dissolved in UO2
- Metallic inclusions
- Second oxide phase
- Alkali metals
- Rare gases
What is the swelling due to solid fission products a function of?
The FP group yields, the burnup, the atomic volume of U, thefission rate per unit volume, and the atomic volue of the group yields which is calculated as the molecular weight of phase containing i divided by the mass density of phase containing i
Are rare earths soluble in UO2? Are rare earths usually release from UO2 fuel? Why
Yes, No, because of low concentration in fuel and low vapor pressures leading to low partial pressures, raults law
Are NOBLE METALS and barium soluble in UO2? Are noble metals usually release from UO2 fuel? Why?
No, they are insoluble, but are still not release despite high concentration due to low vapor pressures
What is the vapor pressure a function of?
is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid’s evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile.
Are rare gases thermodynamically stable in UO2?
No, they have essentially zeor thermondynamic stability in UO2.
What is the result of the low zero thermodynamically stability in UO2? Give three points.
- They remains atomically dispersed in the fuel lattice
- Precipitate into small high pressure bubbles
- Get relased from the fuel by diffusion
Where do high pressure bubbles reside?
inside the grain
What are two secondary effects of fission gas in terms of pressure?
- Due to diffusion the internal pressure rises
2. High pressure bubbles cause swelling
What is the five basic processes involving fission gas bubbles and dissolved gas atoms?
- Nucleation
- Growth
- Re-solution
- Coalescence
- Release
What is nucleation
atom jumbing (diffusing) to form diatoms which serve as embryo nuclei for bubbles
What is growth in terms of the five basic processes involving fission gas bubbbles?
collection of newly created gas atoms by diffusion from the solid to the bubble
What is re solution in terms of the five basic processes involving fission gas bubbbles?
Xenon atoms in a bubble are injected into the solid by passage of a fission fragment
What is coalescence in terms of the five basic processes involving fission gas bubbbles?
when two bubbles join into a single larger bubble
What is release in terms of the five basic processes involving fission gas bubbbles?
gas atoms escape by diffusion to grain boundaries or free surfaces
How does the rate of nucleation of new bubbles behave as the number of xe clusters increases?
As the clusters increase the rate of nucleation ceases and grwoth starts
How are nucleation times relative to temperature
at high temperatures the nucleation time is short and viceversa
How does the bubble size and bubble number density behave as a function of the nucleation period?
the bubble size is small during nucleation period and constant but the bubble number density increases
What parameter has the greatest effect in bubble growth? What other parameters have an effect on grwoth rate?
The diffusion coefficient for Xe in UO2. Other parameteres include the concetration of free Xe atoms and bubble radius
What process can cause bubble grwoth halt and even decrease the number density?
Re solution due to fission fragment collision with Xe atoms dirivng them to the solid.
What is the probability that a FF collides with a gas bubble a function of?
projected area of the bubble and the area of sphere and the mean free path between collisions of FF in bubble.
How can fissiong as swelling cause sever damage to the cladding rapidley?
In a reactivity insertion accident (RIA)
What is a reactivity insertion accident
When there is a reactivity insertion by accidental ejection of a control rod.
What makes fission gas more dangerous than solid fission products release?
Gas is a source of mobile radioactivity if cladding fails
What are the two main types of nuclear fuels?
Metallic fuels and ceramic fuels?
What are the two most popular metallic fuels?
U and Pu-based metal fuels
WHhat are the three main advantage of metallic fuels?
- high thermal conductivity
- high fissile atom density
- good fabricability
what are the 4 main disadvatanges of metallic fuels?
- low melting points
- unstable in irradiation
- poor corrosion resisntance
- incompatibility issues with fuel cladding material
Why did ceramic fuels replaced metallic fuels?
because metallic fuels have low strenght at high temperatures and unfavroable phase transformations
What is the main disadvatange of ceramic fuels?
ceramics can be or become brittle especially at low tempereatures
What are the 4 main advantages of ceramic fuels?
- superior strength at high temps
- low thermal expansion
- good corrosion resistance
- good radiation stability
What is the melting point of UO2?
2800 C
What are the five aspects that make UO2 ideal for NPP?
- High melting point
- chemical stability in water cooled reactors
- compatibility with zircaloy and ss cladding
- excellent irradiation stability
- easy of fabrication
Is the UO2 fuel fabricated to be non porous?
No, it is intentially fabricate with 3 to 7% porosity
What are the atomic mass number where the fission products are centered?
95 and 139
What is the energy release per fission
200 MeV
How is the fission energy distributed?
- Fission fragments (80%)
- Gamma rays and neutrinos (10%)
- Decay from FP (7%)
- Fission chain reaction (3%)
What are the two sources of energy in the pelletes?
local temperature and energy from fission
Where is the temperature higher? Inner or outer? By how much?
Inner temperature is 1200C while outer rim is 800C
Where is the power generation higher? inner or outer?Why?
Outter rim since neutrons easily enter fuel causing fission in the periphery
Why does the rim structure created?
Because of neutron capture of U creates Pu239 especially in the outer region since due to the self shielding effect there is a depression in neutron flux in the center.
Why do we need to underestand the thermal conductivity of the fuel
It is needed for determining the temperature distribution
Is the thermal conductivity good for UO2?
No, its bad and it gets worse with burnup
Does the thermal conductivity of the fuel varies with radius?
Yes, and with burnup
How does the thermal conductivity relate to temperature inteh fuel
The higher the temperature the lower the thermal conductivity
Where is the thermal conductivity lower in the fuel pellet and why?
In the center because the temperature is at its highest there
Why is initial porosity introduced at fabrication? Is there a drawback?
To accomodate fission products during irradiation. The drawback is that porosity decrease thermal conductivity and it becomes worse as burnup goes.
What are the five main effects categories of degradation of thermal conductivity in the fuel?
- Porosity increases and thermal conductivity keeps decreasing
- precipitates of insoluble fission products
- fission products which form solid solution with 𝐔𝐎𝟐
- volatiles and fission gases:
- point defects created by radiation damage
- microstructural evolution
What is the effect of insoluble fission products infuel?
A decrease in thermal conductivity and it varies linearly with their volume fraction, which is proporational to the burnup of the fuel
What three types of elements can form mixed oxides with UO2?
Actinides, rare earths, and transition metal atoms
How do actinides, rare earths, and transition metal atoms end up in the fuel? Name some example elements.
End up completely or partially dissolved in the fuel metrix by forming mixed oxides. Some elements include, Zr, Ce, Nd, Ba, La, Np, Pu
What is the effect of solid solutions with uo2 in thermal conductivity?
it decreases because differences in atomic radius in the lattice results in decrease the overal thermal conductivity due to phonon scattering
What is the effect of point defects in thermal conductivity?
reduction in thermal conductivity
Why does the fuel pellet crack?
because of thermal strains
How is the energy caused by thermal strain release in the fuel pellet?
In fragmentation of the fuel pellet
Why does the pellet take an hour-glassing cylinder shape? What does it mean in terms of pellet, cladding interaction?
Because of the steep radial temperature gradient in the fuel pellets.
This means the interaction starts at the top and bottom edges
Where does zirconium oxide come from in the cladding internal side?
From the fuel pellet cladding interaction, there is interpenetration of species including oxygen due to fission recoil causing zirconium oxide layer to form in the interface.
What is the secondary effect of the zirconium oxide layer formed due to fuel cladding interaction?
The ZrO layer can propagate and implantation of other species including U and Pu creates a chemical bonding between the two materials.
Can the gap reopen in fuel pellet -cladding interaction with a power decrease? Why?
No, due to the chemical bonding caused by the ZrOxide layer. This means the stress is introduced in the fuel pellet periphery.
How do cracks in the fuel pellet propagate to the cladding?
During power ramps and declines it is possible for various FP species to lower the corrosion resistance in the cladding material which combined with the fuel pellet cracking and the close contact between fuel and cladding causes cracking propagated through the pellet.
Other than the oxide layer how is the cladding degraded in the inner region?
Due to transport by fission products in the fuel. For example Iodine formes a gaseous compound ZrI2 which then diffuses to the interior of the fuel and decomposes leaving Zr in the fuel while the Iodine can pick up more Zr metal.
Describe the fuel pellet clock?
- Before burnup, radial cracking begins due to thermal stresses but microstructure remains the same.
- When burnup begins, a center void is created and columnar and large equiaxed grains appear
Why is the central void appear in the fuel?
Initial pores in the material moving towards the center .
where do the columnar grains form in the fuel pellet? How are they formed?
Immediately adjacent to the void formed due to the trails of the pores or fission gas bubbles that migrated to the center of the fuel due to the tempearture gradient
Where are the large equiaxed grains and how do they form?
They are after the void and columnar grains.
Formed due to the ideal temperature for the asfabricated oxide grains to grow to manny times their original size
Why is there almost no microstructure evolution in the outer layer of the fuel pellet?
Due to the lower fuel temperatures.
Why are Xe and Kr the two more problematic FP?
1- virtually insoluble in the fuel matrix
2-gas state rather than a solid meaning rejection from fuel matrix
Where do Xe and Kr end up?
- As part of the gaseous atmosphere of the fuel pin
2. precipitate as small bubbles of gas within the fuel itself
What is the most improtant effect of fp gases like xe and kr? What is the effect in fuel performance? What about thermal conductivity?
affects fuel performance because it promotes fuel-cladding contact, and the resultant stress on the cladding can shorten its lifetime. In addition, the thermal conductivity of the fuel is lowered due to the presence of bubbles, and therefore leads to much higher temperatures in the fuel than desired.
What are the number of gas atoms in a bubble a function of?
the radius of the bubble and the contant exxpressing the volume occupied by the atoms proper
How is the burnup and swelling increase related?
The swelling increases as a funciton of burnup to the power of 3/2.
How much swelling is generated by burnup and FP?
a few percent
Why are metallic oxides found more as burnup increases?
Because the fission of the fuel generates two oxygen atoms per fission and a metallic compound which can form an oxide with the release oxygen.
What happens to fission products with limited solubility?
They migrate to grian boundaries and eventually to the void space within the element.
What is the burnup?
The number of fissions divided by the number of initial heavy metal atoms. In other words
Fission rate * time / initial atoms of U
The average burnup is about X times as large in a fast reactor?
3 times as large
What is a major change in fuel elements in fast reactors ?
A larger plenum region to acoomodate fission gases, Xe, and Kr primarely
What are the four main differences between a thermal and a fast reactor?
- flux is 100 larger
- burnup is about 3 times as large
- fluence much grater causing more radiation damage
- fast reactor has a max operating temperature equal to the melting temperature of the fuel.
What is the bubble growth rate a functio of?
J = 4piRDgC
where C is the coenntration of Xe atoms
R is the bubble radius as time
and Dg is the diffusion coefficient of Xe in UO2
