REGATE

Rod Design Specifications

The geometric input parameters for the FumexII-Regate case are summarized in Table 1.

Operating Conditions and Irradiation History

The irradiation was adjusted by varying the distance of the rig from the SILOE core. The ramp test irradiation history consisted of a pre-condition power step of 19.5 kW/m (peak power) for 48 hours, prior to ramping at 1.0 kW/m/min up to 38.5 kW/m (peak power) which was held for 1.5 hours. The rod average power history during the SILOE irradiation is shown in Figure 2. As the height of the SILOE reactor (0.6 m) is comparable to the segment length (0.44 m), the axial power is not flat during the ramp test, leading to values of of 0.9 and of 0.65.

Figure 2: Rod average power history in the SILOE reactor.

Figure 3: Fast neutron flux history. This history was supplied with the experimental data.

Geometry and Mesh

A 2D-RZ axisymmetric discrete pellet mesh with quadratic elements was used to model this experiment. Each pellet was meshed with 16 axial and 9 radial elements. The clad was meshed with 4 axial elements. Figure 4 shows a section of the meshed with a temperature contour plot during the ramp test.

Figure 4: Section of mesh used for Regate simulation with temperature contour during the ramp test in the SILOE reactor.

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 instantaneous 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. Due to the high misses stress in the clad, plasticity was also used to get the proper deformation during the power ramp in the RisøDR3 reactor.

Fission Gas Release

BISON over predicts FGR after the base irradiation in the Gravlines 5 PWR and under predicts the FGR at the end of the ramp test. The comparisons are plotted in Figure 5.

Clad Diameter

BISON over predicts clad creep down which results in a smaller diameter than measured during PIE. The BISON comparisons to experimental measurements before and after the ramp are shown in Figure 6.