Riso AN3

Rod Design Specifications

Rod CB8-2R 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. A prescribed axial profile for this experiment was provided in the FUMEX-II data (IAEA, 2002-2007). The measured clad surface temperature as a function of time was also provided in the FUMEX-II data (IAEA, 2002-2007) 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 CB8, carried out at Biblis A PWR.

Figure 2: Riso DR3 irradiation period for test AN3 (CB8-2R).

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 7.0 cm. 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 axisymmetric quadratic (Quad8 elements) mesh was used to model the geometry of the rod used in the AN3 experiment. The fuel was meshed considering two fuel pellet types. The first pellet type was 4.1 cm in length with a hole down the center, the second pellet type was 24.5 cm in length with no hole down the center. The first pellet type's mesh consisted of 29 axial nodes and 10 radial nodes (for an aspect ratio of 4.07). The second pellet type's mesh consisted of 166 axial nodes and 13 radial nodes (for an aspect ratio of 3.93). The clad mesh consisted of 131 axial nodes and 3 radial nodes.

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 AN3 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 500K. The fast neutron flux was input as a function of power and scaled to 4.9e17.

Temperature

BISON predicts the fuel centerline temperature well (Figure 3) and is comparable with other well known fuel performance codes. The fuel centerline temperature is taken at a node approximately 36.4 mm from the top of the fuel stack.

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

Fission Gas Release

The calculated integral fuel rod fission gas release is compared to the measured data, as well as with the TRANSURANUS and ENIGMA predictions, in Figure 4. In view of the uncertainties involved in FGR modeling, the predictive accuracy is satisfactory. When compared to other codes, BISON's prediction of total FGR is excellent, with many codes underpredicting the fission gas release at the end of life by more than a factor of 2 (Killeen et al., 2007).

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

Rod Internal Pressure

The fission gas release as a function of time during the ramp test is calculated based off the measured pressure of the rod. When compared to the measured rod internal pressure, BISON slightly over predicts the rod pressure (Figure 5). This is likely due to the conditions of the rod at the refabrication time. It is reported that the fill gas is measured at room temperature, however, the temperature of the gap is higher than that of ambient temperature due to the decay heat of the already irradiated fuel.

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