High Burnup Effects Programme (HBEP) BK363, BK365 and BK370

Operating Conditions and Irradiation History

Rods BK363, BK365 and BK370 were irratited in the BR-3 pressurized water reactor (PWR) in Belgium (HBEP, 2002) at a coolant pressure of 13.73 MPa and coolant inlet temperature of 255 C (IAEA, 2002-2007). Rods BK363 and BK365 were irratiated four cycles to burnups of 66.7 MWd/KgU and 69.4 MWd/KgU, respectively. Rod BK370 was irratiated three cycles to a burnup of 50.9 MWd/KgU. Clad temperatures and local power histories were taken at 10 axial locations and obtained from the FUMEX II data. The plot below, Figure 1, shows the power histories for the BK363, BK365 and BK370 rods. The power histories are similar with BK365 seeing more power in the middle stage. BK370 had a shorter irradiation time than the other 2 rods.

Geometry and Mesh

All three fuel rods were meshed using 2-D axisymmetric models with quadratic elements. For simplicity, the pellet stack was modeled as a single continuous fuel column (smeared mesh). The rods were identical so the same mesh was used for all runs. The fuel pellets had 306 axial elements and 11 radial elements, and the cladding consisted of 312 axial elements and 4 radial elements.

Figure 2: A segment of the fuel and cladding mesh used.

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.

HBEP, BK363

As can be seen above in Figure 3 and Figure 4 the BISON calculated fission gas release is less than the experiment measured amount, about 0.3% and 4%, respectively. As mentioned above the Sifgrs fission gas release model with the transient release model was used in this case. This rod was held at a lower power at the phase that included 5e7 sec (see Figure 1). It is possible that the model did not produce enough fission gas during this phase and thus was not released in the transient phase. Evidence for this theory may be supported by BK365's performance.

HBEP, BK365

As mentioned in the previous subsection, the BISON performance of the BK365 rod against the experiment was better than the BK363 rod. Shown in Figure 5, Figure 3 and Figure 4 the experiment measured amount was about 2.5% and the BISON calculation was about 0.5%. Figure 5 shows a comparison of the linear heat rate to the calculated fission gas release over the time of the experiment.

Figure 5: Comparison of Linear Heat Rate and Fission Gas Release for BK365

HBEP, BK370

There was no experimental data collected from this rod. Other codes did simulate this rod, as did BISON, but since there are no experimental results there will be no discussion about this rod.