Experimental Studies of U²³⁸ Resonance Neutron Capture in UO₂ Fuel Rods

The relative U²³⁸ resonance capture integrals of 0.387-in. diameter UO₂ and uranium metal fuel rods have been measured as well as the spatial distributions of the captures in each type of rod. The effective resonance integral of the UO₂ rod was found to be 1.30 ± 0.02 times that of the . uranium metal rod. This difference is due to the lower density of uranium atoms and the presence of oxygen moderation in the UO₂ fuel rod. The relative importance of each of these two effects was determined by means of U-Zr and U-Al alloy fuel rods. Of the 0.30 excess of the UO₂ resonance capture integral over that of the uranium metal, 0.15 ± 0.02 was contributed by the lower U²³⁸ atom density of the UO₂ rod, and the remaining 0.15 was attributed to oxygen moderation. Work done by Hellstrand (5) gives a value of 1.41 ± 0.03 for the U²³⁸ resonance capture integral for the UO₂ rod relative to that of the uranium metal rod. Furthermore, by evaluating his expression for the resonance integral of a metal rod at a value of S/M corresponding to a “low-density” uranium metal rod (U²³⁸ atom density equal to that of the UO₂ rod) of 0.387-in. diameter, one obtains a density effect of 1.32. These discrepancies can be attributed, at least partially, to a differing flux spectrum at energies above 30 kev in the present experiment as compared with Hellstrand's experiment. Whereas the high-energy flux is depleted in his experiment, there was an appreciable flux peak present in the TRX measurements.