Abstract
Reducing the uncertainty is very important in making better decisions as to whether the measuring target object is radioactive waste or below the clearance levels. This could lead to a reduction of waste classified conservatively as radioactive waste.
One of the factors that overestimates radioactivity level is that the model of weight density distribution used as a conversion factor from measured radiation counts to radioactivity is different from the actual object. In order to solve this problem, this method virtually divides the object into some regions and sets the density distribution for each region.
The density distribution is set by measuring the amount of X-rays transmission in this method. The X-rays transmission amount depends on the thickness of the object in the region. The radioactivity conversion factor for the region is set by evaluating the radiation shielding effect based on the density distribution. The count rate of γ-rays is measured by γ-ray detectors for each region. And then, the activity concentration of the object is evaluated for each region by the weight, γ-rays count rate, density distribution, and radioactivity conversion factor.
The greatest merit of setting the density distribution is that there is no need to store the object in a uniform state to set the conversion factor, and pretreatment and storage work for measuring is greatly reduced.
A device equipped with a unit that sets the density distribution by measuring X-rays is being developed. This device is a box-type measuring one that measures and evaluates activity concentration by storing the object in a container.
More fundamental data is needed for setting the density distribution and the radioactivity conversion factor through experiments to improve the evaluation accuracy.
