Dynamic neutronic and mechanical measurements with a pulse reactor of uranium--molybdenum alloy

The time-dependent behavior of the neutron population in an unreflected, unmoderated cylindrical assembly of 90 wt% uranium (93.2 wt% 235U), 10 wt% molybdenum alloy following rapid establishment of a superprompt criticality with negligible initial neutron population has been studied. Reactivity increases up to 14¢ above prompt criticality resulted in pulses yielding as many as 3.72 × 1017 fissions with reactor periods as short as 12.4 µsec and temperature increases as large as 880°C. In these experiments the reactor produced pulses of 2 × 1017 fissions without any damage. A pulse of 2.37 × 1017 fissions resulted in permanent elongation of the bolts holding the core together, and a pulse of 2.66 × 1017 fissions caused cracks in some of the core parts. Stresses obtained from measurements of the mechanical vibration of the reactor parts were consistent with the observed damage.