Riso II3

Rod Design Specifications

Rod II3 was a re-fabricated rod extracted from a full length rod. The hole for the thermocouple was at the top of the fuel rod and did not penetrate the entire fuel stack. The re-fabricated rod geometry is tabulated in Table 1.

Operating Conditions and Irradiation History

The power history for the base irradiation carried out at the Millstone-1 BWR is shown in Figure 1. The experiment power history carried out at the Riso DR3 facility is shown in Figure 2 and run at BWR conditions by reducing the system pressure to 7.24 MPa. A prescribed axial profile for this experiment was provided in the FUMEX-III data (IAEA, 2008-2012). The measured clad surface temperature as a function of time was also provided in the FUMEX-III data (IAEA, 2008-2012) and used as a boundary condition for this simulation. The other reactor operation parameters are tabulated in Table 2.

Figure 1: Base irradiation history for fuel segment STR014, carried out at Millstone-1 BWR.

Figure 2: Riso DR3 irradiation period for test II3 (STR014-3R).

Geometry and Mesh

The re-fabricated rod geometry was modeled for the entire simulation considering a smeared column of flat ended pellets, with the top pellets containing the hole for the thermocouple. The plenum height was adjusted such that the plenum volume at the beginning of the bump test was approximately 7.41 cm.

A 2-dimensional axisymmetric quadratic (Quad8 elements) mesh was used to model the geometry of the rod used in the II3 experiment. The fuel was meshed considering two fuel pellet types. The first pellet type was 4.2 cm in length with a hole down the center, the second pellet type was 24.9 cm in length with no hole down the center. The first pellet type's mesh is comprised of elements 3.953 mm in the axial direction and 0.3889 mm in the radial direction (for an aspect ratio of 10.16). The second pellet type's mesh is comprised of elements 2.0 mm in the axial direction and 0.3813 mm in the radial direction (for an aspect ratio of 5.244). The clad mesh is comprised of elements 4.278 mm in the axial direction and 0.1775 mm in the radial direction (for an aspect ratio of 24.1).

Material and Behavioral Models

The following material and behavioral models for UO fuel were used:

• UO2Thermal - NFIR: NFIR model for temperature and burnup dependent thermal properties

• ComputeFiniteStrainElasticStress and UO2ElasticityTensor: elastic mechanical behavior

• UO2RelocationEigenstrain: relocation strains, relocation activation threshold power set to 5 kW/m

• ComputeThermalExpansionEigenstrain: thermal expansion with a constant instantaneous thermal expansion coefficient

• UO2VolumetricSwellingEigenstrain: volumetric expansion due to solid and gaseous swelling

• UO2Sifgrs: fission gas release model used with the gaseous swelling model UO2VolumetricSwellingEigenstrain (Pastore et al., 2015)

For the cladding material, a constant thermal conductivity of 16 W/m-K was used and both thermal and irradiation creep were considered using the Limback model (Limbäck and Andersson, 1996). The following material and thermal behavior models were used for the clad:

• HeatConductionMaterial: Thermophysical material properties

• ZryCreepLimbackHoppeUpdate and ZryElasticityTensor: mechanical creep and elastic deformation behavior

• ZryIrradiationGrowthEigenstrain: ESCORE model for volumetric swelling due to irradiation exposure

• ComputeThermalExpansionEigenstrain: thermal expansion with a constant instantaneous thermal expansion coefficient

Details and references for all of these models listed above can be found on the linked BISON documentation pages.

Figure 3: BISON fuel centerline temperature comparison to Riso experimental data

Figure 4: BISON ramp test fission gas release comparison to Riso experimental data.

Figure 5: BISON rod internal pressure comparison to Riso experimental data.

Figure 6: BISON rod outer diameter comparison to Riso experimental data.

Boundary and Operating Conditions

The Riso DR3 irradiation period for the II3 test shown in Figure 2 was appended to the base irradiation power history shown in Figure 1. It was assumed that the clad temperature during the down time between base irradiation and the Riso test was 373K. The fast neutron flux was input as a function of power.

Temperature

BISON predicts the shape of temperature curve well, but fails to reach measured thermocouple temperatures as shown in Figure 3. One possible explanation for the lower predicted temperature is the underprediction of fission gas release as seen in Figure 4. The fuel centerline temperature is taken at a node approximately 38 mm from the top of the fuel stack.

Fission Gas Release

The calculated integral fuel rod fission gas release is compared to the measured data in Figure 4. In view of the uncertainties involved in FGR modeling, the predictive accuracy is satisfactory, falling well within the uncertainty factor of 2 (Killeen et al., 2007).

Rod Internal Pressure

The calculated rod internal pressure matches the measured data set well as seen in Figure 5 following the given shape of the measured curve accurately.

Rod Outer Diameter

The calculated rod diameter seems to predict more cladding creepdown than experiment results suggest as shown in Figure 6. Since BISON currently does not have a cladding option for cold worked Zry2, a SRA Zry4 cladding was chosen instead.