Atomic & Space Age (1940–1970)Nuclear Physics & Energy

Fermi & Szilárd's Nuclear Reactor

US 2,708,656

Heterogeneous Graphite Moderator, Uranium Lattice, and Cadmium Control Rods

Inventor(s)Enrico Fermi, Leo Szilard

Grant Date1955-05-17

Filing Date1944-12-19

LocationSanta Fe, New Mexico & Chicago, Illinois

Fermi and Szilard's pile: lumps of natural uranium in boron-free graphite so neutrons thermalize in the carbon and miss U-238 resonances, with cadmium rods for $k$. CP-1 went critical on 2 December 1942 under Stagg Field.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

Fermi and Szilard solved the central physics puzzle of nuclear fission: how to achieve a self-sustaining atomic chain reaction using un-enriched, natural uranium (which contains 99.3% non-fissionable U-238 and only 0.7% U-235). Their heterogeneous graphite matrix slowed neutrons down without letting them get trapped in U-238 resonance absorption bands.

The Core Breakthrough Mechanism

Fast 2 MeV fission neutrons emitted inside discrete uranium lumps escape into surrounding graphite blocks. After ~114 elastic collisions with carbon nuclei, they slow down to thermal energies (0.025 eV) before diffusing back into a neighboring uranium lump, where they selectively trigger fission in U-235. Movable cadmium rods absorb neutrons to maintain $k_{eff} = 1.000$ .

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Detailed Component Architecture

1Heterogeneous Uranium Fuel Lattice

Discrete uranium metal and oxide cylinders spaced in a 3D grid.

Lumping the fuel prevents fast neutrons from immediately colliding with U-238 atoms at resonance capture energies (5–100 eV). The resonance escape probability $p$ increases from ~0.5 in homogeneous mixtures to >0.85 in a lumped lattice.

19th-C. Term: Bodies of fissionable material disposed in a spaced geometric latticeModern: Fuel assemblies / fuel rods in reactor core

2High-Purity Graphite Moderator Matrix

Blocks of ultra-pure graphite carbon surrounding fuel channels.

Carbon-12 has a low atomic mass (A=12) and an extraordinarily low thermal neutron absorption cross-section ( $sigma_a = 0.0035$ barns), allowing fast neutrons to thermalize through elastic scattering without parasitic loss.

19th-C. Term: Neutron moderating materialModern: Graphite moderator / Light water moderator

3Cadmium / Boron Control Rods

Movable rods containing high neutron absorption cross-section elements.

Cadmium-113 has a massive thermal neutron absorption cross-section ( $sigma_a = 20,600$ barns). Inserting the rods reduces the effective reproduction factor $k_{eff} < 1$ , shutting down the reactor.

19th-C. Term: Neutron-absorbing control elementsModern: Control rod drive mechanism (CRDM) / Scram rods

Governing Physical Equations & Principles

Fermi Four-Factor Formula

k_\infty = \eta \cdot \epsilon \cdot p \cdot f \implies k_{eff} = k_\infty P_{NL}

The infinite multiplication factor is the product of reproduction factor (η), fast fission factor (ε), resonance escape probability (p), and thermal utilization (f). Criticality requires

k_{eff} \ge 1.0

Neutron Moderation Logarithmic Energy Loss

\xi = 1 + \frac{(A-1)^2}{2A}\ln\left(\frac{A-1}{A+1}\right) \approx \frac{2}{A + 2/3}

Average logarithmic energy loss per elastic collision in carbon is ξ = 0.158, requiring ~114 collisions to slow from 2 MeV to 0.025 eV.

Delayed Neutron Reactor Kinetics

\frac{dn}{dt} = \frac{\rho - \beta}{\Lambda} n + \sum_{i=1}^{6} \lambda_i C_i

Delayed neutrons (β = 0.0065) provide a multi-second time constant, allowing human and motorized mechanical control rods to safely balance criticality.

Why It Still Matters

A light-water reactor is a different moderator and a pressure vessel. The control problem is the same: keep $k$ near 1 with delayed neutrons, and have a rod worth you can insert faster than the period.

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Claim #1Independent Master Claim

Verbatim Historical Legal Text

“1. A neutronic reactor comprising a mass of neutron moderating material having a neutron capture cross-section to scattering cross-section ratio less than 0.005, and bodies of a fissionable material disposed in said moderating material in a spaced geometric lattice, the volume ratio of moderating material to fissionable material, the spatial configuration of said bodies, and the purity of said materials being coordinated to provide a neutron reproduction ratio k for an infinite system greater than unity.”

Plain English Engineering Translation

Covers the fundamental architecture of a nuclear fission reactor with a low-capture moderator surrounding a geometric lattice of fissionable fuel bodies designed to achieve a neutron reproduction factor k exceeding 1.0.

Key Protected Innovations:

Heterogeneous moderator-fuel geometric latticeNeutron reproduction factor k > 1.0 in natural uraniumLow neutron capture cross-section moderating medium

Historical Legal Impact:

A heterogeneous lattice of fissionable material in a moderator, with a movable absorber, arranged so k can be held at or above 1.

The Historical Bottleneck

Natural uranium consists of 99.3% uranium-238 and only 0.7% fissionable uranium-235. When a U-235 nucleus fissions, it releases fast neutrons with kinetic energies of ~2 MeV. In pure uranium metal, these fast neutrons are almost entirely captured non-fissionably by U-238 in 'resonance capture' energy bands, extinguishing the chain reaction before a second generation of fissions can occur.

Why Prior Art Failed

•Homogeneous uranium mixtures suffered 100% resonance capture extinction
•Commercial graphite contained boron impurities that absorbed all thermal neutrons
•No controlled nuclear chain reaction had ever been demonstrated in human history

The Breakthrough Insight

“Fermi and Szilard realized that by geometrically separating the uranium into discrete lumps or rods distributed throughout a moderator medium of low atomic weight and negligible neutron absorption (high-purity graphite carbon), fast neutrons escape the uranium lump into the carbon matrix. Through ~114 elastic collisions with carbon atoms, the neutrons thermalize down to 0.025 eV before diffusing back into a neighboring uranium lump, bypassing U-238 resonance traps and preferentially triggering thermal fission in U-235.”

Patent Wars & Legal Litigations

Vs. Manhattan Project / AEC Secrecy OrderInfringement Challenge

Rival Claim & Defense:

United States Government classified atomic inventions under the 1946 Atomic Energy Act

Litigation Conflict:

Filed in December 1944 during the height of the Manhattan Project, the patent was classified as Top Secret. Fermi and Szilard assigned rights to the U.S. government for a nominal sum of $1.00.

Final Resolution & Judicial Outcome:

Declassified and issued publicly on May 17, 1955 under President Eisenhower's 'Atoms for Peace' initiative.

After the Grant

The patent sat classified until 17 May 1955. Fermi and Szilard assigned it for a dollar. Fermi died in 1954 and never saw the issued document. Szilard spent the rest of his life trying to put the bomb back in the political bottle.

Civilizational Impact

2 December 1942, CP-1 went critical under the west stands of Stagg Field. Every later pile, submarine core, and power reactor is a controlled version of that lattice argument: moderate the neutrons, keep k just above 1, and have a cadmium rod you can drop.

Historical Fact

The squash court was unheated. Overcoats and fedoras in the photographs are not style; they are Chicago in December. They toasted with a bottle of Chianti and paper cups. Eugene Wigner handed Fermi the bottle.

Further Context

The graphite had to be boron-free. Ordinary commercial carbon would have poisoned the pile. The National Carbon and Speer lots were a materials project of their own.
Szilard's 1934 chain-reaction patent (British) is the conceptual ancestor. CP-1 is the heterogeneous lattice that made natural uranium work.
Arthur Compton ran the Met Lab. Leona Woods was the only woman present at criticality, on the boron trifluoride counter.