SAFETY AND THE TRINITY TEST
Trinity Test Site (July 1945)
Events: Dawn of the Atomic Era, 1945

The "Trinity" atomic test was the most violent man–made explosion in history to that date.  It also posed the single most significant safety hazard of the entire Manhattan Project.  Understanding this, test planners chose a flat, desert scrub region in the northwest corner of the isolated Alamogordo Bombing Range in south central New Mexico for the test.  This location, 210 miles south of Los Alamos, was only twenty miles from the nearest offsite habitation.  If the explosion was considerably larger than predicted, the dangers could be extreme to the test personnel and surrounding areas. 

During the test, scientists, workers, and other observers were withdrawn almost six miles and sheltered behind barricades.  Leslie Groves and Robert Oppenheimer watched the test from two different sites so that if one was killed in an accident the other might survive to direct continued efforts.  Los Alamos scientists had even discussed the possibility that the atmosphere itself might be ignited and the entire earth annihilated but dismissed this as an unlikely possibility.  Dangers from blast, fragments, heat, and light, once one was sufficiently removed from ground zero, evoked little concern.  The real concern, barring a catastrophic underestimation of the size of the blast, was with radiation.  

Prior to Trinity, scientists were well aware that the blast would create potential radiation hazards.  After all, even basic laboratory or factory work created significant radiation safety issues.  In the case of an explosion, plutonium in the device would fission into other radionuclides.  Neutrons would strike various elements on the ground and turn some into radioactive isotopes.  This radioactive debris would be swept with fission products into a growing fireball and lifted high into the air.  Once in the atmosphere, a cloud of intense radioactivity would form.  Immediate radiation from the explosion and residual radioactive debris initially caused little concern because of dilution in the air and the isolation of the site, but as the test drew closer planners realized, with some sense of urgency, that radioactive fallout over local towns posed a real hazard.  Groves, in particular, feared legal culpability if things got out of hand.  As a result, Army intelligence agents located and mapped everyone within a forty–mile radius.  Test planners set up an elaborate offsite monitoring system and prepared evacuation plans if exposure levels became too high.  

The test was more efficient than expected, and little fallout initially dropped on the test site beyond 1,200 yards of ground zero.  Most radioactivity was contained within the dense white mushroom cloud that topped out at 25,000 feet.  Within an hour, the cloud had largely dispersed toward the north-northeast, all the while dropping a trail of fission products.  Offsite fallout was heavy.  Several ranch families, missed by the Army survey, received significant exposures in the two weeks following Trinity.  The families, nonetheless, evidenced little external injury.  Livestock were not as fortunate, suffering skin burns, bleeding, and loss of hair.  Stafford Warren (below), the Manhattan District's chief medical officer, reported to Groves that "while no house area investigated received a dangerous amount, the dust outfall from the various portions of the cloud was potentially a very dangerous hazard over a band almost 30 miles wide extending almost 90 miles northeast of the site."  The Alamogordo site, Warren concluded, was "too small for a repetition of a similar test of this magnitude except under very special conditions."  For any future test, he proposed finding a larger site, "preferably with a radius of at least 150 miles without population."  The Trinity test had been, as Warren informed Groves, something of a near thing.