The Spectrum of the Angular Neutron Flux in a Heavy-Water Natural-Uranium Lattice

Using the fast chopper at the Stockholm reactor R1, we have measured angular neutron-flux spectra in an arrangement consisting of short uranium rods in heavy water. The rods were mounted in an aluminum container placed in the central vertical channel of the reactor. The axes of the rods were horizontal and approximately located in a vertical plane. The uranium was 29-mm diam, the aluminum can was 1-mm thick, and the spacing was about 140 mm. We measured (E,, in the vertical direction , for various positions . At 1 eV the results obtained were equal to each other within ±7%, but in the thermal region, they differed considerably. Thus, at 0.01 eV, the highest value measured was as much as 4.5 times the lowest one. In spite of the unclean geometry, we have also calculated the angular-flux spectra, starting from scalar-flux spectra obtained from the THERMOS code. The shape of the calculated spectra of the angular flux agreed within a few percent with the experimental spectra. From a combination of calculated and experimental results, we also obtained the spectrum of the incoming neutron current for a fuel rod. This current proved to have a neutron temperature about 7°C above the neutron temperature of the flux halfway between two rods. In the low-energy region (around 0.015 eV) the out-flux from a rod showed a wavy variation, believed to depend on Bragg effects in the metal.