7- 4]
THE HOMOGENEOUS REACTOR TEST (HRE-2)
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the presence of any leaking krypton . This test was inconclusive at the time, however, because of difficulties encountered in purifying the samples for counting . Large samples of the air being tested were stored in gas cylinders ; then the piping was vented and repressurized with helium . After an additional waiting period the sealed volumes were again checked using helium leak detectors sensitive to 1 ppm of helium in air . This test demonstrated that the piping leakage was less than 0 .2 cc/day of helium, with a pressure differential of 15 psi across the leak . These results were subsequently confirmed by krypton data after analytical difficulties were resolved . With the integrity of the reactor piping established, the seal pans were welded in place on the shield roof, and the roof plugs were locked in place . By pressurizing the shield and flooding the roof with water, leaks in the seal pan welds were located for subsequent repair . The major shield leak was found to be through a 16-in . valve in the ventilating duct ; when this was repaired, the total leakage fell from 25 cfm to approximately 1/2 efin . The shield was judged sufficiently tight at this point to proceed with the critical experiments, after which repairs were continued . By painstaking individual checking of all shield penetrations and the use of thermosetting resin to seal the metal lips to which the roof seal pans are welded, the leakage was finally reduced to 4 to 4 .5 liters/ min at 15 prig . 7-4 .5 Nuclear operation [21] . Fuel charging began on December 24, and criticality was achieved on December 27, 1957, with the core and blanket near room temperature and at a pressure of about 800 psig . Nuclear instrumentation for the test consisted of three fission chambers, Viz . . two permanent chambers in the instrument tube outside the reactor pressure vessel and a temporary chamber inside the blanket vessel . An antimony-beryllium neutron source was suspended in the thimble in the center of the core (see Fig . 7-18) . The reactor was brought gradually to the critical condition by the injection of enriched uranium into the fuel solution added to the dump tanks in batches of 100 to 400 g . Fuel feed pumps and purge pumps were operated continuously to provide mixing between the dump tanks and the highpressure system . Following each addition the solution concentration in the high-pressure system was allowed to reach steady state, as indicated by a leveling-off of fission count rates . After 2060 g of U235 had been added and the temperature of the solution lowered to 29 째 C, the neutron source was withdrawn . At this point, the fission count rates continued to rise, indicating that criticality had been achieved . Raising the temperature of the reactor slightly by pumping warm water through the heat exchangers stopped the nuclear chain reaction . By further varying the temperature and concentration of the fuel solution, it was demonstrated that the nuclear reaction could be easily and safely controlled in this manner .