Ultra-low Background Physics in a Deep Geologic Nuclear Waste Repository
Particle astrophysics detectors located in deep underground laboratories require the most radioactivity-free environments humankind can create. The detector materials themselves must be extraordinarily pure and radiation from naturally occurring materials must be shielded using almost Herculean efforts. Listening for nature's secrets, whispered from across the universe, with mega-curies of nuclear waste nearby, might seem like a hopeless cause. However, the converse is actually true.
Personnel/Environmental Monitoring Keeps Background Levels Low
The need for personnel safety and environmental protection in a facility designed for permanent disposal of radioactive waste is paramount. Extensive and sensitive monitoring for contamination from the waste actually helps keep background levels very low. Airborne contamination monitoring in waste disposal areas is used to ensure that potential releases are detected and controlled. Action levels for monitoring waste components are set at levels comparable to naturally occurring backgrounds.
Appropriate separation between the waste disposal areas and the experiment gallery shields physics detectors from radioactive decay products in the waste. Distinct ventilation systems preclude airborne transport from waste disposal areas to experimental areas, and ensure that ultra-low backgrounds are maintained.
All waste containers emplaced in the underground are sealed. They are shipped to WIPP with no detectable external surface contamination from regulated waste management facilities around the DOE weapons complex. Detection levels are limited to the sensitivity of the handheld scanning equipment routinely used in health physics applications. Alpha scintillometers and beta/gamma instruments typically detect levels of a few disintegrations per minute per 100 square centimeter (dpm/100cm^2). Typical soil content of uranium and thorium in the parts per million range result in about the same activity levels. While the waste containers received at WIPP nominally display clean metallic surfaces, background levels of naturally occurring uranium and thorium in a soil smudge on the container would typically be detected and potentially removed before shipment. The WIPP action level to radiologically investigate the nature of surface activity detected during routine scanning for waste acceptance is 20 dpm/100cm^2.
In addition to surface contamination surveys, WIPP employs continuous air monitors (CAMs) both in the surface waste handling building and in various locations in the underground waste disposal rooms. These CAMs are configured to monitor for Pu-239 particulate levels. Because naturally occurring Radon and its daughters have similar alpha spectra as that of Pu-239, a special algorithm is employed to reject the Radon contribution to the airborne activity levels. To put the sensitivity of the CAM's into perspective, it should be noted that typical natural fluctuation of outdoor Radon levels sometimes alarms the CAMs (false positive indication of Pu-239 release). Due to the alpha spectrum overlap between Radon and Pu-239, the detection level for Pu-239 is a function of the naturally occurring Radon contribution. In other words, detection levels of Pu-239 (the primary component of the waste at WIPP) are well below the natural fluctuation of Radon drawn from the outside air.