CRITICAL EXPERIMENTS WITH THORIA-URANIA FUEL IN HEAVY WATER

The nuclear characteristics of a variety of small reactors composed of thoria -urania fuel in heavy water were determined in a program of critical experimentation. The fuel element consisted of ceramic ThO/sub 2/-U/sup 235/ O/ sub 2/ pellets stacked to a height of 1.5 m within 0.787-cm-OD aluminum tubing. The pellets used most frequently were of 0.587-cm diameter and had a Th/U/sup 235/ atom ratio of 25. Rods containing similar pellets with only half as much U/sup 235/ were used to achieve small changes in the U/sup 235/ content of the cores. Some cores were assembled with 0.660-cm-diameter pellets having an atom ratio of 15. All cores were located in a 2-m-diameter tank containing D/sub 2/O. Three distinct core structures were used, allowing measurements with uniformly distributed fuel rods, loading patterns compatible with the EBWR core geometry, and clustered lattic arrangements with D/sub 2/O, H/sub 2/O, and air surrounding the clustered fuel. Most of the cores assembled had some amount of radial D/sub 2/O reflector. A 0.3-m-thick bottom reflector composed of D/sub 2/O and aluminum was always present. For most of the assemblies, the control rods were fully withdrawn, criticality being achieved by adjustment of the water level. Observed critical dimensions and the results of the conventional reactivity, foil activation, and fuel substitution experiments used to determine core parameters such as p, EPSILON , f, and T, are reported. lnformation on void and temperature coefficients and control rod worth is included. No comparison of the experimental results with theoretical predictiors is made nor are observations made on flux-trap systems of this composition included. (auth