Riso AN2

Rod Design Specifications

Fuel pin CB6 was the upper-middle segment of four, 675 mm-long barrier clad segments (from top CB9, CB6, CB7, CB8) which together with a top and a bottom segment constituted a fuel rod stringer. CB6 was bump tested in the unopened condition. The CB6 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 was 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 CB6, carried out at Biblis A PWR.

Figure 2: Riso DR3 irradiation period for test AN2 (CB6).

Geometry and Mesh

The CB6 section rod geometry was modeled for the entire simulation considering a smeared column of flat ended pellets. 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 AN2 experiment. The fuel was meshed so that the total active fuel length would equal 0.54 m, leaving a total upper plenum length of 61 mm. The fuel mesh consisted of elements 2.31 mm in the axial direction and 0.4115 mm in the radial direction (for an aspect ratio of 5.613). The clad mesh consisted of elements 2.57565 mm in the axial direction and 0.19375 mm in the radial direction (for an aspect ratio of 13.29).

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

• ZryThermalExpansionMATPROEigenstrain: thermal expansion with an instantaneous thermal expansion coefficient that varies according to the MATPRO cthexp model

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