IFA-681 Rods 1, 2, and 3

Overview

The Halden integral test IFA-681 was loaded in the Halden Boiling Water Reactor (HBWR) in January 2005 and operated till February 2012 with a completion of 15 cycles of irradiation. The rig contained two UO fuel rods and four UO-GdO rods with varied gadolinia content from 0.0 to 8.0wt.%, which were manufactured by Institutt for Energiteknikk (IFE)-Kjeller. An overview of the fuel rod specifications of the IFA-681 tests is provided in Table 1. In practice, the rod designs were very similar except U enrichment and Gd concentrations. The experiment was conducted under standard HBWR conditions, with the rig operating at a coolant pressure of approximately 3.5MPa, and a coolant temperature of approximately 235. Achieved burnup varied between 31 and 45 MWd/kgUO. The aim in the experiment was to evaluate the impact of Gd concentration to the oxide solution on the fuel thermal behavior, dimensional changes, and fission gas release.

Test Description

Rod Design Specifications

The IFA-681.1 rods 1, 2, and 3 were solid rods and were enriched to 4.70wt.%U for rods 1 and 2, and 2.80wt.%U for rod 3. The fuel and cladding specifications are tabulated in Table 1.

Table 1: IFA-681.1 Test Rod Specifications

Fuel RodRod 1Rod 2Rod 3Unit
Fuel stack height400.4400.0400.2mm
Fuel pellet height10.259.929.97mm
Fill gas compositionHeHeHe
Fill gas pressure1.01.01.0MPa
FuelRod 1Rod 2Rod 3Unit
MaterialUOUO-GdOUO-GdO
GdO0.02.08.0wt.%
Enrichment4.704.702.80
Density95.696.196.0
Inner diameter0.00.00.0mm
Outer diameter8.198.198.19mm
Pellet geometrysolidsolidsolid
Insulator PelletRod 1Rod 2Rod 3Unit
Materialnatural UO/AlOnatural UO/AlOnatural UO/AlO
Inner diameter1.801.801.80mm
Outer diameter8.198.198.19mm
Pellet geometryhollowhollowhollow
CladdingRod 1Rod 2Rod 3Unit
MaterialZy-4Zy-4Zy-4
Outer diameter8.368.368.36mm
Inner diameter9.509.509.50mm
Wall thickness1.141.141.14mm

Operating Conditions and Irradiation History

The HBWR operating conditions are tabulated in Table 2. The reactor power history is shown in Figure 1. The measured reactor coolant temperature was used as the boundary temperature on the cladding outer surface.

Table 2: Operational input parameters.

Average coolant temperature235
Coolant pressureMPa3.5

Figure 1: Through life power histories for IFA-681.1 rods 1, 2, and 3.

Model Description

Geometry and Mesh

The assumed geometry and mesh are shown in Figure 2 and Figure 3. The fuel pellet stack was modeled as a smeared column with merged insulator pellets (natural UO and AlO). The analysis in BISON was performed for the three out of six rods, IFA-681 rods 1, 2, and 3. Note that top 11 pellets are hollow where the thermocouple were located. The integral responses from the experiment were obtained as follows: fuel temperature via a fuel centerline thermocouple, rod internal pressure via pressure transducer, fission gas release (FGR) via the online pressure measurements, and rod power via four neutron detectors installed in the rig. A two-dimensional axisymmetric geometry was used.

Figure 2: A front-view of the IFA-681.1 rod 1.

Figure 3: Close-up view of the IFA-681.1 rod 1, a cut from the top of the fuel rod.

AlO hollow insulation pellets in upper end in IFA-681 rods 1, 2, and 3, followed by natural UO hollow pellets. 11 pellets of hollow enriched UO fuel, followed by 28/29 pellets of the solid enriched UO fuel. Similarly, the lower end, natural UO solid pellets, followed by AlO solid insulation pellets.

Material and Behavioral Models

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

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:

For the natural UO material, a constant thermal conductivity of 3.0 W/m-K and a constant specific heat capacity of 300 J/kg-K were used. The following material and thermal behavior models were used for the cladding:

For the natural AlO material, a constant thermal conductivity of 18.0 W/m-K and a constant specific heat capacity of 880 J/kg-K were used. The following material and thermal behavior models were used for the cladding:

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

Input Files

The supplementary histories, from Halden data, such linear heat rate (LHR), fast neutron flux, coolant pressure, and coolant inlet temperatures are provided to BISON. Fuel Rod Analysis Toolbox is utilized for the condensation of the data prior to setting the BISON inputs. Gadolinium doping is performed using the BurnupFunction after this capability was added in FY23 (Sweet et al., 2023).

The BISON input and all supporting files (power histories, axial power profile, cladding surface temperature boundary condition, fast neutron flux history, etc.) for this case are provided with the code distribution at bison/assessment/LWR/validation/IFA_681/analysis.

Simulation Parameters and Assumptions

As mentioned in the Geometry and Mesh section, the mesh used is not an exact representation of the experiment. The insulator pellets were not included in the heat source term. The expansion thermometer was physically neglected in this mesh. This was done to alleviate troubles with thermal and mechanical properties between the thermometer and the fuel/insulator stack. The plenum length of the fuel rod was adjusted to account for the extra gas volume made from the voided thermometer. The simulation initial gas volumes were as listed in (Khattout, 2013). The value of 8.5 was used for the initial grain size, 1.0 for the fuel surface roughness, and 2.0 for the cladding surface roughness.

Results Comparison

BISON's predictions are compared to the experimental measurements for each rod in the following sections. A Python postprocessor script was used to extract the centerline temperature at the thermocouple (TC) position and the plenum pressure from the BISON simulations. It's important note that the thermocouples and pressure transducers failed, yielding BISON to go to the full time of the average linear heat rate (ALHR) while the experiment doesn't.

Temperature

BISON's temperature predictions are compared to the estimated values derived from the pressure measurements, which are shown for IFA-681.1 Rods 1, 2, and 3 in Figure 4, Figure 5, and Figure 6, respectively.

Figure 4: IFA-681.1 rod 1 temperature histories.

Figure 5: IFA-681.1 rod 2 temperature histories.

Figure 6: IFA-681.1 rod 3 temperature histories.

Rod internal pressure

BISON's rod internal pressure predictions are compared to the measurements, which are shown for IFA-681.1 Rods 1, 2, and 3 in Figure 7, Figure 8, and Figure 9, respectively.

Figure 7: IFA-681.1 rod 1 rod internal pressure histories.

Figure 8: IFA-681.1 rod 2 rod internal pressure histories.

Figure 9: IFA-681.1 rod 3 rod internal pressure histories.

Discussion

The new thermal conductivity model along with the improved gap conductance modeling results in a good aggreement with the measured values for larger gadolinia concentrations. Furthermore, BISON's rod internal pressure predictions are slightly overestimating the expected behavior.

References

  1. F Khattout. The Gadolinia fuel test IFA-681: Overview of in-pile measurements from beginning of irradiation to unloading. Technical Report HWR-1038, OECD Halden Reactor Project, 3 2013.[BibTeX]
  2. M. Limbäck and T. Andersson. A model for analysis of the effect of final annealing on the in- and out-of-reactor creep behavior of zircaloy cladding. In Zirconium in the Nuclear Industry: Eleventh International Symposium, ASTM STP 1295, 448–468. 1996.[BibTeX]
  3. Ryan Terrence Sweet, Daniel J. vanWasshenova, Daniel Schwen, and Aysenur Toptan. Compare mechanistic predictions for doped uo2 mechanical response and other properties with empirical models and experimental measurements. Technical Report, Idaho National Laboratory, 9 2023.[BibTeX]