Superprompt-Critical Behavior of an Unmoderated, Unreflected Uranium—Molybdenum Alloy Reactor

The time-dependent behavior of the neutron population in an unreflected, unmoderated cylindrical assembly of 90 wt.% uranium (93.2 wt.% U²³⁵), 10 wt.% molybdenum alloy following a rapid establishment of superprompt critical conditions with negligible initial neutron population has been studied. Reactivity increases up to 11 cents above prompt critical resulted in bursts yielding as many as 1.8 × 10¹⁷ fissions and peak power up to 100,000 Mw with periods as short as 16 µsec and temperature increases as large as 400°C. For bursts greater than about 6 × 10¹⁶ fissions the safety block—a piece of the core held in place by an electromagnet—is driven out by pressure waves about 225 µsec after the peak of the burst.