IFA-431 Rods 1, 2, and 3

Operating Conditions and Irradiation History

The reactor was operated with a coolant pressure of 3.4 MPa and an inlet temperature of 510 K. The power history was provided by experimentalists from Halden (Wiesenack, 2012).

Geometry and Mesh

All three fuels rods were meshed using 2-D axisymmetric quadratic elements. For simplicity, the pellet stack was modeled as a single continuous fuel column. The thermocouple holes were modeled as closely to the experiment as possible at the top and bottom of the fuel rod. Figure 1 shows a scaled view of the mesh for rod 1. Rods 2 and 3 were identical with exceptions to the thermocouple hole length and the pellet-clad gap width.

Figure 1: Scaled view of the finite element mesh for rod 1 (aspect ratio scaled 10x).

Material and Behavioral Models

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

• UO2Thermal - NFIR: 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 instanteous 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 clad 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 fast neutron flux used in the irradiation creep model was 1.6e12 n/m-s per W/m (Killeen et al., 2007). This value was multiplied by the power history (W/m) and the axial peaking factors to approximate the fast neutron flux. The following material and thermal behavior models were used for the cladding:

• HeatConductionMaterial: Thermophysical material properties

• ZryCreepLimbackHoppeUpdate and ZryElasticityTensor: mechanical creep and elastic deformation behavior for Zircaloy-4

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

• ZryThermalExpansionMATPROEigenstrain: thermal expansion of Zircaloy with the MATPRO model

Boundary and Operating Conditions

The clad outer wall temperature was assumed constant at 513.3 K. The input BOL power histories for Rods 1, 2, and 3 are shown in Figure 2.

Figure 2: Input BOL power history

Centerline Temperature at Beginning of Life

Initial comparisons were made to centerline fuel temperature measurements during the first rise to power, or the period referred to as the Beginning of Life (BOL). Comparisons during this period are important since they isolate several important aspects of fuel rod behavior before complexities associated with higher burnups are encountered. For example, good prediction of BOL centerline temperature requires accurate models for the unirradiated fuel thermal conductivity, gap gas conductivity, thermal expansion of both the fuel and clad materials (which set the gap width), clad conductivity, and fuel relocation.

Figure 3, Figure 4, and Figure 5 show centerline temperature comparisons at BOL for Rods 1, 2, and 3, respectively.

Figure 3: Measured vs. BISON predicted fuel centerline temperatures at BOL for Rod 1.

Figure 4: Measured vs. BISON predicted fuel centerline temperatures at BOL for Rod 2.

Figure 5: Measured vs. BISON predicted fuel centerline temperatures at BOL for Rod 3.

Differences between the measurements and the predictions are quantified in terms of the validation metrics. The results are tabulated in Table 2. Comparisons are in good agreement.