Radioactive Densometer Shielding Design To Meet New Regulatory Requirements
Authors: Buchanan R. J. 
Published: December 2007

Most of the services performed for the petroleum industry require accurately determining the density of the fluids being pumped. Experience has shown that one of the most reliable methods of measuring the density of fluids in field operations is by determining its radiation attenuation properties. This is achieved by the use of a device known as a radioactive densometer, which utilizes a radioisotope source on one side of the pipe and a radiation detector on the other. The density of the fluid is inversely proportional to the count rate of the detector and the unit can be calibrated to obtain a high degree of accuracy when the components are held in constant geometry as in an integral inline device. While inherently a safe device, the radioactive densometer must meet certain regulatory requirements. These requirements have been met in the United States and elsewhere by a shielding design that has proven very successful. However, recent regulatory changes in Europe have required the redesign of the radioactive densometer shielding to meet new exposure limits. This paper will address the exposure limits which are applicable to the radioactive densometer, the design constrains, the experimental methods used, and the resulting shielding design specifications.

Some of the European countries are requiring that density gauges have a maximum instantaneous dose rate of 0.75 mremh-1 (7.5 uSvh-1) if they are to be used in an area that is not controlled. It is not specifically stated in the regulations if this dose equivalent rate limit applies to a whole body dose equivalent rate, an extremity dose equivalent rate, the dose equivalent rate as measured by a survey instrument with the probe on the surface of the shielding, or the actual surface dose equivalent rate as measured by an infinitely small dose measuring device. To prevent the possibility of having to redesign the shield in the future, the new radioactive densometer shielding design will be based on the latter criterion, that of a maximum of 0.75 mremh-1 (7.5 uSvh-1) on the actual surface of the device. To provide for a common frame of reference in this article, the term "dose equivalent", will be used as the final measured quantity, after the appropriate conversion factors are applied, even though an instrument may initially read out in terms of exposure, dose, etc.

The new design must be such that it provides the necessary shielding, but does not overshield, causing excessive weight, size, and cost problems. Therefore, the desired and the most cost effective design is one that is based on the most accurate information available, as opposed to always being on the conservative side. However, to allow for contingencies, the values of the activity of the commonly used Cs-137 sources in the radioactive densometers were increased by 10% to account for source supplier variations in manufacturing. In addition, the dose rate on the surface of the radioactive densometer was limited to 0.5 mremh-1 (5 uSvh-1) so that if the overall experimental error in dose equivalent rate determination was 20% low and the european regulatory agency error in dose equivalent rate determination is 20% high, then their dose equivalent rate measurement will still not exceed 0.75 mremh-1 (7.5 uSvh-1) at the surface of the device.