EVALUATING UNCERTAINTIES IN CELLULOSE, PLASTICS, AND RUBBER INVENTORIES IN THE WASTE ISOLATION PILOT PLANT

The Waste Isolation Pilot Plant (WIPP) is located in southeastern New Mexico and operated by the U.S. Department of Energy (DOE) as a disposal facility for transuranic (TRU) waste. The WIPP must comply with various environmental regulations, including 40 CFR 191, Subpart B, which describes waste containment requirements for the WIPP. The DOE conducts a probabilistic risk analysis, termed performance assessment (PA), of releases of radionuclides to the accessible environment during the 10,000-year regulatory period to demonstrate the WIPP’s compliance with containment requirements. The microbial gas generation model is a fundamental component of the WIPP PA. In this model, microbes consume cellulose, plastic, and rubber (CPR) materials and consequently produce carbon dioxide (CO2). The DOE emplaces magnesium oxide (MgO) in the repository along with the waste to mitigate the adverse effects that the CO2 has on repository performance. The DOE estimates the mass of CPR materials in a waste container by two different methodologies: Real Time Radiography (RTR) and Visual Examination (VE). Radiography is a nondestructive qualitative and quantitative technique that involves x-ray scanning of waste containers to identify and verify waste container contents. RTR estimates are used as the basis for determining the amount of MgO to be emplaced in a waste disposal room. Visual examination consists of either observing the filling of waste containers or opening full containers and physically examining their contents. VE is presumed to provide a much better estimate of CPR quantities than does RTR, but VE is not performed routinely. Accurate (i.e., unbiased) and precise room-scale estimates of CPR are important to the long-term performance of the repository. The purpose of this study is to evaluate the bias and uncertainty present when RTR is used to estimate CPR mass. To evaluate bias in RTR measurements we assume that VE estimates represent the true masses of CPR materials. RTR and VE estimates are compared on a container-by-container basis using a Student’s-t test. We conclude that the RTR estimates were unbiased. To evaluate precision in RTR measurements, we propagate the uncertainty in CPR quantities for a single container to an entire waste room via analytical methods and a Monte Carlo simulation. We determine that the relative uncertainty in CPR quantities in an individual room is expected to be less than 0.3 %.