Metallic HCF BISON Model: 3D cycle fuel performance

Mesh

moose_hcf_mesher.i is used to generate a HCF mesh with first-order hexahedral elements (HEX8) for BISON simulation. The geometry can be split into several curves, made of 1D edge elements, and those can be represented with an explicit (parameterized) formulations, which can be created by MOOSE using the ParsedCurveGenerator. After that, those curves are stitched to form an enclosed boundary using StitchMeshGenerator and the 2D mesh is generated by filling with 2D elements between the two curves using the FillBetweenCurvesGenerator. To extrude the mesh with an axial twist, the AdvancedExtruderGenerator is used with the twist_pitch parameter. Finally, to generate the sidesets and nodesets which are being used to apply boundary conditions, BoundingBoxNodeSetGenerator, SideSetsFromNormalsGenerator and ParsedGenerateSideset are used for various surfaces. The user can control the mesh density through several parameters defined at the top of the input file, such as n_seg_lobe, n_seg_valley, n_seg_l2v, n_center, n_fuel_radial, n_clad_radial, and n_per_axial_pitch. Furthermore, detailed geometrical specifictions can be controlled through addtional input file parameters such as r_fuel, r_clad, R_fuel, R_clad, c_lobe_clad, c_lobe_valley, and c_valley. Figure 2 shows the definition of geometric parameters and Table 1 summarizes the geometric parameters used in mesh generator and specific values used in this model.

Figure 2: Definition of geometric parameters of HCF.

Any MOOSE applicaton can be used to generate a HCF mesh using this input as all the objects used are part of the core MOOSE framework. For instance, a BISON user may execute the following command on any of INL's HPC Cluster:

module load use.moose bison-openmpi
bison-opt -i moose_hcf_mesher.i

This will generate the HCF mesh file (mesh.e), which is used in BISON simulations.

Functions

BISON is capable of modeling U-Pu-Zr alloy fuel for fuel performance analysis. However, the metallic fuel models in BISON are based on the experimental data for fast reactor applications where the fuels were U-rich alloys, which are mostly in -phase with less than approximately 10 wt.\% of Zirconium content. Given that U-50Zr alloy will operate in a different phase (-phase), there is a high uncertainty in using existing models for this application.

To reduce uncertainties associated with applying models for different phases, the material properties are implemented through Functions to better represent the characteristics of the U-50Zr alloy, using published studies of U-Zr alloys. Specifically, thermal properties including thermal conductivity, heat capacity, and thermal expansion are implemented based on experimental data from Idaho National Laboratory (INL) (Beausoleil et al., 2021), where a preliminary thermophysical and mechanical evaluation of unirradiated U-50Zr was conducted. INL's experimental data exhibit good agreement with previously reported measurements for U-50Zr, whereas BISON's models, which are specifically developed for U-rich U-Zr alloys, show noticeable discrepancies. This highlights the importance of applying phase-appropriate thermal properties to ensure accurate fuel performance analysis.

[Functions]
  [u50zr_thermal_conductivity]
    type = PiecewiseLinear
    data_file = U50Zr_thermal_conductivity.csv ## Temperature [K] vs. thermal conductivity (W/m-K)from INL report
    format = columns
  []
[]

[Functions]
  [u50zr_heat_capacity]
    type = PiecewiseLinear
    data_file = U50Zr_heat_capacity.csv ## Temperature [K] vs. heat capacity (J/kg-K) from INL report
    format = columns
  []
[]

[Functions]
  [u50zr_therm_expan]
    type = PiecewiseLinear
    data_file = U50Zr_thermal_expansion.csv ## Temperature [K] vs. thermal expansionfrom INL report
    format = columns
  []
[]

CoolantChannel BISON syntax

This model uses normal operating conditions of the 250 MWth NuScale Power Module (NPM). Figure 3 shows the linear heat generation rate (LHGR) history of the hot fuel pin obtained from a NPM replica design developed at the Massachusetts Institute of Technology (MIT) (Halimi and Shirvan, 2025). The pin-wise peak LHGR is used in this model to simulate the most limiting condition. Table 2 summarizes the thermal-hydraulic parameters used in BISON through the CoolantChannel action. The flow area corresponds to the subchannel area with a rod pitch of 1.26 cm. The heated diameter is the cylinder-equivalent diameter of the HCF rod, and the heated perimeter is the perimeter of the outer fuel surface. Lastly, the hydraulic diameter is computed using Eq. (1). Note that additional turbulent mixing from the helical fuel shape is not taken into account in the 1D axial coolant model.

Figure 3: Linear heat generation rate history of the hot fuel.

(1)

Materials

(Ahn et al., 2016) reported that orders-of-magnitude differences in the size of gas bubbles between the U-0.1Zr alloy (-U phase) and U-40Zr alloy (-UZr phase) following 140 keV He ion irradiation. In the irradiated U-40Zr alloy, the average bubble diameter was approximately 6nm and remained nearly constant across different irradiation doses. To capture this phase-dependent bubble size effect, the fuel volumetric swelling model in BISON (UPuZrVolumetricSwellingEigentrain) was modified by incorporating the fission gas bubble radius as an input parameter (bubble_radius). In this model, the bubble radius is fixed at 10 nm, which is about twice the reported size, as a conservative assumption. This assumption should be revisited once experimental data on volumetric swelling under neutron irradiation become available. Lastly, it is worth noting that zirconium redistribution in the fuel, which could affect the thermal properties and radial fission rate distribution in the fuel, is not yet considered in this model due to the lack of validation of existing models for three-dimensional case.

[Materials]
  [fuel_swelling]
    type = UPuZrVolumetricSwellingEigenstrain
    eigenstrain_name = 'fuel_swelling_strain'
    block = 'fuel'
    burnup = burnup
    hydrostatic_stress = hydrostatic_stress
    fission_rate = fission_rate
    initial_porosity = 0.1
    bubble_radius = 10e-9 # new input paramter for fission gas bubble radius
  []
[]