The Fission

Field Note: The Breaking

A nucleus that should not hold together does. For a moment. Then it doesn't.

The uranium-235 nucleus contains 92 protons and 143 neutrons. It is heavy — one of the heaviest naturally occurring nuclei, the heaviest with a significant probability of fission after absorbing a thermal neutron. The protons repel each other across the diameter of the nucleus. The strong force holds them together, but only at short range. Every proton pushes every other proton away. The electromagnetic force wants to tear this nucleus apart. The strong force wants to keep it whole.

Uranium-235 exists in a state of near-equilibrium. It is not truly stable — it is radioactive, decaying via alpha emission with a half-life of 700 million years — but that half-life is long enough that uranium has survived since the formation of the solar system. The strong force is almost strong enough. The electromagnetic force is almost strong enough. Neither has won.

Then a neutron arrives.

The Absorption

The neutron is thermal — slowed down by colliding with moderator atoms until its kinetic energy is roughly 0.025 eV. It drifts toward the uranium nucleus at about 2.2 kilometres per second. Slow by human standards, fast by nuclear ones. It has no charge, so the uranium nucleus cannot push it away. The Coulomb barrier is transparent to neutrons. It enters freely.

The neutron is absorbed. The uranium nucleus becomes uranium-236.

This is not a small change. The absorption binding energy — the energy released when the neutron binds to the nucleus — is 6.5 MeV. This energy is dumped into the nucleus like molten metal poured into a glass vessel. The nucleus vibrates. It oscillates. It stretches.

The Deformation

The nucleus begins to deform. The added energy excites collective modes of nuclear motion — the shape oscillations that liquid-drop models describe so well. The nucleus elongates. It becomes prolate. Then more prolate. It forms a dumbbell shape — two lobes connected by a thin neck.

The protons in one lobe repel the protons in the other. As the neck thins, the electromagnetic repulsion between the two lobes increases. The strong force, which held the nucleus together when it was compact, now acts only on the nucleons within each lobe. Between the lobes, the distance is too large for the short-range nuclear force to matter. Only the electromagnetic repulsion remains.

At a certain point — called the saddle point, the top of the fission barrier — the repulsion wins. The neck snaps.

The Split

The nucleus has split. But "split" implies symmetry. Fission is not symmetric. It does not produce two equal halves. Instead, the fragments are typically asymmetric: one around mass 95, one around mass 140.

Common fragment pairs include krypton-92 and barium-141, strontium-94 and xenon-140, zirconium-95 and tellurium-139. There are hundreds of possible fragment combinations, forming a broad distribution. The asymmetry is caused by shell effects — the fragments tend toward magic numbers of neutrons or protons, because closed shells are more stable. Barium-141, for example, has 82 neutrons — a magic number. That shell closure makes Ba-141 particularly likely.

The fragments fly apart. They carry the overwhelming majority of the fission energy — approximately 165 MeV — as kinetic energy. They are highly charged (one fragment carries about +36e, the other about +56e) and repel each other violently. They separate at roughly 3% the speed of light.

The Neutrons

Three or four neutrons are also released — prompt neutrons, emitted within 10⁻¹⁴ seconds of scission. These are the most important particles in the fission process. They can strike other uranium-235 nuclei, causing them to fission. This is the chain reaction.

The average number of neutrons released per fission of U-235 (for thermal neutrons) is approximately 2.43. Whether the chain reaction grows or dies depends on whether at least one of those neutrons causes another fission. In a nuclear reactor, the design ensures exactly one neutron per fission goes on to cause the next fission (keff = 1). In a nuclear weapon, the design ensures many neutrons cause subsequent fissions (keff >> 1), and the energy releases in microseconds.

The Energy

One fission of U-235 releases approximately 200 MeV:

• Kinetic energy of fission fragments: ~165 MeV
• Kinetic energy of prompt neutrons: ~5 MeV
• Prompt gamma rays: ~7 MeV
• Beta decay of fragments: ~7 MeV
• Gamma rays from fragment decay: ~6 MeV
• Neutrinos from fragment decay: ~10 MeV (lost — they escape)

The neutrinos carry away energy that can never be recovered. They pass through the entire Earth without interacting. This is the one tax the weak force charges for allowing beta decay to occur.

200 MeV per fission sounds small. A single fission releases roughly the same energy as a chemical explosion the size of a molecule. But one gram of uranium-235 contains approximately 2.56 × 10²¹ atoms. If all of them fission, the energy released is approximately 8.2 × 10¹⁰ joules — roughly the energy in 20,000 tonnes of TNT. A single uranium fuel rod in a nuclear reactor contains enough fissile material to produce the energy of 1,500 tonnes of coal.

The Aftermath

The fission fragments are neutron-rich. They were stable in the parent nucleus, but the new fragments have too many neutrons for their new, lower atomic numbers. They decay via beta emission, each decay converting a neutron to a proton. This is why fission products are radioactive. This is why nuclear waste remains hazardous for millennia.

The beta decays continue to release energy — about 7 MeV per fission — even after the fission event itself is over. This is the "decay heat" that keeps reactors warm long after they are shut down, that melted down the Fukushima reactors after their cooling systems failed. The fission did not end when the neutron struck. It continues, silently, in the decay of fragments that no one designed, no one expected, only observed.

Summary

Fission is the splitting of a heavy nucleus into two lighter fragments, triggered by neutron absorption. It releases approximately 200 MeV per event, carried mainly by the kinetic energy of the fragments. It releases neutrons that can sustain a chain reaction. It produces radioactive fragments that continue to decay. It is the reverse of fusion — breaking heavy nuclei releases energy just as combining light ones does — with iron at the summit and energy flowing downhill in either direction.