NiCE LWR Object Model

In NiCE, a Light Water Reactor (LWR) is modeled as a hierarchical tree data structure with the reactor core as the root node. The fuel rod assemblies, which form the core, are modeled as children of this root node. In a similar fashion, each assembly is itself a tree node with different types of rods as its children. Geometrically, the children of each node are arranged using a pre-specified lattice structure. All nodes are realized by objects in 3 dimensional space. The initial LWR model will focus solely on Pressurized Water Reactor (PWR) types.

Table of Contents

• Reactor Core Model
  • Incore Instrument Model
• Fuel Assembly Model
  • Spacer Grid Model
• Rod Cluster Assembly (RCA) Model
  • Rod Cluster Control Assemblies (RCCAs)
• Reactor Rod Model
  • Fuel Rods
  • Burnable Absorber and Control Rods
• Instrument and Guide Tubes
• Time Evolution and State Points

Reactor Core Model

The reactor core model provides the radial arrangement of fuel assemblies and Rod Cluster Assemblies (RCAs) in the LWR reactor vessel. Additionally, it includes the description of materials outside of the core that may influence the neutronics or coolant conditions within the core. Lastly, it also includes descriptions of the control rod banks (or groupings) to be used for axial positioning control.

Objects of the reactor core include:

• Fuel Assemblies - Nuclear fuel components, purchased from fuel vendors and shuffled around each fuel cycle. A typical fuel assembly is loaded fresh with either integral or discrete burnable absorbers, is operated for three fuel cycles in a different core location each cycle, and is discharged to the spent fuel pool once a substantial amount of its initial fissile material is depleted.
• Rod Cluster Assemblies - Clusters of (typically neutron-absorbing) rods when are placed in and moved between fuel assemblies during refueling outages.
• Incore Detectors - Used for power distribution monitoring inside of the reactor core. Typically, there are movable fission chambers that create electronic signals from neutron flux fields. Incore detectors have to be replaced as needed, when their sensitivity decreases substantially due to depletion of the fissile material.
• Core plates - Structural components which support the weight and radial placement of all fuel assemblies, and prevent movement of the assemblies. The core plates are never changed for a particular nuclear reactor.
• Baffle - A thin steel shroud for the reactor core, providing radial support for the fuel assemblies. There are two main types of core baffles or shroud. Only the first is handled here initially.
• Core Barrel - A steel cylindrical shell which separates the reactor core (with coolant moving upwards) from the coolant moving in the downcomer region of the vessel.
• Thermal shield - No description.
• Vessel - The carbon steel container for all of the reactor core and coolant and providing a pressure boundary for the reactor coolant system in the containment building.

Properties of the reactor core include

• Type (PWR)
• Size - number of fuel assemblies across the core
• Start and stop date - each fuel cycle has a documented start and stop date, typically taken when the power is significant enough to place the reactor on the power grid. This information is often used for calculating decay time for the nuclear fuel.
• Cycle number - each fuel cycle is given a unique sequential number for tracking purposes. The initial cycle for a reactor is Cycle 1. Most reactors in the US today are up to the Cycle 10-20 range, with the oldest ones likely in the mid-twenties. A fuel cycle is typically 18-24 months.
• Reactor rated thermal power - The (typically maximum) design heat generation in the reactor core
• Reactor rated flow - the (typically maximum) design coolant flow rate for the reactor vessel
• Core bypass flow fraction - the fraction of coolant flow that does not contribute to significant heat removal from the fuel rods, including flow that goes around the core and flow that goes through instrument and guide tubes.
• Number and location of assemblies (by ID) - the positioning of fuel assemblies in the core, also called the core loading pattern.
• Number and location of RCAs (by ID) - the positioning of control rods, discrete burnable poisons, and thimble plug in the core fuel assemblies.
• Number and location of incore instruments - the instrumented core locations, which are fixed for all cycles of a typical reactor
• Assembly position labels (X/Y) - Labels by which to refer to fuel assemblies and components by core location
• Control rod bank identifiers and locations - Character identifiers with group RCCAs and allow the operator to move several symmetric control rod clusters simulataneously
• Fuel Assembly pitch - the distance between assemblies in the core, determined by the seating location in the core plates.
• Core height - Distance between core plates that is filled by the fuel assemblies.
• Baffle properties - Material, thickness, gap, and type.
• Core Plate properties - Material, thickness (top and bottom), volume fraction of solid/coolant
• Barrel properties' - Inner and outer radius, thickness, and material.
• Vessel properties' - Inner and outer radius, thickness, and material.

Representative values (defaults) are shown below:

• Type = PWR
• Size = 15
• Start/Stop dates = optional
• Cycle number = 1
• Rated power = 3411 MW
• Rated flow = 145 MLbs/hr
• Bybass flow fraction = 7%

• Locations of Assemblies (octant symmetry):

  1
  2 1
  1 2 1
  2 1 2 1
  1 2 1 2 2
  2 1 2 1 2 3
  1 3 1 3 3 3
  3 3 3 3

• Locations of RCAs (octant symmetry):

  C
  20P T
  C 24P C
  20P T 20P C
  C 20P T 20P C
  20P C 16P C 24P 12P
  C 24P C 16P C T
  12P T 8P T

• RCCA Bank Locations (quadrant symmetry):

  D - A - D - C -
  - - - - - SB - -
  A - C - - - B -
  - - - A - SC - -
  D - - - D - SA
  - SB - SD - - -
  C - B - SA -

  ---

• Detector Locations (full symmetry):

      - - 1 - - 1 -
  1 - - 1 - 1 - - - - -
  

  • • • • • • 1 - 1 - 1 - 1
              1 1 - - - - 1 - - - - - -
  • • • • 1 - - - 1 - 1 - 1 - -
          1 - 1 - - 1 - 1 - - - - - 1 -
  • • • 1 - - 1 - - 1 - - 1 - -
        1 - 1 - 1 - 1 - - 1 - 1 1 1 -
  • 1 - - - - - - 1 - 1 - - - 1
  • • • • 1 - 1 - - - - 1 - - -
          1 - - - 1 - - 1 - - 1 - - - 1
  • • • • 1 - - 1 - - 1 - -
  • 1 - 1 - - 1 - - - - - 1
    1 - - - 1 - - 1 - 1 -
    1 - - 1 - - -

• Assembly x-labels = R P N M L K J H G F E D C B A

• Assembly y-labels = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
• Fuel assembly pitch = 21.5 cm
• Core Height = 406 cm
• Baffle properties: SS, 1.125" thick, 0.064" gap
• Upper Core Plate: SS, 7.6 cm, 50% SS/Coolant
• Lower Core Plate: SS, 5.0 cm, 50% SS/Coolant
• Barrel properties: None
• Vessel Properties: None

Incore Instrument Model

The incore detector system at most PWRs functions by inserting an empty thimble tube into the instrument tube of selected assemblies. This tube is driven into the core from underneath the pressure vessel after the fuel is loaded. The incore detectors travel through the thimble tube and may be movable or fixed during operation. However, the dominant effect of the instruments for the majority of fuel and the majority of time is simply the presence of the SS thimble tube. In the future, this object can be used to provide data for the actual detector itself.

Properties of the incore instrument (thimble tube only) with default values:

• identifier
• material = stainless steel
• height = assumed to be the same as the instrument tube
• Inner radius of 0.258 cm and outer radius of 0.382 cm.
• The instrument thimble is a boundary for the reactor system, containing air (vacuum)

• The location of the instrument in each PWR lattice (octant symmetry):

  1
  0 0
  0 0 0
  0 0 0 0
  0 0 0 0 0
  0 0 0 0 0 0
  0 0 0 0 0 0 0
  0 0 0 0 0 0 0 0
  0 0 0 0 0 0 0 0 0

It is not clear yet whether the instrument location in the lattice should be provided by the instrument model or the fuel assembly model.

Fuel Assembly Model

Each reactor contains several hundred fuel assemblies arranged in a predetermined core loading pattern. Each fuel assembly provides placement of an array of fuel rods and instrument/guide tubes arranged in a square lattice. The fuel assembly structure is provided by the lower and upper nozzles (or end fittings) connected by the fuel assembly guide tubes. These nozzles provide the connections to the lower and upper core plates. The assembly also contains several spacer grids which provide radial and axial support for the fuel rods and in some cases improve the thermal performance of the fuel by increasing coolant flow mixing.

Objects of the fuel assembly include

• Fuel Rods
• Guide tubes
• Instrument tubes
• Spacer grids
• Nozzles

Properties of the fuel assembly include:

• Identifier
• Description
• Size - the number of fuel rods across the assembly
• Rod pitch - the distance between centers of adjacent fuel rods in the fuel lattice
• Number of and location of fuel rods and instrument/guide tubes'
• Inter-Assembly gap (calculated) - half of the distance between adjacent assemblies
• Total fuel mass - mass of fuel (typically UO2) in the entire assembly
• Fuel rod labels (X/Y) - fuel rod coordinates for the fuel lattice
• Nozzle properties - Top and bottom nozzle height, material, mass
• Fuel Rod properties - See below.
• Instrument/Guide tube properties - See below.
• Spacer grid types and axial locations - distance from fuel assembly seat to the bottom of each spacer grid

Reasonable values for these properties are provided below:

• Size=17
• Rod pitch = 1.26 cm

• Rod and tube layout (octant symmetry):

  2              ! 17x17, octant symmetry
  1 1            ! 0 = guide tube
  1 1 1          ! 1 = fuel rod
  0 1 1 0        ! 2 = instrument tube
  1 1 1 1 1
  1 1 1 1 1 0
  0 1 1 0 1 1 1
  1 1 1 1 1 1 1 1
  1 1 1 1 1 1 1 1 1
  

• Assembly gap = calculated by (assembly_pitch - Size*Rod_pitch)/2 (i.e. half gap)

• Fuel mass = 522 kg
• Fuel rod X labels: A B C D E F G H I J K L M N O P Q
• Fuel rod Y labels: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
• Bottom Nozzle: SS, 6 cm high, 6 kgs
• Top Nozzle: SS, 8.8 cm high, 6 kgs

• Spacer grid axial locations:

  End 13.884 cm
  Intermediate 75.2 cm
  Intermediate 127.4 cm
  Intermediate 179.6 cm
  Intermediate 231.8 cm
  Intermediate 284.0 cm
  Intermediate 336.2 cm
  End 388.2 cm

Spacer Grid Model

Spacer grids provide lateral support for fuel assemblies and hold the fuel rods in place axially. They are composed of orthogonal straps that have tabs that contact each rod and tube in the fuel rod array. Properties of the spacer grid include:

• identifier
• material
• height
• mass

Reasonable default values are:

• material = zircaloy-4
• height = 1.5"
• mass = 875 grams

In addition to the grid itself, additional spacer sleeves may be used to attach the grid to the guide tubes. These sleeves may not be the same material as the grid itself. For mixing grids, angled mixing vanes create better T/H conditions for the fuel rods downstream of the grid. There are usually 2 or 3 types of grids, placed axially, in a single fuel assembly:

• end grids (support)
• intermediate grids (support and mixing)
• mixing grids (mixing only)

Rod Cluster Assembly (RCA) Model

Non-fuel rods such as discrete burnable absorbers, control rods, thimble plugs, and others (TPBARs) are inserted into PWR fuel assemblies from the top of the assembly during fuel-handling operations. These rodlets are attached to a central hub called a spider assembly which provides the radial and axial positioning of the insert rodlets. The spider assembly rests on the top nozzle of the fuel assembly. The RCAs can be removed from an assembly during refueling, and can even be reinserted into another assembly between fuel cycles.

Objects

• Rodlets (Control, Poison, Thimble plug, Insert/Generic, etc.)

Properties of RCAs:

• Identifier
• Rodlets properties (see Rod, Burnable Absorber/Control rod, etc.)

• Radial layout of rodlets such as (in octant symmetry)

  0
  0 0
  0 0 0
  1 0 0 2
  0 0 0 0 0
  0 0 0 0 0 2
  2 0 0 1

  where 1 represents a pyrex rodlet and 2 is a thimble plug.

Rod Cluster Control Assemblies (RCCAs)

Some of these clusters which contain control rods are connected to control rod drives which allow the movement of these RCAs axially in the assembly. These clusters are called RCCAs (Rod Cluster Control Assemblies) and have additional properties referring to their movement.

Additional properties of RCCAs:

• Movement step size, maximum number of steps, or stroke length

Axial location defined for the rod is considered to be fully inserted (or zero steps withdrawn)

The default layout for an insert rod cluster is as shown above. For RCCAs, all the rodlets will be the same type. For discrete burnable absorbers, the cluster will use a combination of pyrex and thimble plugs, for example. It is important to ensure that the cluster is defined in a way that can be inserted into the assembly guide tubes.

Default RCCAs cluster properties are:

• Step size = 0.625"
• Maximum number of steps = 230 (0 is fully inserted)
• Maximum stroke length (fully inserted to fully withdrawn) = 144"

Reactor Rod Model

Each rod contained in an assembly is modeled as a set of concentric and/or stacked cylinders of varying width, length, and materials. A typical rod consists of a fuel or poison column contained in cladding. The inner material is smaller than the tubing, providing thermal and irradiation growth space (and created what is known as the pellet-clad gap). An upper (and possibly lower) plenum exists to provide a capture volume for gases produced during operation. The clad tubing is sealed on both ends with caps (or plugs) via some variant of a welding process. In each plenum there is a stainless steel spring which provides compression on the inner material and prevents misplacement during shipment.

All rods must be one of the following types:

• Fuel rod (Ex. UO2, MOX, GAD, IFBA)
• Discrete Burnable Absorber (aka Poison) rod (Ex. WABA, PYREX, B4C-Al2O3)
• Control rod (Ex. RCCAs such as B4C, AIC, or gray rods such as Inconel or stainless steel)
• Generic insert rod (such as thimple plugs, TPBARs, or other)

The fuel assembly is composed of fuel rods and instrument/guide tubes. For PWRs, the other types of rods will be associated with Rod Cluster Assemblies (RCAs). These rods are somtimes referred to as rodlets for differentiation from the enter cluster. These rods can be inserted and removed from the fuel assembly guide tubes at any point during assembly handling operations (i.e. not during operation), and control rods can be moved axially during cycle operation.

Objects of a rod

• Inner fuel or poison material
• Outer cladding or tubing (which may be solid)
• A gas (usually helium) filling the stack/clad gap and plenum
• Upper plenum with spring
• Lower plenum with spring (optional)
• Lower end plug
• Upper end plug

All rod properties

• Identifier
• Description
• Axial location and height of fuel or poison stack - Typical rods are positioned axially according to the axial location of the bottom of the fuel/poison stack relative to the FAS (fuel assembly seat), which is also the top of the bottom core plate.
• End cap height, material, mass/volume
• Upper and lower plenum heights
• Upper and lower plenum spring material mass/volume
• Cladding material, inner and outer radius
• Fill gas material and pressure

The default rod can be considered as a solid zircaloy rod with the following properties:

• Axial height of 385 cm located at 11 cm above the FAS (11 cm is the top of the lower end plug)
• Solid zircaloy cladding with inner radius = 0 cm and outer radius = 0.475 cm
• 1 cm zircaloy endcaps with same outer radius as cladding
• Helium fill gas at 250 psi - in this case with zero volume
• Plenum heights/volumes are zero. Springs are non-existent in this case.

It may be desirable to separately define an object for the fuel/poison column in the rod.

It may be desirable to give the default rod a default fuel/poison column capability of N numbers of concentrate rings of materials.

Fuel Rods

Fuel rods are a special instance of a rod. Fuel rods contain a stack or column of fuel pellets. This pellets are typically UO2 and may include integral burnable poisons. Gadolinia (Gd2O3) can be mixed with the fuel homogeneously in concentrations up to ~8%. IFBA (ZrB2) can be sprayed onto the outside of the fuel pellets prior to being loaded into the rod cladding, in predetermined thickness and boron loadings. IFBA and Gadolinia are exclusive from one another, meaning they are not both used in the same fuel assembly. The pellets that make up a fuel stack can be axially varying, such as low enriched on the ends, high enriched in the central region, and IFBA coating in only a portion of the central region. Also, fuel rods may vary in the assembly itself.

Additional objects of a regular UO2 fuel rod

• Fuel pellets in a central stack - The fuel pellets are stacked into 12' column, which can be considered as a solid fuel column for simple models. These pellets are typical UO2 with a density of 95% of theoretical density (10.96 g/cc). Also, there is a fudge factor which can be applied to account for the fuel volume lost due to pellet dishes and chamfers.

It may be desireable in the long run to explicitly represent the fuel stack as a class containing a stack of pellets. However this is unnecessary for now.

Properties of a regular UO2 fuel rod (in addition to the base rod properties):

• Fuel stack properties - materials(z), inner and outer radius and heights(z)

The default fuel rod will have the following properties:

• Axial height of 385 cm located at 11 cm above the FAS (11 cm is the top of the lower end plug and bottom of fuel stack)
• UO2 Fuel stack inner radius of 0 cm and outer radius of 0.4096 cm.
• UO2 Fuel stack height of 12 feet. The lower 6" and upper 6" of the stack have different U-235 enrichments (called axial blankets)
• No lower plenum. Upper plenum height of 16 cm with a stainless steel plenum spring.
• Zircaloy cladding with inner radius = 0.418 cm and outer radius = 0.475 cm
• 1 cm zircaloy endcaps with same outer radius as cladding
• Helium fill gas at 250 psi - in this case with zero volume

Properties (in addition to the UO2 Fuel Rod specification) of an integral burnable absorber IFBA rod.

• Boron-10 loading
• Boron-10 isotopic abundance
• Coating thickness
• Poison properties - Material, height, axial location.

The default IFBA rod is the default fuel rod with the following additional properties:

• ZrB2 coating 10 microns thick with 2.355 mg/in B-10 loading
• 50% B-10 in B
• Coating height of 126" centered axially in the fuel stack. (Bottom = 12'/2 - 126"/2 + 11cm)
• The axial blankets of the IFBA rods (upper and lower 6") have annular fuel pellets with 0.2 cm inner radius.

Properties (in addition to the UO2 Fuel Rod specification) of an integral burnable absorber GAD rod.

• Poison properties - Material, concentration of Gadolinia

The default Gadolinia rod is the default fuel rod with the entire central stack region (12'-2*6") contained Gd2O3 homogenously mixed into the fuel material.

• Gd2O3 concentration of 5.0 wt%
• Gadolinia height of 11' located in the central region of the fuel (11cm + 6")

Burnable Absorber and Control Rods

Discrete burnable absorber rods (or rodlets) and control rods are connected to spider assemblies and can be inserted into fuel assembly guide tubes in various radial arrangements. This allow for the insertion and removal of neutron absorbers during fuel handling procedures throughout the life of the fuel assembly. There are several common burnable absorber rod types, such as Pyrex (borasilicate glass), Boron Carbide (B4C), Silver-Indium-Cadmium (AIC), Boron-Carbide and Aluminum Oxide (WABA), Inconel, etc.

Some discrete burnable poisons have annular poison material, while some are solid stacks of pellets or simply solid rods. The annular poisons may also have an inner clad in addition to the outer cladding.

Properties of a solid burnable absorber rod (in addition to the rod properties):

• poison materials(z) and concentration/loading
• poison radius
• poison heights(z) and axial locations(z)

Additional properties of an annular burnable absorber rod (in addition to the solid burnable absorber properties):

• inner tube/clad material, inner and outer radius
• poison inner radius
• inner region material (either plenum or coolant)

It is important to ensure that the outer radius of the burnable absorber rod is not greater than the inner radius of the guide tube in which it is inserted.

Instrument and Guide Tubes

Instrument and guide tubes are hollow tubes in PWR reactor fuel assemblies which allow for the insertion of discrete poison rodlets (Guide Tubes) and instrument thimbles (Instrument Tube). These tubes also serve as the axial support structure for the fuel assembly connecting the lower and upper nozzles, while the fuel rods are allowed to "float" in the assembly (connected only by spacer grids to the tubes). An unobstructed instrument/guide tube allows reactor coolant to flow through it.

Objects of an Guide Tube

• Insert (see RCA model)

Objects of an Instrument Tube

• Incore detector and thimble (See Incore Instrument model)

Properties of an Instrument/Guide Tube:

• Identifier
• Material, inner and outer radius, and height.

The default instrument/guide tube has the following properties:

• material = zircaloy-4
• height = nozzle to nozzle, ~155 inches
• Inner radius of 0.561 cm and outer radius of 0.602 cm.

Time Evolution and State Points

State point data of the reactor include

• Inlet coolant temperature (input)
• Inlet coolant density (input)
• Average coolant temperature (output)
• Percent power and percent flow (input)
• Reactor pressure (input)
• Fuel temperature (input)
• Control rod bank positions (steps withdrawn)
• Eigenvalue (input/output)
• Boron Concentration in the coolant (input/output)
• Power distribution (output)
• Coolant temperature and density distribution (output)