Studies of epithermal neutrons in uranium, heavy water lattices

Abstract

Measurements related to reactor physics parameters were made in three heavy water lattices. The three lattices studied consisted of 0.250-inch-diameter, 1.03 w/o U2 3 5 uranium fuel rods arranged in triangular arrays and spaced at 1.25, 1.75, and 2.50 inches. The following quantities were measured in each of the three lattices studied: the ratio of the average epicadmium U2 3 8 capture rate in the fuel rod to the average subcadmium U2 3 8 capture rate in the fuel rod ([sigma]28); the ratio of the average epicadmium U2 3 o fission rate in the fuel rod7 to the average subcadmium U 35 fission rate in the fuel rod (625); the ratio of the average U2 3 8 capture rate in the fuel rod to the average U2 3 5 fission rate in the fuel rod (C ); the ratio of the average U2 3 8 fission rate in the fuel rod to the average U2 3 5 fission rate in the fuel rod (628); and the effective resonance integral of U2 3 8 in a fuel rod (ER12 8 ).

The results of an investigation of systematic errors associated with these measurements have-led to many changes and adjustments in the experimental techniques and procedure which have improved the general precision of the experimental results. A new method was developed to measure the ratio C * which simplified the experiment, significantly reduced the experimental uncertainty associated with the measurement, and avoided systematic errors inherent in the method used to measure C* in earlier work. The value of ER12 8 was also measured by a new method in which the results of measurements made in an epithermal flux which had a 1/E energy dependence are combined with the results of measurements made in a lattice.

The experimental results were combined with theoretical results obtained from the computer programs THERMOS and GAM-I to determine the following reactor physics parameters for each of the three lattices studied: the resonance escape probability, p; the fast fission factor, E; the multiplication factor for an infinite system, k [infinity]; and the initial conversion ratio, C. Methods were developed to measure that portion of the activity of a foil which is due to neutron captures in the resonances in the activation cross section of the foil material. The resonance escape probability was determined by a new method, using the resonance activation date, in which the use of cadmium is not necessary.