Nuclear chain reaction

A nuclear chain reaction is a succession of fission events in which neutrons released by one split uranium nucleus can produce further splits. In the book, the concept marks the moment when a newly confirmed physical phenomenon became both a possible source of power and the basis of an atomic weapon. Its history proceeds from Oppenheimer’s startled reassessment of fission to calculations for a deliverable device, and then to the wider problem of how weapons built on that process might reshape military policy.

From Fission to a Chain Reaction

On January 29, 1939, Luis Alvarez read in the San Francisco Chronicle that Otto Hahn and Fritz Strassmann had split the uranium nucleus by bombarding it with neutrons. He hurried to the Radiation Laboratory to tell Oppenheimer, who initially rejected the result and used a blackboard to argue that fission could not occur. The reversal came quickly: the next day Alvarez reproduced the experiment, showing Oppenheimer oscilloscope pulses from fission that were twenty-five times larger than the natural alpha-particle pulses. Within fifteen minutes, Oppenheimer accepted that the reaction was real and reasoned that it released additional neutrons capable of splitting more uranium atoms. That multiplication was the crucial step from fission as a laboratory observation to a process that might generate power or make bombs.

Oppenheimer’s first response combined scientific excitement with a sense of danger. In correspondence with Willie Fowler, he considered unresolved questions about the fission products, including the expected short-lived high-energy beta particles and whether uranium split randomly or in particular ways. He described the discovery as exciting in a practical, rather than merely rarefied, sense. To George Uhlenbeck, he speculated that a ten-centimeter cube of uranium deuteride might blow itself apart. That was a conjecture, not a demonstrated device, but it shows how rapidly the prospect of neutron multiplication carried his attention from the experiment toward explosive consequences.

Early Bomb Calculations

The possibility of a chain reaction entered the thinking of Oppenheimer’s Berkeley circle almost at once. The week fission became known, graduate student Joseph Weinberg arrived at Room 219 in LeConte Hall. Oppenheimer introduced him to Ernest Lawrence, Linus Pauling, and graduate students Hartland Snyder, Philip Morrison, and Sydney M. Dancoff. At lunch, Weinberg, Morrison, and Dancoff discussed a telegram from Niels Bohr about fission and sketched a bomb based on a chain reaction on a napkin. Morrison’s preliminary calculations suggested that, given what they then knew, the reaction would fizzle before an explosion. They did not yet know that uranium could be purified and concentrated enough to alter that assessment. Within a week, Oppenheimer had a rough bomb drawing on his office blackboard.

In subsequent design discussions, Oppenheimer directed colleagues toward a fission device small enough to be delivered militarily. They estimated that a chain reaction might be achieved with a uranium core inside a metal sphere eight inches in diameter. The calculations were demanding and depended on experimental information the group did not yet possess; the amount of fissionable material required remained uncertain. Their estimates nevertheless suggested that the quantity could be twice what had recently been indicated to the president, implying production on an industrial scale rather than in a laboratory. The chain reaction was therefore not just a question of theoretical possibility: making it into a weapon depended on producing enough material and solving difficult engineering problems.

The Berkeley summer-session calculations were later summarized for scientists at Los Alamos by Bob Serber. A bomb might, in their estimate, yield an explosion equivalent to 20,000 tons of TNT. One possible uranium core, made of highly enriched uranium, would be about the size of a cantaloupe and weigh roughly thirty-three pounds. A plutonium core could weigh about eleven pounds and be no larger than an orange. Either would be surrounded by a thick shell of ordinary uranium about the size of a basketball, bringing the device’s total weight to about a ton. These were design estimates, not a completed weapon: the production of sufficient uranium-235 or plutonium required a massive industrial effort, and the scientists still did not know how to ensure an efficient detonation.

The Reaction and the Weapons Debate

The physical chain reaction also made questions of scale unavoidable. In assessing a possible thermonuclear weapon in 1949, Oppenheimer and the General Advisory Committee compared its projected yield—potentially 100 million tons of TNT—with the Hiroshima bomb’s 15,000 tons. They feared that a weapon of that magnitude could destroy 150 to 1,000 square miles or more, and argued that no legitimate military target could justify such destructive capacity. Their objection was not that a chain reaction could not produce an enormous explosion, but that the military and political consequences of building such a weapon could be catastrophic. They urged that the Super never be produced and that refusal might reopen arms-control negotiations.

The committee also considered what would follow if the Soviet Union developed a Super first. Its report argued that undertaking an American crash program would not deter the Soviets, and that the United States’ existing atomic bombs could provide an effective reply. It proposed instead producing fissionable material for smaller tactical weapons and strengthening conventional forces in Western Europe. This was an argument for an arsenal sized around specific military purposes rather than an open-ended accumulation of weapons. Yet the book makes clear that the distinction between a smaller weapon and a safer policy was unstable: Oppenheimer’s preference for tactical weapons could bring nuclear arms onto the battlefield, where their use might act as a tripwire to an exchange of increasingly powerful weapons.

Physical Multiplication and Strategic Escalation

The chain reaction itself is a physical process: neutrons from fission can sustain further fission. The later danger described in the book is political and military rather than a literal continuation of that process. Oppenheimer was alarmed by an Air Force plan for a massive initial strategic assault on Soviet cities, and by 1953 warned that secrecy concealed both the scale of American stockpiles and plans for their use. He contrasted the American twentieth-thousandth bomb with the Soviet two-thousandth, arguing that sheer numerical superiority would not resolve the strategic problem. The distinction matters: a chain reaction made an atomic explosion possible, but it did not dictate how governments would build, stockpile, or use weapons.

Across the book’s account, the concept therefore carries two linked but distinct meanings. Scientifically, neutron multiplication transformed the discovery of fission into a plausible route to power and explosive force. Historically, that possibility helped create an industrial weapons program and a strategic contest whose consequences could not be managed by technical calculations alone. Oppenheimer’s early insight was that fission could multiply; his later policy work confronted the human decision to multiply the weapons and risks built upon it.