Improved Long Pulse Ionization Simulation Using Pulsed Reactors

Recently developed methods to improve the usefulness of pulsed nuclear reactors as sources for simulation of wide pulse ionization environments, generated by weapon produced neutrons, are described. These methods include the use of a special shield material which performs the dual role of attenuating neutrons and enhancing gamma dose rate, tailoring of pulse shapes by modifying the reactor burst characteristics through selective coupling between the reactor core and special reflector configurations, and use of special shield configurations to improve dose uniformity. Data obtained at the White Sands Missile Range Fast Burst Reactor and the Sandia Corporation SPR II for several shield configurations and reactor operating conditions are presented. Gamma radiation dose rates and 1 MeV damage equivalent neutron fluences obtained with and without use of the special shield material are compared. Dose rate enhancements of 5.2 and 3.3 times free fields were achieved behind shield thicknesses of 5 and 12 in., respectively. The respective neutron fluences were reduced by factors of 5.5 and 62. A peak dose rate enhancement of 12.4 times free field was achieved between slabs of shield material. Peak dose rates of 5 × 107, 1 × 107, and 1.2 × 106 rads(Si)/sec were achieved behind the shield material for pulse widths of 52 μsec, 100 μsec, and 1 msec, respectively, while limiting the neutron fluence to approximately 1 × 1011 n/cm2.