ESCI 421 Final Exam Review

how much of each isotope of uranium is found in nature and where does it come from?

U-238: 99.28% (hacker) of natural uranium - solar system and earth

U-235: 0.72% (pro) of natural uranium - solar system and earth

U-234: 0.0057% (noob) of natural uranium - daughter product of U-238

which uranium isotope should be all gone by now?

U-234 “should” be gone because its half life is 246,000 years (UNC)

• but more is always being created

why is there the same ratio of U-235 to U-238 for every uranium mine in the world?

while both isotopes at the time of earth formation were equally abundant, natural uranium consists of 99.28% U-238 and 0.72% U-235 because they were all formed billions of years ago by the supernova

oklo uranium mine

different ratio of U-235 to U-238

• unusually low U-235

  • confirmed by the presence of fission products

could what happened in africa happen again?

NO it’s impossible since the uranium was formed in the supernova long ago

could it happened elsewhere?

we could find another site where it had previously occured

candu reactors use the natural uranium ratio, why can’t that happen in nature?

uranium does not need to be enriched due to the heavy water moderator which doesn’t absorb as many neutrons as light water does

• cannot happen in nature because natural uranium is too dilute

the fission products stayed in place in the uranium mine. what’s the implication of this for spent nuclear fuel in a DGR?

DGR’s work in practice

• when _underground, none of the fission products moved or harmed anymore → can do this w/out fuel

what are the 5 barriers for NWMO’s proposed DGR?

1. fuel pellet

2. fuel bundle/cladding

3. fuel container

4. bentonite clay

5. geosphere (hot rock)

stages of the open nuclear cycle

1. mining and milling 🔨

2. conversion 🔜

3. enrichment 👦🏻

4. fuel fabrication 👔

5. power plant 🏭

6. fuel storage 🛢

7. waste disposal 🗑

8. recycling ♻

mining and milling

commercial ore is mined and milled into yellowcake powder (U₃O₈)

conversion

yellowcake is converted into uranium hexafluoride (UF₆)

enrichment

uranium isotopes are separated in a series of centrifuges to produce enriched UF₆ gas

fuel fabrication

UF₆ gas is chemically processed to form uranium dioxide (UO₂) powder, then compressed into

fuel pellets → rods → bundles

power plant

pellets provide heat to produce electricity, used fuel can be recycled or stored for geologic disposal

fuel storage

spent fuel refers to the nuclear fuel that has been used in a reactor

waste disposal

final disposition of used fuel or HIGH-level waste

recycling

the constituents of the spent fuel are separated into new fuel and waste products

open vs. closed nuclear fuel cycles

open - treats spent fuel as waste 🗑 for direct disposal

closed - reprocesses ♻ spent fuel to recover and reuse U and Pu

which stages provide concerns about nuclear weapons proliferation and why?

uranium enrichment - INC the conc. of isotope U-235

reprocessing/plutonium recycle - reprocessing involves chemical operations and unused uranium from spent nuclear fuel

ores

naturally occuring rock or sediment that contains enough minerals to be economically extracted

tailings

waste material left over after the valuable materials have been extracted from the ore during milling

cascades

series of connected centrifuges during the enrichment process

separate work unit (SWU)

standard measure of the effort 💪 required to separate 🔙 isotopes (enrichment)

yellowcake

solid, powdered uranium oxide concentrate (U₃O₈) produced during the milling process

how fuel is fabricated

1. conversion 🔜

2. pelletizing 🍞

3. sintering 🍪

4. rod loading 🚚

5. assembly 📲

conversion

uranium hexafluoride (UF₆) chemically processed into uranium dioxide (UO₂) powder

pelletizing

powder is pressed into _^small, cylindrical shapes

sintering

shapes are baked at very HIGH temperatures to create ceramic fuel pellets

rod loading

finished pellets are stacked inside long, thin tubes made of corrosion resistant fuel rods

fuel assembly

multiple rods are bundled together into a fuel assembly → ready for loading into the reactor core

can a nuclear reactor blow up like a nuclear bomb?

NO ❌

• nuclear reactors only 3-5% U-235, nuclear weapons are 90%

• reactors naturally slow down the reaction, bomb requires high-speed compression of fuel

where did the hydrogen from fukushima come from and what happened with it?

• lack of cooling from the reactor - tsunami wiped grid and EDGs

• zirconium fuel cladding overheated

• chemical reaction between Zr + stream → H2 gas → explosion

why are these methods put into a cascade as opposed to doing it all in one go?

one stage isn’t enough → _^little uranium is enriched

why are physical (rather than chemical) processes used to enrich uranium?

NO ❌ chemical difference between U-235 and U-238 → use physical properties

• _^small mass difference → makes it possible to seperate isotopes w/ diffusion or centrifuges

• _^small ionization energy difference → due to even vs. odd nature

BWRX-300 vs. regular boiling water reactors (BWRs)

• _^smaller

• built faster

• built modularly

• built in parallel

• passive safety features

how brachyltherapy treats cancer

injects sources of radiation to treat cancer

sources:

• chemically inert

• short penetration depth → reduce exposure to healthy tissue

how do refueling machines in CANDU reactors replace fuel?

pushes one fuel bundle OUT the reactor → catches it → while pushing another fuel bundle in

allows for on-line refueling - MORE fuel can be put in the reactor w/out turning the reactor OFF → longer operation times, HIGH CF

how does a freeze plug work for molten salt reactor?

if fuel in the MSR overheats/power goes out

→ plug will melt → molten salt 🧂 will flow out of the hole → deliberately designed tanks → prevent the chain reaction ⛓‍💥 from continuing

how do freeze plugs improve the safety of the molten salt reactor?

passive safety system w/out

• electricity ❌⚡

• human intervention ❌👷

why wouldn’t freeze plugs work for other reactors?

other reactors use solid fuel rods

what do eVinci reactors use instead of control rods?

control drums 🥁

• rather than inserting the neutron poison in by inserting a rod

  • eVinci rotates a drum 🔄🥁

how do control drums in eVinci reactors work?

1. reactive material rotates out 🔄

2. neutron reflector makes the core “look LARGER” since the neutrons are less likely to escape

3. moving the reflector - changes the # of neutrons getting reflected back in

4. rotating the drum 🔄🥁 brings the neutron absorber (poison) into the reactor → slowing/shutting down 🔽 the rate of fission events

how is this different than the fusion that’s being pursued, and why is
there less energy available in the fusion that’s being pursued?

• proton-proton fusion is MORE difficult to do than D-T fusion (even D-D)

• D-T and D-D → LARGER CSs

• won’t be able to turn all H in ocean water into He

neutron economy

how effectively a reactor uses its neutrons

• crucial for the economics and sustainability of nuclear power

ionizing vs. non-ionizing radiation

ionizing - remove e-s from the shells of atoms

non-ionizing - DON’T remove e-s

fissionable

able to be fissioned

examples of fissionable nuclides

• U-238

• U-236

• Th-232

• Pu-238

fissionable nuclides in nature (3 gap)

• U-238

• U-235

• Th-232

fissile

ability to split when struck by slow neutrons

examples of fissile nuclides

• U-235

• U-233

• Pu-239

fertile

adding a neutron will transmute the nucleus into a fissile nucleus

examples of fertille nuclides (2 gap)

• U-238

• U-234

• Th-232

if there’s a series of alpha decays from some of the heavy nuclides we find in nature, what will follow and why?

the INC ratio of neutron to protons makes the need for beta decays to occur

if a nuclear reactor is shut off, what is the neutron poison?

Xe-135 has a HUGE absorption CS

• neutron count is 81, wants to be 82

• nuclide builds ^up → neutron economy is bad → reactor can’t run

why does it take time for the neutron poison to build up?

we don’t Xe-135 directly → need to wait 🛑 for it to form

multi-stage decay: Te-135 → I-135 → Xe-135 (Tel…)

4% limit

how much uranium undergoes fission

• mass still present but not uranium anymore → fission daughter products

0.086% limit

what gets converted with E = mc²

• mass is disappearing

raise bore mining

a way of getting ore out of solid rock

1. two tunnels are dug, one ^above and one _below the deposit

2. _^small hole drilled through the deposit of uranium

3. reamer drill attached to bottom and drills upwards into orebody

4. grinds uranium ore deposit → _^smaller rocks

5. fall into the lower tunnel

6. ore is scooped and delivered

mcarthur river operation

world’s LARGEST high-grade uranium mine

neutron cross section

measures the probability of a neutron interacting with a nucleus

why does Xe-135 have so many barns?

HIGH neutron absorption CS

_^low energy resonance - efficient at capturing thermal neutrons

diatomic molecules

rotate, vibrate and e- change level

advantages of nuclear power

• safe + clean 🧼

• reliable + plentiful

• CO₂ free ❌💨

• easy to handle waste 🗑

disadvantages of nuclear power

• slow to build 🏗

• HIGH technology 💻

• HIGH upfront cost 💸

• terrifies people 🤡

CANDU reactor

canada deuterium uranium

heavy-water 💧 natural uranium reactor designed to generate electricity ⚡ through nuclear fusion

CANDU fuel channel

fuel pellets inside pressure tubes inside calandria

passive safety system of a BWRX-300

allows itself to cool for at least 7 days without

• external power ⚡

• operator intervention 👷

• AC power 🔋

small modular reactor

advanced nuclear fission reactors

• _^smaller in size 👌

• _^smaller power capacity 🪫

• _^smaller physical footprint 🦶

tracer

1. isotope injected or ingested 😋

2. follows biological processes 🦠

3. emits radiation through decay 🩻

4. detectors capture photons, creating an image 🖼

PET scanning

• B⁺ decay

• positron + electron → annihilate

• produces 2 gammas in opp. dir.

SPECT scanning

• uses technitium 🪨 for gamma decay

• detects 🕵‍♂️ gamma rays directly

• _lower accuracy 🎯 than PET

  • but accessible/cheaper $ 💵

linear accelerated radiation beam (LINAC)

• works alongside PET scans

• turns e-s into x-rays 🩻

• able to penetrate exact depth

• _^minimizes healthy tissue damage

microreactor

• operates independently as part of an electric grid

• part of a _^microgrid to generate up to 20 MW_th

parts of an eVinci reactor

• control drum 🥁

• heat pipes 🚬

• graphite core 📝

• fuel 🛢

• shutdown rods 🔽

why use control drums?

• MORE control

• INC safety

graphite core block (eVinci)

• graphite → moderator 👾

• provides structural stability 🏛

• contains

  • fuel pellets 🍪

  • heat pipes 🚬

  • shutdown rods 🔽

HALEU TRISO fuel

HIGH-assay _^low-enriched uranium tri-structural isotropic fuel

• 5-20% enrichment

layers of HALEU TRISO fuel

1. outer pyrolytic carbon

2. silicon carbide

3. inner pyrolytic carbon

4. buffer - porous carbon

5. fuel kernel

sodium heat pipes

• boiling pnt. of Na metal is around 900 degrees C

• air flows over cool end 🆒

  • heating air 😮‍💨

    • spins a turbine 🌪

      • generates power 🔌

molten salt

• melted salt 🧊

• efficient at carrying heat 🔥

• absorbs latent heat but temp. is constant when melting 🧊

• u da coolant, u da fuel

molten salt power generation

1. nuclear fuel dissolved into molten salt

2. fission heats the salt as it circulates

3. heat is passed to a 2^ndary salt

4. heat → steam → turbine

5. electrwicity :3

molten salt fuels

• atm pressure (P) operations

• HIGH 🍃 temp.

• explosions ❌

fluorine and chlorine salts

_low absorption C.S.

radio-TOXICity

helium sparging

• He _low solubility

• INC eff. of Xe removal

why fusion?

• _^light nuclei fuse and release (‘em) energy 🎈

• environment ✅

• LARGE supply of fuel 🛢

• safe, risk of meltdown ❌

plasma

ionized gas with “collective behaviour”

fusion plasma conditions

• particles can overcome coulombic repulsion 🧲

• fusion energy → plasmas must be contained, energy extracted

• quantum tunnelling DEC required conditions

• we get critical ignition temp. 🌡 from C.S.

types of confinement

1. magnets

2. lasers

magnetic confinement

1. tokamaks

2. stellarators

tokamaks

donut-shaped machine that uses powerful magnetic fields to

• confine 🪤

  • heat 🔥

    • and stabilize 🎧

plasma, creating conditions necessary for nuclear fusion

stellarators

magnetic confinement device that

• contains plasma

• mimicking the energy production process of stars ⭐

inertial confinement fusion

a method to achieve nuclear fusion

• by blasting a tiny 🐛 fuel pellet (D-T)

• with HIGH-power lasers or ion beams 🚓

how do we get the energy out in fusion?

• reactor walls absorb HIGH-energy neutrons

• KE → thermal energy

• heat is extracted via coolant system 😎

• lithium blanket facilitates tritium breeding

tritium breeding (fusion)

• neutrons + Li-6 → H-3 + He-4

• occurs alongside energy extraction process

• necessary due to rarity of tritium

direct energy conversion

• energy → current (w/out boiling water) ❌💧

• utilizes electromagnetic induction

• works best for “aneutronic” fusion