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# Mechanical Fatigue Testing of High-Burnup Fuel for Transportation Applications(NUREG/CR-7198, Revision 1, ORNL/TM-2016/689)

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## Publication Information

**Manuscript Completed:** December 2016**Date Published:** October 2017

**Prepared by:** Jy-An Wang and Hong Wang

Oak Ridge National Laboratory Managed by UT-Battelle, LLC Oak Ridge, TN 37831-6069

Bruce Bevard, ORNL Program Manager

Michelle Flanagan-Bales, NRC Project Manager

NRC Job Code N6686/N6789

Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington DC 20555-0001

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## Abstract

Testing was conducted at Oak Ridge National Laboratory (ORNL) to determine the ability of high burnup (HBU) (>45 GWd/MTU) spent nuclear fuel (SNF) to maintain its integrity under conditions relevant to storage and transport. The cyclic integrated reversible-bending fatigue tester (CIRFT) is an innovative system used in these tests. The CIRFT can impose pure bending loads on SNF test specimens and can measure the in situ curvature of the fuel rod during bending. *Pure bending* is a condition of stress in which a bending moment is applied to a beam without the simultaneous presence of axial, shear, or torsional forces. The CIRFT is composed of a U-frame equipped with load cells. A setup with three linear variable differential transformers (LVDTs) was used.

Since the initial publication of this NUREG in May 2015, several new factors were identified that influence the recorded curvature measurement data. Because these factors were not accounted for in the first publication, the initial quantitative results are not considered valid. This revision provides updated results of the 2015 CIRFT tests.

After the 2015 publication, a new phase of testing was implemented that used samples treated to induce radial hydrides. This revision includes the results of CIRFT tests on segments subjected to hydride reorientation treatment (HRT), thus broadening the applicability of the research results. This first revision entirely replaces the initial May 2015 publication.

The HBU SNF rods with Zircaloy-4 (Zry-4) cladding were studied under static and dynamic (cyclic) bending. They survived static unidirectional bending to a maximum curvature of 2.2–2.5 m-1, or a maximum moment of 85–87 N∙m. The maximum longitudinal cladding strain before failure or before reaching CIRFT displacement capacity was 1.2–1.3%. The segment composite structure of an HBU rod introduces numerous stress riser sites into the rod system, often resulting in specimen fracture at the pellet-to-pellet interface regions under both static and dynamic CIRFT testing. Nevertheless, the static CIRFT test results on HBU rods indicate that fueled rods have a significantly greater flexural rigidity than the calculated flexural rigidity for HBU cladding alone.

During dynamic testing, the fatigue life of HBU rods mainly depended on the level of loading. Under loading with moments of ±5.08 to ±35.56 N∙m, which resulted in an equivalent strain (ε) of ±0.025 to ±0.31% strain at 5 Hz, the fatigue life (N) ranged from 5.5 × 103 to 2.3 × 106 cycles. Considering the complexity and nonuniformity of the HBU fuel cladding system, it was significant to find that the ε-N data for the HBU were characterized by a curve that would be expected for standard uniform materials. The ε-N curve of the HBU rods can be described by a power function of y = 3.839 ∙ x-0.298, where *x* is the number of cycles to failure, and *y* is the strain amplitude (%). It was also significant to find that, if an endurance limit is defined by survival of >107 cycles (as is typical for material fatigue endurance limits), then the HBU rods tested exhibited an endurance limit. The endurance limit for the HBU rods tested is likely between strains 0.03–0.05%.

The maxima of the imposed curvature κ during dynamic testing ranged from ±0.07 to ±0.62 m-1 at 5 Hz. The κ-N curve of the HBU rods can be described by a power function of y = 6.864 ∙ x-0.283, where x is the number of cycles to failure and y is the maxima of clad tensile curvature |κ|max (m-1). An endurance limit is likely between 0.07–0.12 m-1 when it is defined at 107 cycles.

From very limited CIRFT test results of rods that underwent HRT, the following phenomena were observed.
        - HRT rods in static testing showed that the maximum moment decreased in comparison to that of as-irradiated fuel rods at similar curvature levels.
        - Both the HRT static and dynamic flexural rigidity are less than that of as-irradiated baseline data.
        - The load amplitude vs. failure frequency trend indicated that HRT rods had slightly less fatigue life than those indicated in the as-irradiated baseline CIRFT data.

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