Chest wall thickness measurements and the dosimetric implications for male workers in the uranium industry

The Human Monitoring Laboratory has measured the chest wall thickness and adipose mass fraction of a group of workers at three Canadian uranium refinery, conversion plant, and fuel fabrication sites using ultrasound. A site specific biometric equation has been developed for these workers, who seem to be somewhat larger than other workers reported in the literature. The average chest wall thickness of the seated persons measured at the uranium conversion plant and refinery was about 3.8 cm, and at the fuel fabrication facility was 3.4 cm. These values are not statistically different. Persons measured in a seated geometry had a thinner chest wall thickness than persons measured in a supine geometry - the decrease was in the range of 0.3 cm to 0.5 cm. It follows that a seated geometry will give a lower MDA (or decision level) than a supine geometry. Chest wall thickness is a very important modifier for lung counting efficiency and this data has been put into the perspective of the impending Canadian dose limits that will reduce the limit of occupationally exposed workers to essentially 20 mSv per year. Natural uranium must be measured based on the 235U emissions at these type of facilities. The refining and conversion process removes 234Th and the equilibrium is disturbed. This is unfortunate as the MDA values for this nuclide are approximately a factor of three lower than the values quoted below. The sensitivity of the germanium and phoswich based lung counting system has been compared. Achievable MDA's (30 minute counting time) with a four-phoswich-detector array lie in the range of 4.7 mg to 13.5 mg of natural uranium based on the 235U emissions over a range of chest wall thicknesses of 1.6 cm to 6.0 cm. The average achievable MDA is about 8.5 mg which can be reduced to about 6.2 mg by doubling the counting time. Similarly, MDA's (30 minute counting time) obtainable with a germanium lung counting system will lie in the range of 3 mg to 28 mg of natural uranium based on the 235U emissions over a range of chest wall thicknesses of 1.6 cm to 6.0 cm. The average achievable MDA will be about 7 mg which can be reduced to about 4 mg by doubling the counting time. Unfortunately, all of these MDA values are close to, or above, the dose limit. Neither system is sufficiently sensitive to detect an intake of Type S natural uranium if it occurred less than six months prior to the lung count. (author)