This invention relates to a process for analyzing the contents of containers. More specifically, it relates to a process for a digital radiography tilt method for detecting free liquids in hazardous and radioactive waste containers.
Vast quantities of radioactive waste materials have been generated as a result of the production of nuclear energy and defense materials over the past five decades. It has been estimated that there are over 1.4 million drums of buried and stored, high and low level waste, in the United States. There are also hundreds of facilities requiring decontamination and decommissioning that will generate many more drums of radioactive waste. Guidelines have been imposed concerning the safe handling of this waste and on the acceptable forms of radioactive waste materials for storage and disposal.
In an effort to ensure that containment drums do not rust and leak or otherwise release radioactive liquids and vapors into the environment, current regulations for radioactive waste require that drums contain less than one percent free liquids. There are additional requirements on the integrity of the drums and their contents that require a minimum drum wall thickness, and that no pressurized containers or flammable materials such as powdered metals, etc., are allowed without special processing and handling.
To date, the most practical means to certify that waste drums meet waste acceptance criteria is by non-destructive evaluation (NDE) methods. Opening drums for inspection and certification is not practical from either a safety or a cost basis because it is labor intensive and it creates additional waste in the location where the inspection occurs. The prior art in NDE methods used up to now have had some success, but it is widely recognized that there is room for improvement and for many types of waste in drums (e.g., cement solidified waste, compacted sludge, and vitrified waste), the capabilities of existing equipment and methods will not work.
The current method used to determine the presence of free liquids in drums is real-time radiography (RTR) and the method used for measuring drum wall thickness is ultrasound.
RTR imaging depends on X-ray attenuation measurements. RTR utilizes screen scintillators and image intensifiers for its measurements. Because RTR produces an image of a portion of drum in fractions of a second, moving fluids are visualized as they slosh in their container while the drum is rotated. The average time required by RTR to inspect an entire drum is ten minutes. It has been reported that RTR can detect as little as one teaspoon of water in a drum.
Some major disadvantages of RTR are: (1) containers that are full of liquids are not detectable because the liquids do not slosh, (2) the limited dynamic range of screen scintillators and image intensifiers (typically less than 100 intensity levels) have such limited contrast resolution that it is not possible to resolve common materials (i.e., plastics, glass, and metals) in the same image, (3) quantitative measurements of densities, atomic numbers, free liquid volumes, drum wall thickness, etc., are not possible, and (4) RTR systems have only been equipped with 420 kV X-ray tubes which will not penetrate drums filled with cement, sludge, or other dense waste forms.
Further, for quantitative measurements of drum wall thickness, prior art use of ultrasound measurements depend on the travel time of reflected acoustic signals from an output source back to a detector. Typically, wall thicknesses around the drum circumference at eight heights is measured. This measurement process usually requires 30 minutes. The ultrasound measurements are reported to be accurate within 0.001 inch.
The disadvantages of ultrasound are: (1) running water on the drum surface is required for good contact with the acoustic sensors, (2) ultrasound does not work on rusty drum surfaces, and (3) excessive measurement time is required, as indicated above.
Thus, there is a need in the art for a non-destructive means for detecting free liquids in waste containers and identifying the characteristics of the contents that is fast, conclusive, and cost-effective. Further, there is a need for a more convenient way to archive and recall the results for analysis at a later date. It, therefore, is an objective of this invention to provide a process for analyzing the contents of containers, such as drums containing radioactive wastes and the like, that overcomes the disadvantages of RTR and ultrasound and that is faster and provides results that are more readily interpreted than the results of prior art methods.