Gamma Scan Confirmation of Lead Pour in a Type B Cask

Packaging Technology, Inc., a subsidiary of the French nuclear consortium Areva, has been tasked with manufacturing six RH-72B (72B) casks for the Department of Energy’s Carlsbad Operations Office. The 72B transportation cask will be used to transport remote-handled (RH) transuranic wastes to the Waste Isolation Pilot Plant (WIPP) located in New Mexico. Certification of each 72B cask includes a gamma scan of the cask lead shielded wall to verify that no significant voids form within the lead subsequent to the lead pour. Voids in the lead would be revealed as spikes in the gamma scan measurements. The radioactive isotope Iridium-192 was used as the source for the gamma scan measurements. To determine the maximum and minimum expected values for the cask gamma scan, a test fixture was required to be developed with flat plate shields that matched the maximum and minimum thicknesses of the steel-lead-steel cask wall. Design of the test fixture was a non-trivial exercise due to the influence of backscatter radiation, which if unshielded resulted in unreasonably high test fixture radiation doses. To properly shield the backscatter radiation, a collimator is required around the source. The measured dose rates using the test fixture is highly sensitive to the diameter of the collimator penetration, as a collimator penetration diameter that is too narrow results in artificially low dose rate measurements when compared to the cask measurements. To assist in the design of the collimator, the Monte Carlo N-Particle (MCNP) gamma transport code was employed. Using MCNP computer simulations, it was determined that a collimator diameter of 6 inches was sufficient to properly mimic the cask configuration.