A GAMMA RAY TRANSMISSION GAUGE FOR DETERMINATION OF HEAVY AND LIGHT METAL LOADING OF FUEL ELEMENTS*

Multi-line gamma sources and a gamma-ray spectrometer were used to determine heavy and light metal loading of fuel rods. Knowing the absorption coefficients for the chosen lines and some geometrical factors and measuring the transmission of an unknown sample and a standard sample, one can determine the mass of heavy and light elements in the former relative to the corresponding masses in the known standard which has the same geometrical features as the unknown sample. A prototype multiple gamma energy gauge has been built. This device has been optimized to the problem of determining the amount of Th loading in the HTGR fuel rods. Using a set of attenuation coefficients, which were known to an accuracy of about 2%, the absolute amount of the heavy elements (approximately 92%-95% Th and 5% to 8% U-238) was made in a 2-minute measurement to an average accuracy of 1. 5%. Higher accuracies, if desirable, can be obtained by using better known standards.