Radiological Monitoring and Control During the Quehanna Decommissioning Project

K.M. Kasper (Scientech, LLC)

The Quehanna Hot Cell Decommissioning Project involved unique and extreme radiological conditions. Airborne concentrations could reach hundreds of DAC (derived airborne concentration). Removable surface contamination was quantified in terms of Rad per hour per 100 square centimeters by measuring a standard cloth smear with an open-window ion chamber. Beta dose rates from the primary contaminant, Sr-90, could be found at levels in the tens of thousands of Rad per hour. The radiological levels were so severe that a special robotic system had to be designed, engineered, and developed to safety dismantle the most dangerous components, which were in Cell 4. Other hot cells were outfitted with modern manipulator arms to consolidate and package thousands of curies of Co-60. Even though the most hazardous work at Quehanna was done without direct human contact, extensive monitoring was absolutely necessary to ensure that the dangers were confined. Positive confinement of the radiological hazards was achieved primarily through three layers of containments that provided multiple, defined and separated air spaces. This prevented the migration of airborne and or surface contamination. Positive control of airflow was also imperative. This was accomplished by using a configured high efficiency particulate air (HEPA) filtered air handling system. This system had two stages of air purification, a backup primary unit, and redundant power supply system. To verify airflow, continuous, alarming, air monitors were stationed at each of the three openings to the primary containment. Personnel dose monitoring and control was an ongoing challenge during the course of the project. Extreme dose rate gradients existed because of the high specific activity Sr-90 material. Multiple dosimetry requirements were established for nearly all of the hot cell work tasks. Internal dosimetry techniques were also necessary but were not straightforward.

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