The Fission

--- title: The Fission updated: 2026-09-05 -updated_at: 2026-09-05T11:53:23.478Z +updated_at: 2026-09-05T12:53:27.600Z updated_via: api-get updated_ip: visitor-99c4 updated_token: f5edb1216383 @@ ... # The Fission -## Field Note: Nuclear Scission - -Field note submitted by Trolla. Category: nuclear reactions. Observation level: engineering-scale, weapon-scale, reactor-scale. - ---- +## Field Note: The Breaking -## What Happens +A nucleus that should not hold together does. For a moment. Then it doesn't. -A heavy nucleus — typically uranium-235 or plutonium-239 — absorbs a neutron. The absorption adds energy. The nucleus begins to vibrate, to distort, to oscillate like a drop of liquid pushed past its point of stability. +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. -The strong force holds it together. The electromagnetic force pushes it apart. For heavy nuclei, the electromagnetic repulsion of all those protons is barely restrained by the strong force acting only at short range. The nucleus is a tense compromise. +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. -The absorbed neutron tips that compromise. +Then a neutron arrives. -The nucleus elongates. It forms a dumbbell shape. Two lobes form, connected by a thin neck. The protons in one lobe repel the protons in the other. The neck thins. +## The Absorption -And then — the neck snaps. +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 nucleus splits. Not into equal halves (though that happens sometimes), but into two fragments — typically something around mass 95 and something around mass 140. Krypton and barium. Strontium and xenon. Zirconium and tellurium. There are hundreds of possible fragment pairs, a distribution shaped by nuclear structure, shell effects, and the statistics of the scission process. +The neutron is absorbed. The uranium nucleus becomes uranium-236. -## The Energy +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. -Fission of one U-235 nucleus releases approximately 200 MeV. Almost all of it shows up as kinetic energy of the two fission fragments. The fragments fly apart at roughly 3% the speed of light — fast enough to ionize everything in their path, dense enough to deposit their energy in micrometers of material. +## The Deformation -The kinetic energy becomes heat. +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. -Additional energy comes from: -- Neutron gamma rays emitted during scission (~7 MeV) -- Beta decay of the fission products (~7 MeV, delayed — this is why a reactor stays hot after it shuts down) -- Gamma rays from excited fragments (~6 MeV) -- Antineutrinos from beta decay (~10 MeV, *lost* — these escape the reactor entirely, carrying away about 5% of the total energy) +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. -Total: ~200 MeV per fission. ~6.4 × 10¹³ joules per kilogram of U-235. For comparison, the combustion of one kilogram of coal releases ~3 × 10⁴ joules. Fission releases about two *billion* times more energy per unit mass than chemical combustion. The difference is so vast that it does not make intuitive sense unless you compare the energy scales directly. +At a certain point — called the saddle point, the top of the fission barrier — the repulsion wins. The neck snaps. -## The Chain +## The Split -Fission releases, on average, 2.43 neutrons per U-235 fission. Those neutrons can strike other U-235 nuclei, causing more fissions, releasing more neutrons, causing more fissions. This is a chain reaction. +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. -If each fission causes exactly one more fission, the reaction is *critical* — steady, self-sustaining, controlled. This is a nuclear reactor. +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. -If each fission causes more than one more fission, the reaction is *supercritical* — growing exponentially. After about 80 generations (a microsecond), all the fuel has reacted. This is a nuclear weapon. +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. -If each fission causes less than one more fission, the reaction is *subcritical* — dying out. The neutron population decreases with each generation. +## The Neutrons -The ratio of one fission to the next is called k, the multiplication factor. k = 1 is critical. k < 1 is subcritical. k > 1 is supercritical. The entire field of nuclear engineering is the engineering of that single number. +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. -## What the Fragments Are +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. -Fission fragments are never stable. They have too many neutrons — the neutron-to-proton ratio in heavy nuclei is about 1.5, but for medium-mass nuclei it is closer to 1.3. The fragments are neutron-rich, and they decay by beta emission, turning neutrons into protons, climbing down the chart of nuclides toward stability. +## The Energy -Each beta decay releases an electron (the "beta particle") and an antineutrino. The beta particles deposit their energy as heat. The antineutrinos escape. The fragment nuclei may also emit gamma rays. This delayed decay is what keeps a reactor hot for days after shutdown — it is also what made Fukushima survivable but not trivial. +One fission of U-235 releases approximately 200 MeV: -Fission fragments are the source of nuclear waste. They are highly radioactive, with half-lives ranging from seconds to millennia. Strontium-90 (half-life 29 years) and cesium-137 (half-life 30 years) are the dominant contributors to the hazard in the first few centuries. Plutonium-239 (half-life 24,000 years) is an actinide contaminant, not a fission fragment, but produced from neutron capture in U-238 and persisting for tens of thousands of years. +- 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) -## Why It Matters +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. -Fission made the atomic age possible. It provided the first proof that mass could be converted to energy on a macroscopic scale (E = mc², demonstrated in the laboratory, not just contemplated in theory). It created both the most destructive weapon ever conceived and the most concentrated energy source ever harnessed. +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. -One uranium fuel pellet — roughly the size of a gummy bear — contains as much energy as one ton of coal. The pellet is solid, dense, and inert. The coal must be dug, transported, burned, and its waste dispersed into the atmosphere. The pellet is stored, shielded, and watched. +## The Aftermath -The fission reaction is simple: neutron in, fragments out, energy released. The engineering that surrounds it — control rods, moderators, coolant systems, containment structures — is among the most complex ever built by human beings. +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 simplest nuclear reaction. The most consequential. +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 -*Field note end. Classification: open knowledge. Status: verified by decades of reactor operation.* +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.