Pellet clad interactions Flashcards
How does time effect the temperature gradient?
Huge temperature gradient in radial fuel pin
Gap decreases over time
pellet thermal conductivity decreases with radiation
What are the three key effects of gap closure?
Densification of pellet (rapid)
Swelling of pellet (continuous)
Creep slow down of the cladding (slow)
To model the heat transfer across the gap which coefficients are considered?
radial
conductive
press
oxide
How does radial cracking occur?
During the initial rise to power due to extreme temperature gradient
Initial gas gap closes due to creep - coolant pressure of cladding, swelling of fuel pellet
Pellet - hour-glassing from thermal expansion -> bamboo shaped fuel
Radial fuel pellet cracks at triple points
How do cracks occur in AGR fuel?
UO2 pellets in Nb stabilised austenitic stainless steel clad
Nb forms carbides to provide pinning particles to reduce creep rates
During rise to power pellet cracks
Clad creeps down onto pellet as pellet expands
pellet clad gap closes - bonding occurs
During down rates - pellet contracts and sliver of bonded fuel remains
During rate changes pellet fragments move
Radial displacement and opening/closing of radial crack occurs
Strain concentrated ahead of crack due to sliver of bonded fuel
-> formation of clad bore cracks ahead of radial cracks
problem is made worse by loss in ductility of clad
How does carbon deposition affect in AGR?
Impairs the heat transfer between cladding and coolant, increasing cladding temperature increases failure due to creep
How does helium embrittlement occur in the AGR?
Helium due to neutron irradiation of nickel leading to loss of ductility due to irradiation embrittlement
Nano-bubbles cause creep cavity nucleation sites at grain boundaries
What are the main parameters for I-SCC of PCI in PWR?
Iodine stress corrosion cracking
Friction between clad and pellet
Stress concentration in clad due to pellet clad hour-glassing
Clad strains due to opening of pellet radial cracks
What is the mechanism of I-SCC?
Initiated as intergranular crack and can change morphology into transgranular crack
TG at interfaces of course slip bands and gbs, in grains not orientated for prismatic slip and adjacent to grains that are
Recrystallised microstructure show higher resistance
What are PCI resistant fuels?
Pure Zr barriers to make more I-SCC resistant
Chromia doped - larger grain size and more cracking and greater creep - more cracks reduce stress in pellet and clad due to better distribution of strain
MOX - more PCI tolerant