First Deck Flashcards
What is Compton Scattering?
Derived from energy deposited into detector by gamma-ray scattering at variety of different angles, leading to a Compton edge being formed across a large range of energies between the maximum and minimum energies transferred
Scintillator Detector Basics
Interaction of an ionising particle results in many photoelectron-hole pairs, Hole drifts towards activator site ionization, Electron travels through material until it reaches an ionised activator site, Photomultiplier tube converts weak light output to electrical signal
Pros:
o Very good (Sub ns) time resolution (count rate)
o High efficiency
o Good (mm) position resolution (precision)
Cons:
o Poor energy resolution
o Hygroscopic
Scintillator Detector Examples
NaI(Tl), LSO, BGO, Plastics
Organic Vs Inorganic Scintillators
Inorganic good light output, slow response time
Organic fast response time, poor light output
Semiconductor Detector Basics
Utilise the photoelectric effect to induce current across the detector
Pros:
o Better resolution
o Can measure both alpha and beta
o Can do low and high energies
Cons:
o Requires cooling
o Expensive
Semiconductor Detector Examples
HPGe, Silicon
Neutron Detector Basics
Determined by the energy deposited into the detector by induced nuclear reactors within it. Energy peaks dictated by Q value of the reaction
Cons:
o Wall effects arising from products deposited into the wall
Neutron Detector Examples
Common examples: BF3, He-3
Helium is inherently more likely to have an interaction, Boron more energy so gives better readings
He-3 + n -> H-3 + H-1 + 0.764 MeV
B-10 + n -> Li-7 + He + 2.310 MeV (94%) or 2.792 MeV (6%)
Solvent Extraction Basics
Where a compound transfers from one solvent to another owing to differences in solubility or distribution coefficient of the two solvents
Purex Process
- Chop up fuel assembly and dissolve in HNO3
- Centrifuge out solids
- Primary separation using TBP to extract U & Pu (must reduce Pu(IV) to Pu(III))
4a. Uranium purification decontaminates the U stream and the solvents by heating (to make fission products unextractable by changing the oxidation state) then using solvent extraction (TBP). Tc can be mixed in.
4b. Plutonium purification decontaminates the Pu stream and solvents, undergoes Pu nitrate evaporation by using solvent extraction (TBP) and backwashing the Pu from the solvent with a nitrate. Np can be mixed in.
5a. Convert the Uranium nitrate to oxide powder (denitration). Uses fluidised bed reactor.
5b. Convert Pu nitrate to Pu oxalate (precipitate) using oxalic acid when is then decomposed to PuO2 using a 2-stage furnace.
Actinides basics
5f elements, early actinides have variable oxidation state (U -> 4,6; Pu -> 4 (also 3, 5, 6, 7), later all 3, soft metals, very reactive, pyrophoric
Actinyls
Actinyls: (ex. UO22+) do not undergo hydrolysis in aqueous solution. Hydrolysis occurs when a metallic ion dissociates water around it until precipitating out
Hydrolysis of Actinides
- An4+ have smaller ionic radius with high charge, hence high charge density. Hydrolysis will occur and precipitation of An(OH)4
- (Actinyls) An5/6+ have larger ionic radius with lower charge density, hydrolysis will not occur
- Degree of hydrolysis depends on pH of solution (Strong acids/bases fully dissociate)
Fission Mechanisms
Fission happens from vibrations in nucleus, vibrations get too much it splits and scatters cause of Coulomb force in to fission fragments, they then gamma emit and neutron evaporate until they are stable. Higher energy neutrons make it more likely to get symmetrically fission
Water as a Moderator
Pros: Cheap, abundant, easy to control, great as a coolant
Cons: larger than desired absorption x-section, tritium activation can occur, boils
Heavy Water as a Moderator
Pros: Small absorption x-section, easy to control, great as a coolant
Cons: Expensive, rare (unless you’re at the poles), tritium activation can occur, boils
Graphite as a Moderator
Pros: Small absorption x-section, abundant, solid matrix (won’t boil)
Cons: Subject to radiation damage, C-14 activation can occur, burns, can’t be used as coolant
UO2 Production
- Ore crushed and mixed with sulphuric acid to oxidise U -> U6+
- Concentration increased by ion exchange/solvent extraction and precipitated out to UOC (yellowcake (mostly U3O8))
- Dissolve UOC in HNO3, filter UNL, solvent extract, concentrate denitrate to UO3
- Hydrate UO3 then dehydrate to UO2, then react in kiln with HF to form UF4
- React UF4 with F2 to get UF6
- Enrich using centrifuge
- Burn enriched UF6 in steam to get UO2F2, then raise temp to 550-750 °C to reduce to UO2
- Condition powder then press into pellets, then sinter
Magnox Basics
Fuel: Natural U metal (0.71% U-235)
Cladding: Magnox (Mg + 1% Al + ..)
Coolant: CO2 under pressure to improve heat transfer
Moderator: Graphite
Fuel rods: 1m long, 2.8 cm D, 3500 channels, online refuelling, finned rods, variable reactor to reactor
AGR Basics
Fuel: Ceramic Oxide fuel (MOX)
Cladding: Stainless Steel (necessitates higher fuel enrichment)
Coolant: CO2
Moderator: Graphite
Fuel rods: groups of 36 per element, stainless steel spacer grid, individual pellets ~ 1x1cm
PWR Basics
Fuel: Ceramic Oxide fuel (MOX)
Cladding: Zircalloy
Coolant: Water
Moderator: Water
Fuel rods: 3.5m rods, 17 x 17 array, 10 x 8 mm pellets
Enrichment via Centrifuge
Centrifuge UF6. One isotope of Fluorine, so can use mass difference of U-235 and U-238 to separate
Fertile Materials
Materials that can be made fissile e.g. U-238 (-> Pu-239), Th-232 (-> U-233)
Fissile Materials
Materials that can undergo spontaneous fission e.g. U-235, Pu-239
Metal Coolant Basics
Pros: you can run your reactor far hotter than a traditional water reactor, also metals are liquid over far larger temperature ranges, so are much better at dissipating heat spikes etc, better passive safety. Cons: they are corrosive as fuck and nasty to deal with, plus solidification can become an issue
Multibarrier Concept
Wasteform: Glass/Ceramic (sometimes direct disposal if you’re using KBS-3)
Containers – Primary: For transport and storage (Steel); Overpack: Corrosion resistance (Stainless Steel, Copper (KBS-3))
Buffer Materials – Buffer between container and environment (often bentonite clay or cement)
Natural/Geological Barrier – GDF concept stuff, big load of rock
Glass Components
Glass Formers: SiO2, B2O3, P2O5
Network Stabilisers: Li2O, Na2O, K2O, CaO, SrO etc
Intermediates: Al2O3, Fe2O3, PbO
Why Vitrify?
Volume reduction
Chemically durable
Flexible
Ceramics Basics
Highly ordered, crystalline solids. Useful for immobilization as radionuclides can take lattice spots within the ceramic
Magnox Sludge Challenges
It’s a mix of highly radioactive, mystery material, much of which is liquid
Lawson Criterion
Figure of merit in nuclear fusion, used to assess the parameters needed for the ignition of a specific fusion reaction, accounting for plasma density, size, confinement time, operating temperature, power loss etc.
Fusion Triple Product
nTτE
Where:
τE = Confinement time, T = Plasma temperature, n = Plasma density
Tritium Breeding
Tritium rare so must be bred for reactor usage. Breeder blanket breeds in-situ from Li, using Be as a neutron multiplier
Li6 + n -> H3 + He
Li7 + n -> H3 + He + n (requires more energy)
Stochastic vs Deterministic Radiation Effects
Deterministic: Damage occurring above a threshold dose
Stochastic: Delayed damage due damaged genetic material
Absorbed Dose
(Gy) Dose of ionizing radiation deposited into matter in terms of radiation per unit mass (Gy = Jkg-1)
Dose Equivalent
(Sv) Measure to equate damage from different types of ionizing radiation, calculated using a weighting factor to compare (beta, gamma, X-rays = 1, neutrons = 2 < x < 5, protons = 2, alpha = 20)
Collective Dose
Sum of all effective doses, measured in man-sieverts or person-rem, also assumes Linear No-Threshold approach to problem
Problem Radionuclides
Tc-99 (VII), Sr-90, H-3; essentially high mobility beta emitters.
Alpha emitters tend to be quite nasty but much lower mobility
Monitored Natural Attenuation
Some radionuclides are naturally attenuated by the sediment in the subsurface, others may have a tolerable radiotoxicity.
By monitoring the natural attenuation, you can maintain the site without significant intervention and have the ability to act if there is a change in concentration or mobility
Bio-Stimulation
Microbial reactions can affect contaminant behaviour by breaking down solid organic matter, destroying/ forming inorganic compounds and chemically modifying the subsurface.
This can be used to transform radionuclides from a mobile to immobile form by pumping an electron donor (i.e. acetate, lactate) into the subsurface which can stimulate anaerobic (no oxygen) metabolism in the already present microorganisms. One disadvantage is that this process can be slow.
Zero Valent Iron Addition
Using the addition of zero-valent Fe to alter the oxidation states of the radionuclides.
o Fe(0) -> Fe2+, pH increased given OH- production
Ideal detector requirements
- Good Energy Resolution (Light yield/charge collection)
- High Efficiency (Z)
- Good Position Resolution
- Good Timing Resolution
Synroc Constituent Crystal Structures
Hollandite (AxB8O16)
Perovskite (ABO3)
Zirconolite (ABC2O7)
