Oppenheimer's Gamble - The Plutonium Crisis

Scientists counting alpha particles from reactor-bred plutonium samples in isolated Pajarito Canyon cabin discovered catastrophically higher spontaneous fission rates than cyclotron-produced plutonium.

Oppenheimer called emergency meeting announcing gun-type plutonium bomb design—focus of hundreds of scientists’ work—would not function.

Higher spontaneous fission rate meant gun-type bomb design—firing one plutonium mass into another—would suffer predetonation, becoming “billion dollar dud.”

Seth Neddermeyer led small Los Alamos group quietly developing entirely different bomb approach: imploding subcritical plutonium mass to criticality rather than assembling separate masses.

Neutron chain reaction growth rate depends not only on total mass but also on density—subcritical plutonium mass can reach criticality through rapid compression.

Neddermeyer’s early experiments using steel surrogates surrounded by TNT invariably produced “mangled and twisted pieces”—compression wasn’t symmetric enough. Feynman famously announced from back row: “it stinks.”

Months before plutonium crisis, Oppenheimer reached out to genius mathematician John Von Neumann for implosion help—“brilliant strategic foresight” demonstrating “always have a backup” principle.

Von Neumann proposed “brilliant new design” borrowing from optical lens principles: shape explosive arrangement so detonation wavefronts match spherical plutonium mass, achieving symmetric compression.

Von Neumann divided explosives into lenses, placing slow-velocity explosive cone inside fast-velocity explosive region within each lens to equalize wavefront arrival times.

To implement lens design in 3D, Von Neumann designed “huge soccer ball” with pentagon and hexagon shaped lenses fitting together around plutonium core.

Oppenheimer knew Von Neumann’s design assumption—constant detonation wave velocity in given material—was not true. Velocity is “nonlinear and nonlocal,” depending on boundary shape and overall lens scale.

Within 2 weeks of plutonium crisis, Oppenheimer completely reorganized Los Alamos laboratory to focus on implosion, redeploying hundreds of scientists from gun design.

X and G divisions pursued seven simultaneous testing strategies; high-speed X-rays allowed scientists to see inside imploding bomb, capturing density changes as detonation wave propagated.

Robert Serber proposed clever improvement: place radioactive lanthanum-140 inside bomb core, measuring gamma ray absorption at multiple external detectors for continuous compression data.

Serber commissioned two military tanks to protect scientists during RaLa tests; lanthanum sources emitted very unsafe radiation levels. First test: forgot forest would catch fire, scientists fled scene in tanks.

RaLa tests revealed “myriad of problems”: lens firing poorly synchronized due to slight manufacturing differences between explosive detonation wires, causing problematic timing differences “on order of microseconds or more.”

Luis Alvarez led ignition system redesign, switching from explosive wires to new electrical detonator design, ultimately achieving synchronization “within a few hundreds of a microsecond” (hundreds of nanoseconds).

Manufacturing explosive lenses presented “huge range of challenges”: complex 3D molds, crack-prone high explosives, bubbles and imperfections. Over 18 months, facility delivered 20,000+ lenses to test sites, “many times this number rejected due to poor quality.”

With no analytical solutions for governing hydrodynamics equations, Von Neumann proposed iterative numerical approach using Lagrangian formulation where bomb modeled as “small masses connected by springs.”

Von Neumann’s numerical method was “enormously time consuming,” initially performed at Los Alamos by “groups of human computers using mechanical desk calculators” led by Richard Feynman, later performed on “IBM punch card accounting machines.”

Von Neumann’s improved methods remained “Limited in their predictive abilities,” leaving “painfully slow iterative trial and error approach as primary path to working implosion design.” Months of development led to “hard-fought improvements.”

At 5:29 AM July 16, 1945, “all of Manhattan Project’s science, mathematics, and engineering came together in blinding flash” in New Mexico desert—“most violent explosion humans had ever created,” equivalent to 20,000 tons of TNT. “Oppenheimer’s gamble on implosion bomb had paid off.”