Radiation With Matter Flashcards
General affects of irradiation on matter
Swelling due to fission fragments and helium gas due to alpha decay
Volume increase due to Landing in interstitials and vacancies when they can’t fit
Phase changes, radiation induced phase
Distortion at constant volume under irradiation is referred to as growth
Redistribution of alloying elements known as radiation induced segregation
Explain the radiation damage event
Neutron produced by fission collided with a lattice particle.
The atom displaced becomes a primary knock on atom
The pka transfers energy to other atoms, losing energy as it moves.
It will eventually come to rest in an interstitial (frenkel pair)
Advantages of metallic fuel
Good thermal conductivity Low melting point Anisotropic swelling Fabrication is cheap and easy High fuel content
Disadvantages of metallic fuel
Change in phase due to temperature
Huge swelling due to this
Advantages of ceramic fuel
More stable Less swelling due to inherent and engineered porosity Lower density Higher melting point Phase stability
Disadvantages of ceramic fuel
Lower thermal conductivity due to pores, ff and presence of Pu
Low fracture strength
Low heat transfer to coolant
Limited diameter due to heat transfer
Main steps in fuel fabrication
Milling and mining UO2 powder with binder and lubricant
Granulating to form free flowing pellets in a spherodiser
Cold compaction in automated press (green pellets)
Heating removes lubricant and hinder
Sintering
Why is fabrication necessary (pores)
Fabricated pores are useful in accommodating the fission fragments, reducing swelling.
Pore formas are added to add uniform porosity
To minimise in fabrication the number of small pores as these collapse under pressure and cause densification
Crystal defects in ceramic fuels
Since UO2 has two ions (U and O2-) defects for both types may exist. Cation ion (U) frenkel pair (smaller type, more common) Anion (O2-) frenkel pair (less common due to size of anion being larger)
What is a neutron flux depression
Neutrons slowed down to thermal neutrons so source of thermal neutrons is on outside of the fuel
Implies a neutron flux depression across the pellet with the flux being greatest near the surface of the pellet
Thermal neutrons won’t be able to penetrate deep into fuel so fission occurs around edges
Pellet design
Pellets are dishes and chamfered to avoid localised stresses and accommodate swelling and isotopic expansion
Centre tends to expand more than surface due to roaches temps
Classic pellet design
Classic pellet design will form hour glass on failure due to anisotropic expansion
Bubbles
Small Filled with exclusively gaseous fission fission products High pressure Spherical Formed as fuel ages Diffusion, random
Pores
Quite large
Mainly helium and other fission products
Low pressure
Disc like shape
Formed as sintering during fabrication
Vapour transport, pores nice towards temp, removes any imperfections
Mechanical properties of UO2
Normal conditions UO2 is brittle
At high temps UO2 exhibits plastic deformation before failure
Yield point separates regions na of elastic and plastic deformation
Ductile-brittle temp: the temp at which measurable plastic deformation prior to failure first occurs.
Purpose of the cladding
Encase the fuel pellets and barrier between the fuel and coolant
Properties of a good cladding
Mechanically stable under conditions of temp pressure etc
Chemically stable
Low capture
Why is zirconium used
Abundant
Cheap
Very low capture
Good corrosion resistance
Fabrication of Zirconium
Isolated from its ore using carbon and Chlorine (ZrCl4)
ZrCl4 reacted with magnesium (Zrmg)
Excess magnesium removed to leave Zr sponge
Sponge creates think rods which are melted in a vacuum arc furnace
Ingot produced and forged into rods
What is cold pilgering
Longitudinal cold rolling press that reduces diameter and thickness
How does cold pilgering Work
A crankshaft drives mill and saddle back and forth
Ring dies rotate
In this motion dies reduce the tube
Relies on tapered cross section between the does and mandrel to reduce diameter and thickness
What is hydrogen embrittlement
HCP phase has low solubility of hydrogen
Excess hydrogen forms as hydride
Leads to embrittlement, delayed hydride cracking and hydride blistering occurs
Hydrogen comes from water from release during fission
Water also gets in due to damage from the cladding
How does hydride cause failure
Hydride forms perpendicular to max stress allowing crack to propagate
New hydride will defuse into the crack continuing propagation
Debris fretting
Debris can build up due to corrosion
Parts of cladding fall off
Debris can accelerated into the cladding by the coolant
Radiation growth
Change in shape of a solid at constant volume
Occurs in solids with crystallographic anisotropy
Explain cladding corrosion
2 layers, pre and post transition
Pre forms protective layer
Post transition loses its protectiveness
Movement of ions due to fission spike
Electrons which are lost move through the oxide layer due to holes
ZrO2 acts as a semi conductor
Due to volume increase, stresses build up
Causes cracks meaning more oxides can build up the layer
Oxide spalling
Oxide layer delaminates and falls off
What is meant by the term cold spot
Due to oxide layer being a poor conductor, this leaves a cold spot
