Sister Rod Nondestructive Examination Annual Report FY2017

This report documents work performed under the Spent Fuel and Waste Disposition for the US Department of Energy (DOE) Office of Nuclear Energy (NE) Spent Fuel and Waste Science and Technology program. This work was performed to fulfill the Level 2 Milestone M2SF-17OR010201021, “Documentation of Non-destructive Tests on Sister Pins,” within work package SF-17OR01020102.

The High Burnup Spent Fuel Data Project, sponsored by DOE-NE, is focused on understanding the effects of long-term storage and transportation on high burnup (HBU) (>45 gigawatt days per metric ton uranium) light water reactor fuel. The goals of the project are to “provide confirmatory data for model validation and potential improvement, provide input to future spent nuclear fuel (SNF) dry storage cask design, support license renewals and new licenses for Independent Spent Fuel Storage Installations (ISFSIs), and support transportation licensing for high burnup SNF” [1]. In support of project goals, 25 sister rods were removed from fuel assemblies at the North Anna Power Station. Nine rods were removed from the project assemblies, and 16 rods were removed from similar HBU assemblies. The 25 sister rods were shipped to Oak Ridge National Laboratory (ORNL) Irradiated Fuels Examination Laboratory (IFEL) in early 2016 for detailed nondestructive examination (NDE) and destructive examination (DE). The detailed examinations will provide essential information on the physical state of the HBU rods and the fuel contained in the rods prior to loading, drying, and long-term dry storage [1]. Similar tests will be performed at the end of the long-term storage period to identify any changes in the fuel rods’ properties during the dry storage period [1].

While the planned NDE tasks are delineated in the sister rod test plan [2], this report provides an annual status of the work completed in FY2017. The NDE scope includes visual examinations of the rods’ external surfaces and gross dimensional measurements. In addition to the sister rod program’s NDE scope, two additional radiation scanning projects performed for the National Nuclear Security Administration (NNSA) and completed using the sister rods are also summarized. Also, a specialized eddy current examination for measurement of cladding hydrogen content was performed on 19 of the 25 sister rods by the Electric Power Research Institute (EPRI), and a summary of that exam is provided.

Visual examinations of all 25 sister rods were completed in May 2017. The images are available in three user-viewable formats:
(1) 784 individual, unprocessed photos per sister rod, for a total of 19,600 photos (the source data for the other two image sets),
(2) 25 compiled user-interactive Shockwave Flash (SWF) collage files (one per sister rod), each containing 784 individual photos per sister rod, with each labeled for searching and observation, and
(3) 96 flattened 40 mm axial segment collages per sister rod (2,400 total images) obtained by filtering and stitching together the individual azimuthal photos.

The images are only available in digital format and are stored on ORNL’s CURIE resource [3] . Waterside surface features relevant to the specification of future NDE and DE were identified via visual examination and tabulated. No weld failures, obvious cladding breaches, or other significant defects were observed. Rod identification and bar codes are visible on all rods except for F35P17 and F35K13 since bar codes were not used when these two rods were fabricated. A typical striated surface texture appearance which is visible in the images as alternating axial light and dark bands is present to varying degrees in all cases, with the more heavily oxidized rods having the appearance of deeper striations. The visual inspection indicated that shallow grid-to-rod fretting (GTRF) marks are common, and a few rods have deep GTRF marks. Several rods appear to have patches of Chalk River Unidentified Deposits (CRUD) and/or spalling oxide. Peeling oxide was identified on M5 and ZIRLO sister rods. Rod insertion (pre-irradiation) and rod removal from the parent fuel assembly produced long axial scratches on most rods. Interactions with grid springs and dimples may have scraped off CRUD or oxides along the length of the rods during assembly removal at the orthogonal grid spring locations.

Three sets of integrated radiation measurements were completed, including (1) one-dimensional scans using the Advanced Diagnostics and Evaluation Platform (ADEPT) sodium iodide (NaI) detector, (2) high resolution gamma spectroscopy using a high-purity germanium (HPGe) detector, and (3) gamma and neutron measurements using a fission/ionization chamber-based fork detector. The HPGe and fork detector measurements were performed for the DOE NNSA Office of Nonproliferation and Arms Control (NPAC) and are summarized here for completeness. There is very good agreement in the general trends and locations of grid burnup depressions. Differences of up 7% were observed between the fork gamma counts and the HPGe total gamma counts near the rod bottom, likely due to the fact that a collimator is not used in the fork measurement. There is good agreement between all three detectors in the higher burnup regions of the rod. Radiochemical assay of selected rod locations will provide additional definitive (within measurement uncertainty) burnup measurements. The one-dimensional gamma scans of all 25 sister rods performed specifically for the sister rod project were completed using ADEPT in 2 energy ranges: 400 to 800 keV for examination of the fuel stack, and 1,100 to 1,600 keV for examination of the structural components. The two energy ranges were collected simultaneously. Data were collected in 1 mm increments along the axis of the rod and were indexed to the bottom of the rod. The scan signal exhibited the expected behavior without any sign of fission product accumulation or migration. The axial profile of the rods was as expected, and depressions in burnup were easily observed at the spacer grid locations. Pellet-pellet interface locations were also observable, so an estimate of the number of pellets in each rod could be made. The spring in the plenum region was visible, so an estimate of the length of the plenum region was also made. Some small fuel stack gaps were observed via the gamma scanning. The largest was estimated as 5 mm on sister rod 6U3P16.

The overall length of each sister rod was measured multiple times and was within the expected range.
Profilometry measurements were taken using two pairs of linear variable differential transformers (LVDTs) to measure the fuel rod diameter as a function of axial location. The two sets of measurements are 90° apart and can provide information on the extent to which the rod is out of round (ovality). Overall, within the accuracy of the device and given the actual surface condition of the rods, no significant ovality (0.04 mm) was noted in the rods. In general, the expected diametrical trends were observed. Some rods had very thick oxide with large spalled areas (see Section 3.1). Rods with the thickest oxide layers appear to have some locally erratic diameter measurements. In particular, assemblies 3A1 and F35 show the expected increasing diameter from the bottom to the top of the rod (referenced to the true bottom for the F35 rods), but there is a large amount of point-to-point variation in the higher rod elevations due to the uneven spalling CRUD/oxide layer. A large (~¾ pellet length) reduced diameter (~0.5 mm) region was noted in the profilometry scan for rod 6U3P16 that is associated with the ~5 mm pellet-pellet gap identified during gamma scanning. Bambooing, which is a small diameter variation with a period of about 10 mm (the pellet length), was observed in all rods.

All sister rods were extensively photographed during the visual examination. The visuals camera was also used to photograph the profilometry calibration rod, and the information obtained was used to resolve boundary pixel criteria and to scale the pixel count in each sister rod visual to a rod diameter. The diameter was measured in 40 mm increments along the length of the rod at 0, 45, 90, 135, 180, 225, 270, and 315-degree rotations. The diameter measurements based on the visuals were compared with the LVDT measurements and the features identified by gamma scan. Although these measurements are not considered to be as accurate as the LVDT measurements in total, they are useful in verifying trends, measuring the ends of the rods that cannot be measured with the LVDTs, and for additional observations around the circumference of the rod. Overall, this method gives reasonable agreement with the LVDT data, and the overall accuracy appears to better than 0.05 mm. Optimization of the optical path and camera optics, along with a fixed reference point for picture-by-picture calibration, could lead to a non-contact fast measurement system.

The sister rod project eddy current measurements and rod surface temperature measurements have not yet been completed. They are scheduled for the September 2017 timeframe. EPRI performed Frequency-Scanning Eddy Current Technique (F-SECT) measurements on the sister rods in the IFEL hot cell. The F-SECT was developed specifically by EPRI to nondestructively estimate cladding hydrogen content. The F-SECT measurement includes point measurements of the combined zirconium oxide and CRUD thickness. It has been tested in both hot cell and poolside environments on zirconium alloys, including channel box, water rod, spacer, and fuel cladding. A total of 19 sister rods were successfully measured over 3 consecutive days. Rods included ZIRLO, M5, Zirc-4, and LT Zirc-4 clad rods. The detailed results will be reported by EPRI and are summarized in this document for information only. To date, only oxide/CRUD thickness data are available; the estimated hydrogen concentration data are expected to be available in November 2017.