Trinity - Completion of the wartime mission

The "gadget"in the Trinity Test Site tower awaiting detonation. Norris Bradbury is on the left. The man on the right is Boyce McDaniel.

The Los Alamos Laboratory was organized in 1943 to design a nuclear weapon that the Army hoped would win World War II. In the course of the next two years, the Laboratory designed a weapon using uranium-235 assembled by firing one part of a critical mass into another, but this technique was found to be inadequate for plutonium, because isotopic impurities of plutonium-240 would cause it to predetonate.

In the second year of its existence, therefore, the Laboratory was reorganized to solve the much more difficult problems of implosion - the uniform compression of plutonium to a super- critical mass that had been proposed by Seth Neddermeyer of the California Institute of Technology, John von Neumann of the Institute for Advanced Study at Princeton and others.

Because of the uncertainties attending almost every phase of the implosion weapon, it was decided almost at the beginning of the effort that the implosion bomb would have to be tested. After various test sites were considered, a location in the Jornado del Muerto desert in central New Mexico was selected. Harvard physicist Kenneth Bainbridge planned and University of Minnesota physicist John Williams supervised the construction of the facilities to support a test there.

Los Alamos Director J. Robert Oppenheimer named the site "Trinity" after a poem by John Donne that he had been reading. To capture the plutonium that might be lost if the bomb fizzled, Manhattan Engineer District Commander Leslie Groves ordered a container, called "Jumbo," to be built at a cost of more than $12 million. Jumbo was the largest item that had ever been shipped by rail, and several trestles on the railroads from the factory that built it in Ohio to the Trinity site had to be rebuilt.

By the time Jumbo arrived, the production of plutonium at the Hanford Engineer Works had increased so that Groves was less chary of it, and Oppenheimer and his colleagues believed that there was less chance of a fizzle. Consequently, the container was relegated to the sidelines and hung not far from Ground Zero to serve as an indicator of the power of the bomb. It emerged unscathed although the tower was destroyed.

The construction of the Trinity site was rapidly accomplished in the winter and spring of 1945, and by June, Bainbridge was ready to calibrate the instruments that would be used to measure the blast, heat and radiation of the "gadget" using a 100-ton stack of high explosives tagged with fission products from the Hanford pile. The 100-ton test was the largest man-made explosion up to that time and made it possible for the Los Alamos scientists to refine their instruments before the much larger blast anticipated from the gadget.

The design of the gadget had been fixed in February 1945 when Groves ordered a design freeze so that the device could be ready by July. A conservative solid-core design by Robert Christy, a member of the Theoretical Physics (T) Division, the gadget required the development of detonators, fuses and high-explosive lenses that were not yet perfected. Given a clear goal, however, Los Alamos scientists and technicians succeeded in producing all of the components of the device successfully by July 13.

On that day, assembly of the gadget began at Trinity. A crew led by Norris Bradbury, a professor of physics at Stanford University who had come to Los Alamos by way of the Naval Reserve and Dahlgren Proving Grounds, assembled the high-explosive lenses that had been brought from V-site at Los Alamos the day before escorted by Harvard professor George Kistiakowsky, who had led the high-explosives effort at the Laboratory since November 1943. Bradbury, Kistiakowsky and five G (gadget) engineers' began their work at 1 p.m. After the tamper and the active material were inserted into the spherical case, the final high-explosives were inserted, "as slowly as the G-engineers wished," said Kistiakowsky.

Saturday, July 14, 1945, the assembled gadget was hoisted to the top of the 100-foot tower on which it would be detonated. The firing unit was wired by late afternoon. Bradbury's schedule for Sunday, July 15, called for the staff to "look for rabbit's feet and four-leaved clovers." The detonation was scheduled for 4 a.m., Monday, July 16.

Meantime, Los Alamos scientists had conducted a test of the implosion assembly at Los Alamos that seemed to indicate that it would not work.

Kistiakowsky was roundly criticized by Groves and Oppenheimer. His peacetime boss, James Bryant Conant of Harvard University, who was the scientific head of the atomic bomb effort, subjected him to a two-hour interrogation as to the causes of the failure of that effort. Kistiakowsky, however, was sure that the assembly would work, and Theoretical Division Leader Hans Bethe got him off the hook when he reported that calculations showed that the detectors used for the Los Alamos test could not have distinguished between success and failure.

As the test approached, the weather worsened, as the meteorologist assigned to predict it had warned. A thunderstorm broke over the site late on July 15, and the test was postponed from 4 a.m. to 5:30 a.m. to avoid the possibility of a rain-out of fission products from the bomb cloud. In nearby settlements, members of the health physics team were ready to evacuate the population should the test greatly exceed expected yields. Although most scientists believed that the yields would be low, Edward Teller, group leader of the Super and General Theory Group (F-1) in F (Fermi) Division, bet that it might exceed 40 kilotons, and Enrico Fermi, head of F Division, was heard taking side-bets that the bomb would incinerate New Mexico. Groves called the governor of New Mexico to alert him that an evacuation of the state might be required.

Oppenheimer was in a state of high tension during the early morning hours, but, as predicted, the weather cleared and the countdown for the test was begun at 5:10 a.m. "As we approached the final minute," Groves recalled, "the quiet grew more intense. I was on the ground (at Base Camp) between [ Vannevar] Bush [chairman of the Office of Scientific Research and Development] and Conant. As I lay there in the final seconds, I thought only what I would do if the countdown got to zero and nothing happened." At the control point, Joe McKibben, who had been with Project Y since the beginning, threw the switch that started the precise automatic timer at minus 45 seconds. Only Donald Hornig, a physical chemist from Harvard University, on the arming party, could stop the explosion.

At 5:29:45 a.m., the gadget exploded with a force of 21,000 tons of TNT, evaporating the tower on which it stood. Groves' deputy, Gen. Thomas Farrell, wrote that the "whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined. Seconds after the explosion came first the air blast pressing hard against the people, to be followed almost immediately by the strong, sustained awesome roar that warned of doomsday and made us feel we puny things were blasphemous to dare tamper with the forces heretofore reserved for the Almighty." Oppenheimer was reminded of the quotation from his favorite Sanskrit text, the Bhagavad-Gita, "I am become Death, the Destroyer of Worlds." To his brother, Frank, who had helped construct the site, he said only, "It worked."

Los Alamos had succeeded in producing a nuclear weapon only two years, three months and 16 days after it was formally opened. The implosion bomb was, however, a vast departure from the nuclear weapon first envisaged. That device, the gun-type uranium weapon, did not need to be tested. Farrell commented to Groves immediately after the Trinity test, "The war is over." "Yes," Groves replied, "just as soon as we drop one or two of these things on Japan."

- Robert W. Seidel

Editor's note: This is the final story in the series commemorating the Laboratory's 50th anniversary. The series began July 31, 1992, with a story on the pre-war years and continued throughout 1992 and this year. Bob Seidel, the author, is a science historian with the Center for National Security Studies (CNSS) at Los Alamos National Laboratory.