Nuclear Technology

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Definitions

Fissionable

A nuclide which can theoretically undergoes nuclear fission as a result of high or low energy neutrons. Examples include $\text{ce}{U}^{2}38$. This encompasses:

Fissile readily undergoes nuclear fission as a result of low energy neutrons. Examples include $\text{ce}{U}^{2}35,P{u}^{2}39$. There are definitions for this which further specify "can be used in a chain reaction" but it is not standard.

A nuclide which

Fissionable also encompasses:

Fertile not fissile itself, but can can capture low energy neutrons to create elements which decay into fissile materials. This makes them suitable for breeder reactors. Examples include $\text{ce}{U}^{2}38andT{h}^{2}32$

A nuclide which is

Fuel Cycle

• The nuclear fuel cycle describes the progression of nuclear fuel from its creation to its disposal.
  • Once-Through Cycle: Used fuel is not reprocessed (USA)
  • Closed Cycle: Spent fuel is reprocessed to recover U and Pu for reuse (e.g., France, Russia).

1. Front-End (Fuel Preparation)

• Mining & Milling: Uranium ore is extracted and processed into U₃O₈ (“yellowcake”).
  1. Convention mining, dig up rocks and grind them up
  2. Heap leach, dig up rocks then perform chemical extraction
  3. In Situ Recovery, inject water + h2o2/na2co3/co2 and pump it up like EOR
• Conversion: U₃O₈ is converted to UF₆ through chemical processes $Reduction:\text{ce}U3O8+2H2\to 3UO2+2H2O$ $Hydrofluorination:\text{ce}UO2+4HF\to UF4+2H2O$ $Fluorination:\text{ce}UF4+F2\to UF6$
• Enrichment: increases the concentration of useful isotope $\text{ce}{U}^{2}35$ in its fluorinated state
  • Concentrations:
    • Natural: 99.3% $\text{ce}{U}^{2}38$, 0.7% $\text{ce}{U}^{2}35$, < 0.01% $\text{ce}{U}^{2}34$
    • Commercial grade (LEU): 3 - 5% $\text{ce}{U}^{2}35$ for LWRs
    • Commercial grade (HALEU): 5 - 20% $\text{ce}{U}^{2}35$ for advanced reactors
    • Weapons grade: 90% $\text{ce}{U}^{2}35$
  • Methods:
    1. Gas diffusion is old and outdated.
    2. Gas centrifuge is standard. Separate based on atomic weight $\Delta$
    3. Laser is cutting edge. Excite Specific isotopes to cause reaction, then separate.
• Fabrication: Enriched UF₆ is turned into UO₂ pellets, packed into fuel rods
  • Conversion to UO2
    Dry;Method:\ Step;One:\ce{UF6^{gas} + 2H2O^{gas} → UO2F2^{solid} + 4HF}\ Step;Two:\ce{UO2F2^{solid} + H2^{gas} → UO2^{solid} + 2HF}\end{matrix}$$
    Wet;Method:\ Step;One:\ce{?UF6^{gas} + ?H2O^{water} + ?NH3 → ?(NH3)2U2O7^{solid} + ?HF}\ Step;Two:\ce{?(NH3)2U2O7^{solid} →[heat] UO2^{solid}}\end{matrix}$$
  • Pressed into pellets, heated, machined
    • Can add poisons like $\text{ce}Gd$ to temporarily slow down reactivity and heat generation
  • Pellets are built into fuel rods ¹
    • Rods are made mostly (95%) of $\text{ce}Zr$ because of its low neutron cross section.
    • $\text{ce}Hf$ is deliberately removed because of high cross section. $\text{ce}Sn,Nb,Fe,Cr$ are added in trace to improve corrosive and structural properties.
    • Filled with pressurized $\text{ce}He$ and welded shut, with a little overhead space maintained for gaseous byproducts.
  • Assemblies vary moderately depending on reactor design.
    • PWR, BWR, CANDU, AGR,

2. In-Reactor (Fuel Use)

• Fission: Fuel is loaded into a reactor where U-235 undergoes fission, releasing energy.
• Transmutation: Some U-238 captures neutrons and turns into Pu-239, which can also fission.

3. Back-End (Used Fuel Management)

• Storage: Spent fuel is stored on-site (wet or dry)
• Disposal: Placed in final repositories (none in the USA)
• Reprocessing: Recover fissile materials (e.g., Pu-239, U-235) for reuse.

Reprocessing

• Isotope Separation
  • Methods
    • Centrifuge using differing mass to separate out materials. Needs stages
    • Mass-spec basically just use a mass spec and collect the atoms as they veer off. Very slow, expensive, low throughput
    • Lasers new idea to selectively excited isotopes for reactions. Not done at scale yet.
• Chemical Separation ²
  • Goals
    • Keep actinides like Am 241 and Cm to aid in fission
    • Remove Lanthanides to reduce cross section, make waste less self-heating
    • Recover unused fuel like U or Th to go in again
    • Extract Pu for weapons production
  • Chemical separation methods
    • Solvent extraction is just doing LLE with spicy metals. Target ions are complexed (solvated) with salts to aid it into the organic phase. They can also be bound ionically to a ligand for organic extraction. Main thing is the distibution coefficient
    • Ion Exchange refers to a static solid phase matrix (like bead resin) that the solvent is flow through. There is then a batch step that separates the target (or unwanted) ions later.
    • Volatility cooking things off is sometimes possible, but very limited.
• Dominant method since 1940s is the liquid-liquid PUREX extraction method
  1. Leech waste in nitric acid. Then filter.
  2. Mix with Kerosene/Phosphates. Actinide waste is in aqueous. U and Pu are in organic.
  3. Selective reduction of Pu that now is soluble in aqueous again. Mix with nitric acid.
  4. Uranium is in kerosene, Plutonium is in nitric. Purify from here.

Reactor Types

These attributes can be mixed and matched, most of them do not exclude each other and hence there is a wide range of possibilities

• Purpose
  • Breeder Reactor: produces more fissile materials than it consumes. More for isotope production than power.
  • Burner Reactor: uses more fissile materials than it consumes. Mainly for generating electricity or heat.
  • Research Reactors: used to produce supply of neutrons. Generally these are slow, burners types but in theory could be different³
  • Propulsion Reactors: does what it says on the tin. Naval or Aero.
  • ==Both breeder and burners do some of each process (burning and generating new fuel). It’s about the ratios, not all or nothing. They both produce usable electricity.==
• Fission speed
  • Slow aka Thermal-neutron reactors
    • Use a moderator material to decrease the temperature/kinetic energy of the neutrons produced from fission. This increases odds of fission in fissile materials.
    • Increased fission cross section decreases enrichment required for sustained reaction.
    • Vastly more common across the world in all cases.
  • Fast-neutron reactors
    • Use neutrons directly as they are produced from fission. They have no moderators and use low moderating coolants. Less common and more expensive.
    • They require higher enrichment grades because of the lower reaction cross section, and degrade hardware faster due to high energy collisions.
    • However, they produce less transuranic waste because they can fission actinides.
• Coolant

Tools & Specifications

• Hotcells & Gloveboxes
  • Hotcells are robotically operated, where as Gloveboxes are for human-operated systems.
    • Hotcells are operated via master-slave controllers. Used to be purely mechanical. Now they allow some electronics for precision, haptics, and automation.
  • Both kinds use negative pressure environments, not positive. They may or may not be inert atmospheres.
    • They open-loop system draws air in and filters it before exhaust.
    • Facilities are designed with negative pressure cascade, so air flows towards the cell.
• NQA-1
  • is an ASME certification for the proper operation of nuclear facilities. This could apply to anyone from power plants, to equipment manufacturers, to fuel handlers, to disposal.
  • Section 2 is for having a QA program. This includes annual internal audits and manual
  • Section 3 details the manual itself. Must be written in plain English. describes the scope, processes, responsibilities, and authority.
  • Section 4 application process.
  • Section 5 audits. Once for initial/renewal, twice over three years, unlimited if triggered for cause or nonconformance.
  • Section 6, 7, 8 details, changes, suspensions
• RELAP5-3D
  • is a simulation tool for modeling transient thermal-hydraulic systems.
  • Purpose
    • Analyzing accidents from loss of coolant and operation transient changes.
  • Capabilities
    • Nonhomogeneous (multiphase with liquid & gas) and nonequilibrium of two or fewer species.
    • Control systems components like PI, valves, and fluid handlers.
    • Models by approximating flow path as one dimensional, where the radial dispersion is considered negligible.
  • Method
    • Hydrodynamics require nodalization, where control volume boundaries are manually chosen throughout the flow path. Boundaries should not be placed in the middle of substantial density changes or flow transitions.
      • Each node is connected through junctions which decide which data to pass on and how the connection should be modeled.
    • Thermodynamics are generally modeled one dimensionally, using finite methods. Analysis connected to the hydrodynamic path can conduct heat transfer calculations normal to the direction of flow (aka how hot do my pipes get)
      • There are 2D simulations for low P core flooding to analyze pressure spikes, but not primary.

Shielding

See also: Radiation Methods and difficulty vary.

• $\dot{D}$ = dose rate (mrem/hr or mSv/hr)

  • Applies before attenuation
  • $\dot{D}=\frac{\Gamma A}{d^2}{e}^{-\mu x}B$
  • $\Gamma$ = specific gamma ray constant (dose rate per unit activity at unit distance)
  • $A$ = source activity (Ci or Bq)
  • $d$ = distance from source (cm)

• $\mu$ = Attenuation coefficient

  • varies significantly with photon energy due to three primary interaction mechanisms:
  • Total: $\mu ={\mu }_{\text{PE}}+{\mu }_{\text{C}}+{\mu }_{\text{PP}}$
  1. Photoelectric effect (dominates at low E): ${\mu }_{\text{PE}}\propto Z^4/E^3$
  2. Compton scattering (dominates at intermediate E): ${\mu }_{\text{C}}\propto Z/E$
  3. Pair production (dominates at high E > 1.022 MeV): ${\mu }_{\text{PP}}\propto Z^2$

• Alpha - trivial

  • Almost never an issue, but you can use a lookup table
  • TL:DR worry more about contamination control

• Beta - intermediate

  • Range of betas in a given medium: $range(cm)=\frac{range(c{m}^2)}{density(c{m}^3)}$. Usually 5 - 10 mm
  • Finding the penetration of brehmsstrahlung produced… much harder
    • $FractionofBrehmfromBeta(Zofmaterial,MaxparticleeV)$
    • Energy fluence (aka flux) from Brehm energy/s
    • Absorption in outside medium (flux, coefficient, medium density)
    • Ion pairs produced per absorbed energy (medium property)
    • Charge produced per ion pair (medium property)
    • Convert to dose rate (Roentgen)
  • TL;DR is Low-Z first to stop betas with minimal brehm then High-Z shields secondary X-rays

• Gamma - moderate

  • Beer-Lambert law ($I=I_{0}B{e}^{-\mu x}$)
    • Can condense exponent into a half-value layer. Found via lookup.
    • Empirical buildup factor $B$ to right side for scattering (waves can re-emit with less E)
  • TL;DR is dense and High-Z materials. Maximize photoelectric effect and pair production.

• Neutron - Hard

  • TL;DR is hydrogenated for moderating, then Boron for absorption. Beware gammas.

Sources

Footnotes

1. https://world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/fuel-fabrication#triso-high-temperature-reactor-fuel ↩

2. https://www.youtube.com/watch?v=wKZBy-w7qr4 ↩

3. https://world-nuclear.org/information-library/non-power-nuclear-applications/radioisotopes-research/research-reactors ↩