Measurements of Delayed Fission Product Gamma-Ray Transmission Through Low-Enriched UO₂ Fuel Pin Lattices in Air

Measurements of delayed fission product gamma-ray transmission through low-enriched UO₂ fuel pin lattices in an air medium were conducted at the Rensselaer Polytechnic Institute Reactor Critical Facility (RCF). The RCF core consists of excess Special Power Excursion Reactor Test fuel pins enriched to 4.81 wt% ²³⁵U and clad in stainless steel. An experimental apparatus was constructed to hold various arrangements of fuel pin lattices. The arrangements consisted of a single activated source pin taken from the reactor core surrounded by inactive fuel pins in an air medium. A sodium-iodide detector and gamma-ray spectroscopy system was used to generate a pulse-height spectrum of the gamma-ray radiation for detector positions outside the lattice. The change in radiation intensity as the detector is rotated about the vertical axis of the lattice, the "channeling effect," was measured. Measurements of the channeling effect were performed for six experimental arrangements: 3 × 3, 5 × 5, and 7 × 7 lattices, with both the corner and the center positions containing the activated pin. The results of the measurements demonstrate that the gamma-ray radiation intensity can vary widely as a function of angle relative to the angle of rotation about the vertical axis of the lattice.