A Gamma-Ray Scanner for Nuclear Spent-Fuel Cask Safeguards

Dr. A.J. Caffrey

Idaho National Laboratory

The increasing use of dry-storage casks for spent nuclear reactor fuel suggests the need for cask safeguards methods in addition to tamper-resistant tags and seals.  At present, if a cask seal fails, perhaps due to weather exposure, its contents cannot be verified without reopening the cask.

At Idaho National Laboratory, we are developing a gamma-ray scanner system to confirm the presence or absence of spent-fuel bundles in dry-storage cask slots.  The monitoring system uses a well-collimated Ge detector to view spent-fuel bundle slots from the cask’s flat end cap.  The cask’s filled and empty slots are distinguished by the peak-to-Compton background ratios of fission and activation product gamma rays.

We have conducted scanner test experiments on a Westinghouse MC-10 spent fuel cask partially filled with pressurized-water commercial reactor fuel bundles.  The peak-to-Compton background ratios for ⁶⁰Co gamma rays have proved to be a reliable indicator of full and empty fuel slots.  Gamma rays from ¹³⁷Cs and ¹⁵⁴Eu fission products also were used to analyze the data and interpret the results.   In addition, the technique may also provide a unique gamma-ray spectral fingerprint of individual spent-fuel bundles to monitor if tampering has occurred since the last cask inspection.