Speaker
Description
Selenium-75 (75Se) is one of dozens of radioisotopes produced at Oak Ridge National Laboratory (ORNL) to support the US Department of Energy’s Office of Isotope R&D and Production. The isotope is produced by irradiating enriched 74Se target material in the High Flux Isotope Reactor (HFIR). 75Se has a half-life of 120 days and is used as an industrial gamma radiography source in remote locations where standard x-ray generators cannot go due to size, weight, and power limitations [1].
Before being loaded into HFIR irradiation capsules, the 74Se subcapsules undergo x-ray fluorescence spectroscopy (XRF) to identify any Se contamination on the subcapsule surfaces. Any subcapsules identified with Se surface contamination are excluded from the assembly. Recently, two 75Se contamination events occurred in the hot cell during routine processing following HFIR irradiation cycles 510 (November 2024) and 511 (March 2025) in which evidence of surface contamination via XRF analysis was not identified prior to irradiation. In both events, elevated 75Se contamination levels were identified via smears taken by radiation control technicians on the subcapsules and various hot cell surfaces. No personnel were contaminated with 75Se during these events; however, the additional efforts that were required to decontaminate the hot cell and identify the leaking Se subcapsule added significant unforeseen time, cost, and complexity to the standard processing schedule. An effort began to conduct a thorough review of the XRF testing process and evaluate other nondestructive examination (NDE) methods to improve the effectiveness in identifying leaking Se subcapsules prior to irradiation.
Four additional NDE/spectroscopy techniques were selected to investigate further: helium (He) bombing/mass spectrometer leak testing (MSLT), bubble leak testing (BLT), x-ray computed tomography (XCT), and laser-induced breakdown spectroscopy (LIBS). Several Se subcapsules that failed MSLT and/or BLT were analyzed via XCT. Figs. 1 and 2 show a 3D rendition and cross-sectional view of the weld surface of one Se subcapsule, respectively. Although obvious leaks were not identified, XCT confirmed significant porosity in these welds that could lead to the release of 75Se during the harsh HFIR irradiation environment.
This investigation determined that XRF is valuable for identifying Se surface contamination on the subcapsules, but additional NDE testing should be performed to evaluate the weld integrity prior to irradiation. Given the small size and large quantity of the Se subcapsules, He bombing/MSLT and BLT are readily available methods at ORNL and appropriate complement XRF to cover a greater range of possible leak rates, thereby reducing the likelihood of future Se hot cell contamination events.
[1] QSA Global Inc., Selenium-75 Sealed Sources for Gamma Radiography (2026).