OSIRIS H09

Rod Design Specifications

The geometric input parameters for the OSIRIS H09 rod are summarized in Table 1.

Operating Conditions and Irradiation History

The H09 rod was irradiated for 4 cycles in the OSIRIS reactor to a final discharge average burnup of 46.06 MWd/kgU. The power mode selected for this simulation is PiecewiseConstant. The average linear powers in the 4 cycles were approximately 22, 20, 18, and 15 kW/m (OECDNEDB, 2002). The average power history for is shown in Figure 1. The power history assumed a 24 hour startup time that was broken into 24 timesteps in one hour increments. Because the axial power shapes and boundary conditions are modeled as PieceswiseBilinear, a ramp time of 360 seconds (0.1 hours) was assumed at each power step for the axial power shape and boundary condition input. The startup time of 24 hours and the ramp time of 360 seconds (0.1 hours) are based on ANATECH's experience with fuel rod modeling for steady state operation and the development of Falcon Verification and Validation cases. They are intended to minimize the introduction of computational artifacts from unrealistic power changes and ramp rates into the analyses. The axial power profile was calculated from the OSIRIS data package (OECDNEDB, 2002) taken from the IFPE database. The cladding outer surface temperature as a function of time was also provided in the OSIRIS data package (OECDNEDB, 2002), and was used as a boundary condition for this simulation. The cladding outer surface temperature ranged from 562.55 K to 611.95 K. The initial fill-gas (Helium) pressure was 3.1 MPa, and the coolant system pressure was 15.5 MPa. The fast neutron flux as a function time was calculated from the data provided in the OSIRIS data package (OECDNEDB, 2002). Operational input parameters are summarized in Table 2.

Figure 1: OSIRIS H09 power history with 24 hours startup

Geometry and Mesh

The rod specifications in Table 1 were used to define the geometry for this simulation. The OSIRIS H09 rod was modeled as a two-dimensional, axi-symmetric linear mesh with quadratic elements. The fuel mesh consisted of 11 radial elements and the cladding mesh consisted of four radial elements to form a clad thickness of 0.575 mm. The fuel stack length is 3.66 m and the plenum height is 0.139 m. The mesh of the top portion of the rod is shown in Figure 2.

Figure 2: OSIRIS H09 mesh (not to scale)

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 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 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

Fission Gas Release

The fission gas release data available for this experiment is from post irradiation examination (PIE) puncture tests. Figure 3 show BISON's comparisons with the end-of-life measurement for the OSIRIS H09 rod. BISON computes a reasonable FGR value that under predicts the measured result.

Figure 3: Fission gas release comparisons for OSIRIS H09

Rod Internal Pressure

The only rod internal pressure data available for this experiment is from PIE puncture tests at the end-of-life. Figure 4 shows BISON's comparisons to the experimental data for the OSIRIS H09 rod. The figure shows BISON over predicts the rod internal pressure.

Figure 4: Rod internal pressure comparison for OSIRIS H09

Cladding Diameter

The calculated final rod diameter as a function of axial position is compared to measured data. A comparison of the computed and measured end-of-life rod diameter data (excluding oxide thickness) is shown in Figure 5. The figure shows BISON over predicts the cladding diameter at the end of life which results in less cladding computed creep down than measured.

Figure 5: EOL average cladding diameter comparisons for OSIRIS H09

Discussion

Based on the data presented above, several observations can be made regarding the results obtained from BISON analyses of the OSIRIS H09 test rod.

• BISON's prediction the EOL FGR is reasonable, but somewhat low.

• BISON over predicts the measured rod internal pressure by a fairly large margin.

• BISON over predicts measured EOL cladding diameter.

  - Based on evaluation of these and other assessment cases, this behavior appears to be related to fuel swelling after fuel/cladding contact. Additionally, other effects on fuel deformation including relocation, densification, fuel creep, etc. could influence the behavioral response in these analyses.

Since cladding oxide thickness and hydrogen concentration data are available for OSIRIS H09 rod, these characteristics should be evaluated in the future once these features are available in BISON.