IFA 597.3 Rod 7 and Rod 8

Rod Design Specifications

As stated in the previous section, both rods were nearly identical and comparisons for both experiments were modeled with one simulation. The specifications for rod 8 were used for the simulation. Rod 8 was a re-fabricated rod extracted from a full length rod. The hole for the thermocouple was at the top of the fuel stack 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 Ringhals BWR is the same for both rods 7 and 8 and is shown in Figure 1. The experiment power history carried out at the Halden boiling water reactor (HBWR) is shown in Figure 2. The measured clad surface temperature as a function of time is shown in Figure 3. The other reactor operational parameters are tabulated in Figure 2.

Figure 1: Base irradiation history for IFA-597.3, carried out at Ringhals BWR.

Figure 2: Halden irradiation periods for rods 7 and 8. The irradiations include IFA-597.2 and IFA-597.3.

Figure 3: Temperature on the outside surface of the cladding for both the base irradiation and Halden irradiations for Rods 7 and 8

Geometry and Mesh

The re-fabricated rod geometry was modeled for the entire simulation. The rod was modeled with two smeared pellet blocks, one annular and one solid, to account for the thermocouple at the top of the fuel rod.

A 2D-RZ axisymmetric quadratic mesh was used to model the geometry of rod 8. The fuel mesh consisted of 128 axial nodes and 14 radial nodes (11 radial elements for the annular section) and the clad was meshed with 4 radial elements through the thickness. A section of the meshed fuel rod at the thermocouple location is shown in Figure 4.

Figure 4: 2D-RZ axisymmetric mesh for IFA-597.3 Rod 8 simulation with temperature contour plot at thermocouple location.

Material and Behavioral Models

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

• UO2Thermal - NFIR: For temperature and burnup dependent thermal properties.

• ComputeFiniteStrainElasticStress and UO2ElasticityTensor: elastic mechanical behavior

• UO2VolumetricSwellingEigenstrain: volumetric expansion due to solid and gaseous swelling

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

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

• UO2Sifgrs: fission gas release model used with the gaseous swelling model UO2VolumetricSwellingEigenstrain

For the clad material, a constant thermal conductivity of 16 W/m-K was used and both thermal (primary and secondary) and irradiation creep were considered using the Limback creep model (Limbäck and Andersson, 1996). 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-2

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

• ZryThermalExpansionMATPROEigenstrain: thermal expansion of Zircaloy with the MATPRO model

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

Temperature

Comparison of the measured and predicted fuel centerline temperature during the first four and final power ramps are shown in Figure 5. Although BISON tends to under predict the temperature, considering uncertainties in the power and temperature measurements the comparisons are reasonable.

Figure 5: BISON fuel centerline temperature comparison to Halden experimental data.

A comparison of the predicted (P) and measured (M) fuel centerline temperatures for the entire IFA-597.3 ramp section is shown in Figure 6. Superimposed on the graph are a M=P, M=P$+$10%, and M=P$-$10% lines to illustrate how well BISON is predicting the fuel centerline temperature. Given the uncertainty in thermal conductivity and measurements of linear power, predictions within $\pm$10% are considered acceptable.

Figure 6: Predicted versus measured temperature throughout the IFA-597.3 Halden ramp.

Fission Gas Release

BISON under predicts the total FGR at the end of base irradiation and at the end of the power ramps. The pressure transducer that was used to measure the FGR reached its maximum operating limit at 68 MWd/kgU. The total fission gas release measured during the PIE puncture test was 15.8%. BISON predicts 1.8%. The BISON results compared to experimental data is shown in Figure 7.

Figure 7: BISON fission gas release comparison to Halden experimental data.

Clad Elongation

The clad elongation was predicted with both frictionless contact between the fuel and clad and with glued contact between the fuel and clad, with the actual clad elongation lying between the two predictions.

Figure 8: BISON clad elongation comparison to Halden experimental data.