Reactor Neutron-Pulse Propagation

An asymptotic velocity of (18.79 ± 0.32) × 10³ cm/sec is reported for the propagation of a TRIGA reactor neutron pulse through a 4 ft × 4 ft × 5.8 ft graphite thermal column. Interpretation in terms of a proposed theory yields Dv = (2.34 ± 0.02) × 10⁵cm²/sec for 1.60 g/cm³ graphite. The propagation velocity was found to be independent of reactivity inserted in the range covered by experiment (to 2.90 dollars). Also, the pulse shape was preserved with an amplitude attenuation length equal to that for the steady-state thermal flux. These results are attributed to the fact that the neutron lifetime in the column was shorter than the pulse width and period. Calculations are presented for the reverse situation where the pulse shape is distorted. Pulse measurements with a Boral curtain lowered into the column are used to test the theory for situations where the slowing-down density is important.