Riso AN4

Rod Design Specifications

ROB CB7-2R 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 Biblis A PWR is shown in Figure 1. The experiment power history carried out at the Riso DR3 facility is shown in Figure 2. The axial profile for this experiment is shown in Figure 3. The measured clad surface temperature as a function of time is shown in Figure 4. The other reactor operation parameters are tabulated in Figure 4.

Figure 1: Base irradiation history for fuel segment CB7, carried out at Biblis A PWR.

Figure 2: Riso DR3 irradiation period for test AN4 (CB7-2R).

Figure 3: Axial power profile for Riso AN4.

Figure 4: Measured clad surface temperature as a function of time.

Geometry and Mesh

The re-fabricated rod geometry was modeled for the entire simulation as a single dished pellet. The entire fuel stack was shifted up from the bottom of the clad by 5.1 mm, which is the height of the insulator pellet at the bottom of the fuel rod.

A 2-dimensional axi-symmetric linear mesh was used to model the geometry of the rod used in the AN4 experiment. The fuel mesh consisted of 141 axial nodes and 9 radial nodes (for an aspect ratio of 5.1), the clad mesh consisted of 113 axial nodes and 5 radial nodes (Figure 5).

Figure 5: 2-D axi-symmetric mesh for Riso AN4 simulation. Note: Mesh plot is scaled axially by 0.05

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 Zircaloy-4 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

• 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.

Boundary and Operating Conditions

The Riso DR3 irradiation period for the AN4 test shown in Figure 2 was appended to the base irradiation power history shown in Figure 1. The power history used as an input parameter for this particular simulation is shown in Figure 6. The clad temperature as a function of time shown in Figure 4 was used as the clad boundary condition for this simulation. The fast neutron flux was input as a function of power and scaled to 4.9e17.

Figure 6: BISON input power history for Riso AN4.

Temperature

The fuel centerline temperature predicted with BISON compared extremely well with the experimental data. The maximum difference between measured temperature and predicted temperature was approximately 72 degress C at the top of the ramp. (Figure 7).

Figure 7: BISON fuel centerline temperature comparison to Riso experimental data.

Fission Gas Release

BISON predicts the total fission gas released well with some over prediction during the first flat power section of the ramp test (Figure 8).

Figure 8: BISON total fission gas release comparison to Riso experimental data.

Plenum Pressure

The calculated plenum pressure increases throughout the entire ramp test (Figure 9). The pressure prediction follows the fission gas release behavior which could be the reason for the constant increase in plenum pressure during the ramp test.

Figure 9: BISON plenum pressure comparison to Riso experimental data.