NiCE SFR Object Model

This page is under construction and can be considered... indecent. All numerical values listed are assumed to be under cold operating conditions.

Table of Contents

• Reactor Core Model
• Reactor Assembly Model
  • Fuel Assembly
  • Control Assembly
  • Reflector Assembly
  • Shield Assembly
  • Test Assembly
• Reactor Pin Model
  • Fuel Pin
  • Control Pins
  • Shield Pins

Reactor Core Model

The SFR core is the highest level structure provided in the NiCE object model hierarchy for sodium-cooled fast reactor modeling. The SFR reactor core is modeled as a radial arrangement hexagonal assemblies. Convention dictates assemblies are ordered with the center-most assembly being "row 1," the subsequent ring of assemblies surrounding it being "row 2," and so forth, iterating in concentric circles outwards.

The radial view of a SFR core is typically divided into five regions:

• inner core
• outer core
• radial blanket (optional)
• control assemblies
• radial reflectors

In addition to these, the entirety of the SFR core is contained within a shielding structure called the radial shield. The shield's purpose is to limit the amount of radiation that passes between the internal and external environments of the reactor core, although is not always depicted as part of a reactor core model. Additional assemblies may be included for fuel testing and materials testing.

In addition to fissile materials, an SFR can also contains fertile materials (such as U-238 or Pu-240) to be converted to fissile materials via neutron absorption (a process referred to as fuel breeding). The blanket regions of a reactor core are reserved exclusively for this purpose. A homogeneous core layout entails an even distribution of fertile material throughout the core fuel region by including an axial level of blanket within each assembly. This differs from a heterogeneous core layout which employs alternating concentric rows of pure fissile and fertile materials within the core region.

The presence of blanket materials inside a liquid-metal cooled reactor is the distinguishing factor between a breeder- and burner-type reactor. A breeder reactor contains fertile materials for fuel breeding, whereas a burner reactor does not, and simply burns through it's available fuel supply. At this time, the NiCE SFR object model is assumed to handle burner-type reactors. However, the ability to include blanket assemblies is included for future modeling possibilities.

Objects of the reactor core include:

• Fuel Assemblies - Nuclear fuel components including inner core, outer core and—optionally—blanket assemblies. Inner and outer cores regions can contain both fissile and fertile materials, while the blanket region materials are exclusively fertile.
• Control Assemblies - Includes primary control rods, secondary (shutdown) control rods. Primary and secondary control rods are made of neutron-absorptive materials.
• Test Assemblies - Any fuels or materials included in the reactor core for the purpose testing during operations.
• Reflector Assemblies - A structure made of neutron-reflective material, causing subcritical fuels to become critical.
• Shield Assemblies - A stainless steel structure to shield the reactor core and external environments from irradiation.

Properties of the reactor core include:

• Type (SFR)
• Size - Total number of assemblies contained in the reactor core.
• Number and Location of Fuel Assemblies - Position of inner and outer core fuel assemblies at the core view, also called the core loading pattern.
• Number and Location of Control Assemblies - Position of control/shutdown rod assemblies at the core view.
• Number and Location of Testing Assemblies - Position of material and/or fuel testing assemblies at the core view.
• Number and Location of Reflector Assemblies - Position of radial reflector assemblies at the core view.
• Number and Location of Shield Assemblies - Position of shield assemblies at the core view.
• Inter-assembly Gap (cm) - Distance between the external walls of adjacent assemblies to allow for interstitial coolant flow.
• Lattice Pitch (cm) - Also called assembly pitch. The shortest distance between the centers of adjacent assemblies (also equivalent to the assembly outer flat-to-flat plus inter-assembly gap).
• Thermal Power (MWth) - Maximum heat the reactor is designed to generate.
• Electrical Power (MWe) - Maximum electricity the reactor is designed to generate.
• Thermal Efficiency - The ratio of thermal power to usable electrical power generated.
• Primary Coolant - Liquid sodium coolant.
• Inlet Temperature (°C) - Coolant temperature as it enters the bottom inlet of an assembly.
• Outlet Temperature (°C) - Coolant temperature as it exits the top outlet of an assembly.
• Reactor Temperature ΔT (°C) - Temperature difference between assembly inlets and outlets.
• Core Average Temperature (°C) - Average of inlet and outlet temperatures.
• Cycle Length - The length of time corresponding to one iteration of a fuel cycle.

Typical values are shown below:

Due to the migration of our wiki articles from MediaWiki to Markdown, the "locations" will not be displayed correctly. We will soon move back to MediaWiki, at which point this injustice will be righted.

• Type = SFR
• Size = 199

• Locations of Fuel Assemblies (1/3rd symmetry of reactor core):

  • • • O - I I -
    • • • O I - I I
  • • • O - I I - I
    • • • O O - I I -
    • • • • O O - O O
    • • • • • O O - -

    ---

      - - - - - - - -
         - - - - -
    

I = Inner core assembly (24 total); O = Outer core assembly (30 total)

• Locations of Control Assemblies (1/3rd symmetry):

  • • • • • • • P

    ---

  • • • • P - - S -

    ---

    • • • • • • P - -

    ---

    ---

      - - - - - - - -
         - - - - -
    

P = Primary control assembly (7 total); S = Secondary control assembly (3 total)

• Location of Test Assemblies (1/3rd symmetry):

  • • • • M - - -
    • • • • • F - -

  ---

  - - - - - F - - M
   - - - - - - - - -
    - - - - - - - - -
     - - - - - - - - -
        - - - - - - - -
           - - - - -
  

F = Fuel test assembly (6 total); M = Material test assembly (3 total)

• Locations of Reflector Assemblies (1/3rd symmetry):

  • R R - - - - -
    • R R - - - - -
  • R R - - - - - -
    • R R - - - - - -
    • R R R - - - - -
    • R R R R - - R R
    • • R R R R R R R
      • • R R R R - -

        ---

R = Radial reflector assembly (78 total)

• Locations of Shield Assemblies (1/3rd symmetry):

  S - - - - - - -
  S - - - - - - -
  S - - - - - - - -
  S - - - - - - - -
  S - - - - - - - -
  S - - - - - - - -
  S S - - - - - - -
  S S - - - - S S
  S S S S S

S = Shield assembly (48 total)

• Inter-assembly Gap = 0.4000 cm
• Lattice Pitch = 14.5980 cm
• Thermal Power = 250 MWth
• Electrical Power = 95 MWe
• Thermal Efficiency = 38%
• Primary Coolant = Na
• Inlet Temperature = 355°C
• Outlet Temperature = 510°C
• Reactor Temperature ΔT = 155°C
• Core Average Temperature = 432.5°C
• Cycle Length = 4 months

Reactor Assembly Model

A SFR core can contain anywhere from a few dozen to several hundred assemblies (sometimes referred to as sub-assemblies), all of which can be categorized into one of five different types:

• fuel
• control
• test
• reflector
• shield

In the NiCE SFR object model, these assemblies can be categorized into two types: pin-containing assemblies, and rod-containing assemblies. Pin-containing assemblies constitute the majority of assemblies in a SFR core, and include fuel, control, test and shield assemblies. Reflector assemblies differ in that they contain solid rods of steel (rather than pins), and are thus labelled as a rod-containing assembly.

The SFR assembly consists of an external hexagonal stainless steel duct containing an array of cylindrical pins or rods arranged in a triangular lattice, known as a pin/rod bundle. Pins and rods are indexed within the assembly in a manner similar to assemblies at the core level: the center-most pin/rod is labeled "row 1," the concentric circle around it being "row 2," and so forth. Each pin/rod has a stainless steel helical wire wrap around the exterior. This wire wrap provides uniform spacing of the pins/rods within the assembly through which sodium coolant can flow, in place of a grid spacer like one would typically find in an LWR.

Basic objects of all SFR assemblies are:

• Duct - A hexagonal stainless steel tube housing the pins or rods in a triangular lattice.
• Nosepiece - Slotted ports at the bottom of the assembly, allowing coolant to flow upwards and through the assembly structure.
• Handling Socket - A structural end-cap located at the top of an assembly, allowing the assembly to be handled from above.

Properties of all SFR assemblies include:

• Size - Number of pins or rods contained inside a SFR assembly. Each assembly contains <math>\scriptstyle 3\times(r^{2}~-~r)~+~1</math> pins/rods, indexed in concentric circles starting with the center-most pin/rod being "row 1."
• Assembly Type - Either fuel, control, test, reflector or shield assembly.
• Duct Material
• Nosepiece Material
• Handling Socket Material
• Duct Outer Flat-to-Flat Distance (cm) - The distance between parallel exterior duct walls of an assembly.
• Duct Wall Thickness (cm) - Thickness of the hexagonal steel duct enclosing each fuel assembly.
• Duct Inner Flat-to-Flat Distance (cm) - The distance between parallel interior duct walls of an assembly.
• Handling Socket Height (cm)
• Nosepiece Height (cm)
• Total Fuel Assembly Height (cm)

The default SFR assembly can have the following values:

• Duct Material = HT-9 steel
• Nosepiece Material = SS-316 steel
• Handling Socket Material = SS-316 steel
• Duct Outer Flat-to-Flat Distance = 14.1980 cm
• Duct Wall Thickness = 0.3000 cm
• Duct Inner Flat-to-Flat Distance = 13.5980 cm
• Handling Socket Height = 30.00 cm
• Nosepiece Height = 38.00 cm
• Total Fuel Assembly Height = 328.000 cm

It is assumed that all assemblies in the NiCE SFR object model inherit the above parameters and values. Typical values not listed above are implemented on a per-assembly-type basis, as outlined in the following sections.

Fuel Assembly

The fuel assembly is a pin-containing assembly, and contains fissile materials, namely oxide (PuO2, UO2) or mixed-oxide (PuO2-UO2) fuels. Different types of fuel assemblies can be distinguished from one another by their inclusion in either the inner and outer core regions. This distinction usually corresponds to differences in fuel material concentrations (for example, 14.6 wt% Pu and 17.0 wt% Pu in the inner and outer cores, respectively), but can also include factors such as differences in coolant flow rates or power densities.

Additional objects of the fuel assembly include:

• Fuel Pins - Also known as the fuel pin bundle, can include inner core, outer core and blanket pins.

Properties of a fuel assembly include:

• Pin Type - Either inner core, outer core or (optionally) blanket assembly.
• Pin Pitch (cm) - Distance from a reactor pin center to an adjacent pin's center.
• Duct-Bundle Clearance (cm) - The gap separating the pin bundle lattice from the duct, along each wall of the assembly.
• Outer Duct Perimeter (cm) - Total perimeter of the assembly duct's exterior wall.
• Inner Duct Perimeter (cm) - Total perimeter of the assembly duct's interior wall.
• Lattice Perimeter (cm) - (?)
• Lattice Area (cm2) - (?)
• Flow Area (cm2) - (?)
• Wetted Perimeter (cm) - The total perimeter in a cross-sectional view of the assembly that is in contact with the liquid coolant. Equal to

<math>P~=~\sum\limits_{i=1}^\infty~l_{i}</math>,

where <math>l_{i}</math> is the perimeter of each surface in contact with coolant.

• Hydraulic Diameter (cm) - A term useful in calculating coolant flow in non-cylindrical structures. Equal to

<math>D_{H}~=~\frac{4A}{P}</math>,

where <math>A</math> is the total flow area, and <math>P</math> is the wetted perimeter.

• Pin Bundle Height (cm)

Reasonable values for a fuel assembly are as follows:

• Size = 217 (9 rows)
• Pin Pitch = 0.9080 cm
• Pin Bundle Height = 260.00 cm

Control Assembly

Control assemblies are a pin-containing assembly type, and can be considered a specialized extension of the fuel assembly with two discernible differences. The first difference is the presence of a second inner duct, and the second being absorptive (rather than fuel) materials housed inside the pin bundles.

The purpose of primary control assemblies is two-fold: to provide neutronic start-up and shutdown, and control over the neutron population during normal operation. The construction of a control assembly is very similar to that of a fuel assembly: the cross-sectional view consists of a hexagon-shaped wrapper tube (or duct) encasing a triangular lattice of absorber pins with sextant symmetry. Like fuel pins, absorber pins are also wrapped in a helical wire to provide uniform spacing.

Primary control assemblies can be constructed from a myriad of neutron-absorptive materials, but are typically made with natural boron carbide (B4C) pellets. The process of neutron absorption releases energy as heat and produces helium gas, which necessitates the need for coolant flow and gas plenums in a control assembly similar to a fuel assembly.

Secondary control assemblies, on the other hand, are intended for rapid shut-down in emergency situations. Shutdown control assembly pellets are often made of 10B boron carbide.

Additional objects of the control assembly include:

• Control Pins - Also known as the control pin bundle, can include primary and secondary control pins.
• Inner Duct - A secondary hexagonal stainless steel tube, internal to the main duct.
• Follower - A stainless steel rod, positioned beneath the control pin bundle inside an empty region of the assembly.
• Lower Reflector - A stainless steel reflector, external to control pins, for the purpose of neutron reflection.

Properties of a control assembly include:

• Pin Type - Either primary or secondary control assembly.
• Pin Pitch (cm) - Distance from a reactor pin center to an adjacent pin's center.
• Follower Material
• Inner Duct Outer Flat-to-Flat (cm) - The distance between parallel outer walls of the inner duct.
• Inner Duct Thickness (cm) - Thickness of the inner duct.
• Inner Duct Inner Flat-to-Flat (cm) - The distance between parallel inner walls of the inner duct.
• Pin Bundle Height (cm)
• Follower Height (cm)
• Follower Radius (cm)
• Empty Region Height (cm)
• Lower Reflector Height (cm)

Typical values for a control assembly:

• Size = 91 (6 rows)
• Pin Pitch = 1.2484 cm
• Follower Material = SS-316 steel
• Lower Reflector Material = HT-9 steel
• Inner Duct Outer Flat-to-Flat = 12.7980 cm
• Inner Duct Thickness = 0.3000 cm
• Inner Duct Inner Flat-to-Flat = 12.1980 cm
• Pin Bundle Height = 116.00 cm
• Follower Height = 16.80 cm
• Follower Radius = 5.00 cm
• Empty Region Height = 85.10 cm
• Lower Reflector Height = 42.10 cm

Reflector Assembly

Reflector assemblies are the only rod-containing assembly type in the NiCE SFR object model, and are perhaps one of the simplest in terms of axial structure. Reflector assemblies consist of simply a nose-piece, rod region, and a upper handling socket.

Additional objects of the reflector assembly include:

• Rods - A cylindrical object inside reflector assemblies, typically made of steel, for the purpose of neutron reflection inside the core region.

Properties of the reflector assembly include:

• Rod Material
• Rod Pitch (cm) - Distance from a reflector rod center to an adjacent rod's center.
• Rod Radius (cm) - Radius of the reflector rods.
• Rod Height (cm)

The default reflector assembly can be considered to have the following values:

• Size = 91 (6 rows)
• Rod Material = HT-9 steel
• Rod Pitch = 1.4067 cm
• Rod Radius = 0.7026 cm
• Rod Height = 260.00 cm

Shield Assembly

The shield assembly is a pin-containing assembly type. Much like a reflector assembly, the shield assembly has the simplest axial geometry.

Additional objects of the shield assembly include:

• Shield Pins - Also known as the shield pin bundle.

Reasonable values for a default shield assembly are:

• Size = 19 (3 rows)
• Pin Pitch = 3.0441 cm
• Pin Bundle Height = 260.00 cm

Test Assembly

A test assembly is a specialized extension of a fuel assembly, used for the purpose of fuel or materials testing. The structure of a test assembly is no different from that of a fuel assembly, only differing in the composition of pin materials. For the purpose of the NiCE SFR object model, these differences are not evident at the assembly structure level.

Reactor Pin Model

A reactor pin consists of a cylindrical stainless steel cladding containing pellets; the cladding's purpose is to separate pellet materials from coming in direct contact with coolant. Additionally, the pellets and cladding are separated from one another by a thin gap of helium (called the pellet-clad gap), allowing for any thermal or irradiation expansion. A He-filled gas plenum is located at the very top (and sometimes bottom) end(s) of the reactor pin for gassing of any reaction products. A spring located inside the upper plenum exerts force upon the pellet stack, keeping all fuel and absorber pellets in place during transport.

Each reactor pin has a stainless steel helical wire wrap around the cladding's exterior. This wire wrap provides uniform spacing of the pins within the assembly through which sodium coolant can flow, in place of a grid spacer like one would typically find in an LWR. Reactor pins are secured inside the reactor assembly by locking onto a grid from the bottom.

Objects of the SFR reactor pin include:

• Pellets - Cylindrical units of fuel or absorber material stacked vertically on top of one another. For the sake of simplicity, pellet-pellet interactions are often ignored in SFR modeling, and the pellet stack is modeled as one continuous column.
• Cladding - Cylindrical tubing that houses all the components of a reactor pin.
• Gas - For filling of the pellet-clad gap.
• Wire Wrap - Helical wire wrapped around the reactor pin exterior to provide uniform spacing of pins within an assembly.
• Spring - Placed at the top of the pellet stack, for holding pellets in place during transportation.
• Upper Gas Plenum
• Lower Reflector
• Upper End Cap
• Bottom End Cap

Properties of a default SFR reactor pin include:

• Pin Type - Inner core fuel, outer core fuel, blanket, primary control, secondary control, or shield pins.
• Pellet Material
• Cladding Material
• Wire Wrap Material
• Gas Type - Gas contained in the pellet-clad gap.
• Pellet Radius (cm) - Radius of the pellet column.
• Pellet Smear Density (%) - Concentration of pellet material if it were to be smeared uniformly throughout the inside of the pin cladding. Pellet smear density takes into account the cladding-fuel gap, as well as pellet interstices, to create an overall-representative value of the fuel density.
• Cladding Outer Radius (cm) - Outer radius of the pin cladding.
• Cladding Thickness (cm) - Thickness of pin cladding.
• Cladding Inner Radius (cm) - Inner radius of the pin cladding.
• Wire Wrap Radius (cm) - Radius of the wire used for helical wire wraps.
• Wire Wrap Pitch (cm) - The vertical distance along the length of an assembly for a wire wrap to complete one rotation.
• Spring Height (cm)
• Gas Plenum Height (cm)
• Active Core Height (cm)
• Lower Reflector Height (cm)
• Upper End Cap Height (cm)
• Bottom End Cap Height (cm)

The default SFR pin can be considered to have the following values:

• Cladding Material = HT-9 steel
• Wire Wrap Material = (?)
• Gas Type = Helium

It is assumed that all pins in the NiCE SFR object model inherit the above parameters and values. Typical values not listed above are implemented on a per-pin-type basis, as outlined in the following sections.

Fuel Pin

Fuel pins are a special instance of a SFR pin that contain all fissile and fertile materials necessary for reactor's fission. Fuel pins typically contain oxide or mixed-oxide (MOX) fissile materials, such as UO2, PuO2 or PuO2-UO2. If the reactor is designed to breed fuel, it will also contain axial blanket regions (usually composed of fertile U-238) directly above and below, sandwiching the fissile material (hence the term "blanket"). This axial blanket would not usually be present in a burner reactor.

Additional objects of a fuel pin include:

• Upper Axial Blanket (optional) - Axial level above the fissile core material in a pin, containing fertile material for fuel breeding.
• Lower Axial Blanket (optional) - Axial level below the fissile core material in a pin, containing fertile material for fuel breeding.

Properties of a fuel pin:

• Upper Axial Blanket Height (cm) (optional)
• Lower Axial Blanket Height (cm) (optional)

The default fuel pin can be considered to have the following values:

• Pellet Material = U-TRU-10Zr (16.5% TRU inner core, 20.7% TRU outer core)
• Pellet Radius = 0.3014 cm
• Pellet Smear Density = 75%
• Cladding Outer Radius = 0.4000 cm
• Cladding Inner Radius = 0.3480 cm
• Cladding Thickness = 0.0520 cm
• Wire Wrap Radius = 0.0515 cm
• Wire Wrap Pitch = 20.3200 cm
• Upper Axial Blanket Height = 0 cm
• Lower Axial Blanket Height = 0 cm
• Spring Height = 0 cm
• Gas Plenum Height = 120.00 cm
• Active Core Height = 80.00 cm
• Lower Reflector Height = 60.00 cm
• Upper End Cap Height = 0 cm
• Bottom End Cap Height = 0 cm

Note: only present in breeder-type reactors. At the time of writing this document, NiCE assumes a burner-type reactor, however, these parameters are still mentioned here for a complete description of sodium-cooled fast reactors in general.

Control Pins

Control pins are a special instance of the reactor pin, sharing many similar characteristics to the fuel pin, but instead contain absorber materials instead of burnable ones. Control pins also lack axial reflector structures.

The default control pin can be considered to have the following values:

• Pellet Material = B4C (boron carbide)
• Cladding Outer Radius = 0.5552 cm
• Cladding Inner Radius = 0.4852 cm
• Cladding Thickness = 0.0700 cm
• Wire Wrap Radius = 0.0665 cm
• Wire Wrap Pitch = 20.3200 cm
• Pellet Radius = 0.4473 cm
• Pellet Smear Density = 85%
• Spring Height = 0 cm
• Gas Plenum Height = 31.00 cm
• Absorber Height = 85.00 cm
• Lower Reflector Height = 0 cm
• Upper End Cap Height = 0 cm
• Bottom End Cap Height = 0 cm

Shield Pins

The default shield pin can be considered to have the following values:

• Pellet Material = B4C (boron carbide)
• Pellet Radius = 1.1442 cm
• Pellet Smear Density = 81%
• Cladding Outer Radius = 1.5213 cm
• Cladding Inner Radius = 1.2713 cm
• Cladding Thickness = 0.2500 cm
• Absorber Height = 260.00 cm
• Gas Plenum Height = 0 cm
• Spring Height = 0 cm
• Lower Reflector Height = 0 cm
• Upper End Cap Height = 0 cm
• Bottom End Cap Height = 0 cm