A focused series of tests was needed to address the most serious problems. These tests were designated the MIDDLE NORTH Series. Again, early shots under this series were smaller and were meant to obtain data on basic blast and shock phenomena, target response, and to develop new nuclear simulation techniques. By 1968 the need for another big event was apparent so that good scaling relationships could be established between various sized yields and for verification and improvement of computer models and techniques. Operation PRAIRIE FLAT, a 500-Ton HE event was staged at the Defence Research Establishment Suffield (DRES)- an isolated, 1000 square mile area in Alberta, Canada consisting entirely of barren prairie.
A modified section of an underground communication building was constructed at the Defence Research Establishment, Suffield, and experiments were conducted to determine blast-valve performance, overpressures in the air shaft and down-stream plenums, and overpressure damage to air ducts. The air ducts were installed in order to give an indication of the expected damage to air-conditioning equipment which would be caused by overpressure pulses. The experiments were conducted in the PRAIRIE FLAT and DIAL PACK Events. Both experiments employed 500 tons of TNT to produce the blast effects.
In the PRAIRIE FLAT Event, the test structure was engulfed by a peak free-field blast overpressure of 34 psi. Two louver blast valves were mounted in the structure. One was closed directly by the blast forces and suffered no damage. That was not the case for the air ducts located in the downstream plenum. The other blast valve was properly controlled to close before blast arrival.
Eighteen microbarographs recorded airblast from PRAIRIE FLAT and three 1. 2-ton HE blasts. Sensors were spaced at 1- and 2-mite intervals at distances from 119 to 141 miles west of the bursts, to record ozonosphere ducted propagations, hopefully in a caustic. Results showed that there were humps in the amplitude-versus-distance curve which were observed to pass through the array and changed from shot to shot. These appear to be the caustic, which was broken up by atmospheric irregularities, and resulted in approximately double amplitudes over 6- to 8-mile bands. Amplitudes were proportional to the 0.425 power of apparent blast yield, which was close to the 0.40 value previously measured.
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