First Deck

Question 1

What is Compton Scattering?

Answer 1

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

Question 2

Scintillator Detector Basics

Answer 2

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

Question 3

Scintillator Detector Examples

Answer 3

NaI(Tl), LSO, BGO, Plastics

Question 4

Organic Vs Inorganic Scintillators

Answer 4

Inorganic good light output, slow response time
Organic fast response time, poor light output

Question 5

Semiconductor Detector Basics

Answer 5

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

Question 6

Semiconductor Detector Examples

Answer 6

HPGe, Silicon

Question 7

Neutron Detector Basics

Answer 7

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

Question 8

Neutron Detector Examples

Answer 8

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%)

Question 9

Solvent Extraction Basics

Answer 9

Where a compound transfers from one solvent to another owing to differences in solubility or distribution coefficient of the two solvents

Question 10

Purex Process

Answer 10

1. Chop up fuel assembly and dissolve in HNO3
2. Centrifuge out solids
3. 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.

Question 11

Actinides basics

Answer 11

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

Question 12

Actinyls

Answer 12

Actinyls: (ex. UO22+) do not undergo hydrolysis in aqueous solution. Hydrolysis occurs when a metallic ion dissociates water around it until precipitating out

Question 13

Hydrolysis of Actinides

Answer 13

• 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)

Question 14

Fission Mechanisms

Answer 14

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

Question 15

Water as a Moderator

Answer 15

Pros: Cheap, abundant, easy to control, great as a coolant
Cons: larger than desired absorption x-section, tritium activation can occur, boils

Question 16

Heavy Water as a Moderator

Answer 16

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

Question 17

Graphite as a Moderator

Answer 17

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

Question 18

UO2 Production

Answer 18

1. Ore crushed and mixed with sulphuric acid to oxidise U -> U6+
2. Concentration increased by ion exchange/solvent extraction and precipitated out to UOC (yellowcake (mostly U3O8))
3. Dissolve UOC in HNO3, filter UNL, solvent extract, concentrate denitrate to UO3
4. Hydrate UO3 then dehydrate to UO2, then react in kiln with HF to form UF4
5. React UF4 with F2 to get UF6
6. Enrich using centrifuge
7. Burn enriched UF6 in steam to get UO2F2, then raise temp to 550-750 °C to reduce to UO2
8. Condition powder then press into pellets, then sinter

Question 19

Magnox Basics

Answer 19

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

Question 20

AGR Basics

Answer 20

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

Question 21

PWR Basics

Answer 21

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

Question 22

Enrichment via Centrifuge

Answer 22

Centrifuge UF6. One isotope of Fluorine, so can use mass difference of U-235 and U-238 to separate

Question 23

Fertile Materials

Answer 23

Materials that can be made fissile e.g. U-238 (-> Pu-239), Th-232 (-> U-233)

Question 24

Fissile Materials

Answer 24

Materials that can undergo spontaneous fission e.g. U-235, Pu-239

Question 25

Metal Coolant Basics

Answer 25

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

Question 26

Multibarrier Concept

Answer 26

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

Question 27

Glass Components

Answer 27

Glass Formers: SiO2, B2O3, P2O5
Network Stabilisers: Li2O, Na2O, K2O, CaO, SrO etc
Intermediates: Al2O3, Fe2O3, PbO

Question 28

Why Vitrify?

Answer 28

Volume reduction
Chemically durable
Flexible

Question 29

Ceramics Basics

Answer 29

Highly ordered, crystalline solids. Useful for immobilization as radionuclides can take lattice spots within the ceramic

Question 30

Magnox Sludge Challenges

Answer 30

It’s a mix of highly radioactive, mystery material, much of which is liquid

Question 31

Lawson Criterion

Answer 31

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.

Question 32

Fusion Triple Product

Answer 32

nTτE
Where:
τE = Confinement time, T = Plasma temperature, n = Plasma density

Question 33

Tritium Breeding

Answer 33

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)

Question 34

Stochastic vs Deterministic Radiation Effects

Answer 34

Deterministic: Damage occurring above a threshold dose
Stochastic: Delayed damage due damaged genetic material

Question 35

Absorbed Dose

Answer 35

(Gy) Dose of ionizing radiation deposited into matter in terms of radiation per unit mass (Gy = Jkg-1)

Question 36

Dose Equivalent

Answer 36

(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)

Question 37

Collective Dose

Answer 37

Sum of all effective doses, measured in man-sieverts or person-rem, also assumes Linear No-Threshold approach to problem

Question 38

Problem Radionuclides

Answer 38

Tc-99 (VII), Sr-90, H-3; essentially high mobility beta emitters.
Alpha emitters tend to be quite nasty but much lower mobility

Question 39

Monitored Natural Attenuation

Answer 39

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

Question 40

Bio-Stimulation

Answer 40

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.

Question 41

Zero Valent Iron Addition

Answer 41

Using the addition of zero-valent Fe to alter the oxidation states of the radionuclides.
o Fe(0) -> Fe2+, pH increased given OH- production

Question 42

Ideal detector requirements

Answer 42

• Good Energy Resolution (Light yield/charge collection)
• High Efficiency (Z)
• Good Position Resolution
• Good Timing Resolution

Question 43

Synroc Constituent Crystal Structures

Answer 43

Hollandite (AxB8O16)
Perovskite (ABO3)
Zirconolite (ABC2O7)