Reaction Rates in Representative Plutonium-Recycle Fuel Lattices

Measurements have been made of relative nuclear-reaction rates within sub-critical water-moderated plutonia-urania fuel lattices, under conditions considered typical for plutonium recycle in central-station power reactors. Measurement conditions included water:fuel ratios of 3:1 and 2:1; temperatures of 70, 235, 330, 430, and 540°F; and three positions within the unit cell. Nuclear reaction rates measured included relative fission rates in ²³⁵U, ²³⁹Pu, and ²⁴¹Pu, as well as relative capture rates in ¹⁷⁶Lu (principal resonance at 0.143 eV), ¹⁶⁸Yb (0.597 eV), ¹⁹¹Ir (0.654 eV), ¹⁹³Ir (1.303 eV), ¹⁹⁷Au (4.906 eV), ¹³⁹La (73.5 eV), and ⁶³Cu (1/v detector). To facilitate comparison with predicted values, the experimental resonance absorption-rate ratios were normalized to ratios measured within a pure water spectrum. Experimental reaction-rate ratios were compared with values predicted using the THERMØS code in conjunction with a modified version of EPITHERMØS; and agreement varying from fair to good was observed. The internal consistency of the measurements suggests their future utility for evaluating methods of calculating neutron spectra and relative reaction rates within lattices of the type considered.