Fast-Pulse Reactor Operation with Reflector Control and a 106-mm-diam Glory Hole

The Army Pulse Radiation Facility Reactor has been pulsed to 17 cents above prompt criticality using external reflector control. This is a novel method of fast-pulse reactor operation. The purpose of this work is to provide a high neutron fluence uniform over a 106-mm-diam, 198-mm-high in-core irradiation cavity or “glory hole,” in both pulse and steady-state modes of operation. The 106-mm-diam glory hole is obtained by removing from the standard core a cylindrical center fuel element, the “safety block,” and replacing it functionally by three 50.8-mm-thick, 305-mm-high scramable copper reflectors positioned 5.3 mm from the reactor shroud. The cost of this modification was favorable since fabrication of new fuel pieces was unnecessary. To date this assembly has been successfully pulsed to yields as high as 1.83 × 1017 fission/pulse. There is an ∼38% increase in prompt neutron lifetime in the reflected core due to the central cavity and the reflectors. The prompt negative shutdown coefficient is decreased only slightly so that the reflected core can be pulsed with requisite safety and satisfactory reproducibility. At the routine pulse level of 1.5 × 1017 (±2%) fissions, the pulse width is 66 µsec, the neutron fluence in the glory hole is 5.0 × 1014 n/cm2 (>10 keV), where the peak neutron flux is 6.4 × 1018 n/(cm2 sec) and the gamma-ray dose is 1.6 × 105 R. With a thermal-neutron flux trap, the peak thermal-neutron flux is 1 × 1018 n/(cm2 sec). With a neutron-to-gamma-ray converter, the peak gamma-ray emission rate is 3 × 109 R/sec. Operation at 10 kW in a steady-state mode produces a neutron flux (>10 keV) of 1012 n/(cm2 sec). Experiments have been performed previously to evaluate the use of reflectors as control and pulse rods. The present method of operation extends the use of reflectors to provide the principal mechanical shutdown mechanism in superprompt critical pulse operation.