The filming of many scenes will be long remembered in Hollywood. For instance, the reenactment of the "brickpile" test underneath Stagg Field at the University of Chicago, an event which one atomic expert, Dr. Henry T. Wensel, prophesied would "2000 years from now, when all our present sources of energy are exhausted. become recognized as possibly the most important date in all history."   Keen interest also was displayed by workers and actors alike in scenes taking place in such famed atomic laboratories as those at Columbia University. the University of Chicago and Los Alamos. for these. actual atomic equipment valued at more than $100,000 was rented by the studio, and additional thousands spent in duplicating instruments that could not otherwise be obtained. Included in the rented equipment were various types of Geiger-Muller counters. for detecting radioactivity, and a cathode ray oscillograph for measuring energy and indicating fission.   The greatest excitement on the set came with the filming of the New Mexico bomb test at Alamogordo. and the Hiroshima raid. To depict the blinding flash of the atom bomb, greater than mid-day light, it is estimated that over two-and-a-half million watts of light were used for each scene, the equivalent of more than 42,000 average house bulbs. More than a million watts powered a battery of huge skylights surrounding the set with a million-and-a-half watts coming from 100 spot lamps. An intricate system demanding ten miles of cable connected the set with the studio's main generator, bringing a simultaneous blinding flash from all the lamps and throwing a maximum load on the generator..   For these and all other scenes showing the atom bomb, iall persons not directly connected with the film were barred from the stage. The one exception was Dr. Oppenheimer, who visited Hollywood at the precise moment when Actor Hume Cronyn was depicting him in one of the most dramatic events of the New Mexico test. The young, thinnish physicist, in distinct contrast to the popular conception of bearded. dignifieJ scientific genius, saluted his screen prototype by doffing his hat and tossing it to Cronyn, shouting as he did so, "Hello, Oppy!"   "It's equal to the destructive power of 20,000 tons of TNT!"