Editing Nuclear weapon design (section)

===Fission===
{{Main|Nuclear fission}}
When a free neutron hits the nucleus of a fissile atom like [[uranium-235]] (<sup>235</sup>U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons (for <sup>235</sup>U three about as often as two; an average of just under 2.5 per fission). The fission chain reaction in a supercritical mass of fuel can be self-sustaining because it produces enough surplus neutrons to offset losses of neutrons escaping the supercritical assembly. Most of these have the speed (kinetic energy) required to cause new fissions in neighboring uranium nuclei.<ref>{{cite web |title=nuclear fission {{!}} Examples & Process {{!}} Britannica |website=britannica.com |url=https://www.britannica.com/science/nuclear-fission |access-date=2022-05-30}}</ref>

The uranium-235 nucleus can split in many ways, provided the atomic numbers add up to 92 and the mass numbers add up to 236 (uranium-235 plus the neutron that caused the split). The following equation shows one possible split, namely into [[strontium|strontium-95]] (<sup>95</sup>Sr), [[xenon|xenon-139]] (<sup>139</sup>Xe), and two neutrons (n), plus energy:<ref>Glasstone and Dolan, ''Effects'', p. 12.</ref>
:::<math>\ {}^{235}\mathrm{U} + \mathrm{n} \longrightarrow {}^{236}\mathrm{U}^{*} \longrightarrow {}^{95}\mathrm{Sr} + {}^{139}\mathrm{Xe} + 2\ \mathrm{n} + 180\ \mathrm{MeV}</math>

The immediate energy release per atom is about 180 million [[electron volt]]s (MeV); i.e., 74 TJ/kg. Only 7% of this is gamma radiation and kinetic energy of fission neutrons. The remaining 93% is kinetic energy (or energy of motion) of the charged fission fragments, flying away from each other mutually repelled by the positive charge of their protons (38 for strontium, 54 for xenon). This initial kinetic energy is 67 TJ/kg, imparting an initial speed of about 12,000 kilometers per second (i.e. 1.2&nbsp;cm per nanosecond). The charged fragments' high electric charge causes many inelastic [[coulomb collision]]s with nearby nuclei, and these fragments remain trapped inside the bomb's fissile [[pit (nuclear weapon)|pit]] and [[tamper (nuclear weapons)|tamper]] until their kinetic energy is converted into [[heat]]. Given the speed of the fragments and the [[mean free path]] between nuclei in the compressed fuel assembly (for the implosion design), this takes about a millionth of a second (a microsecond), by which time the core and tamper of the bomb have expanded to a ball of [[Plasma (physics)|plasma]] several meters in diameter with a temperature of tens of millions of degrees Celsius.

This is hot enough to emit [[black-body radiation]] in the X-ray spectrum. These X-rays are absorbed by the surrounding air, producing the fireball and blast of a nuclear explosion.

Most fission products have too many neutrons to be stable so they are radioactive by [[beta decay]], converting neutrons into protons by throwing off beta particles (electrons), neutrinos and gamma rays. Their half-lives range from milliseconds to about 200,000 years. Many decay into isotopes that are themselves radioactive, so from 1 to 6 (average 3) decays may be required to reach stability.<ref>Glasstone, ''Sourcebook'', p. 503.</ref> In reactors, the radioactive products are the nuclear waste in [[spent fuel]]. In bombs, they become radioactive fallout, both local and global.<ref>{{cite web |title=Nuclear explained – U.S. Energy Information Administration (EIA) |website=eia.gov |url=https://www.eia.gov/energyexplained/nuclear/ |access-date=2022-05-30}}</ref>

Meanwhile, inside the exploding bomb, the free neutrons released by fission carry away about 3% of the initial fission energy. Neutron kinetic energy adds to the blast energy of a bomb, but not as effectively as the energy from charged fragments, since neutrons do not give up their kinetic energy as quickly in collisions with charged nuclei or electrons. The dominant contribution of fission neutrons to the bomb's power is the initiation of subsequent fissions. Over half of the neutrons escape the bomb core, but the rest strike <sup>235</sup>U nuclei causing them to fission in an exponentially growing chain reaction (1, 2, 4, 8, 16, etc.). Starting from one atom, the number of fissions can theoretically double a hundred times in a microsecond, which could consume all uranium or plutonium up to hundreds of tons by the hundredth link in the chain. Typically in a modern weapon, the weapon's pit contains {{convert|3.5|to|4.5|kg}} of plutonium and at detonation produces approximately {{convert|5|to|10|ktTNT}} yield, representing the fissioning of approximately {{convert|0.5|kg}} of plutonium.<ref>{{cite web |title=NWFAQ: 4.2.5 Special Purpose Applications |last=Sublette |first=Carey |website=Nuclearweaponarchive.org |url=https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html#Nfaq4.2.5 |access-date=11 August 2021 |quote=Modern boosted fission triggers take this evolution towards higher yield to weight, smaller volume, and greater ease of radiation escape to an extreme. Comparable explosive yields are produced by a core consisting of 3.5–4.5 kg of plutonium, 5–6 kg of beryllium reflector, and some 20 kilograms of high explosive containing essentially no high-Z material.}}</ref><ref>{{cite web |title=NWFAQ: 4.4.3.4 Principles of Compression |last=Sublette |first=Carey |website=nuclearweaponarchive.org |url=https://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html#Nfaq4.4.3.4 |access-date=11 August 2021 |quote=A simplistic computation of the work done in imploding a 10 liter secondary in the "W-80" ... the primary actually produced (5 kt)...}}</ref>

Materials which can sustain a chain reaction are called [[fissile]]. The two fissile materials used in nuclear weapons are: <sup>235</sup>U, also known as [[highly enriched uranium#Highly enriched uranium (HEU)|highly enriched uranium]] (HEU), "oralloy" meaning "Oak Ridge alloy",<ref>{{cite web |title=Atomic Glossary  |publisher= Nuclear Museum |url=https://ahf.nuclearmuseum.org/ahf/history/atomic-glossary/ |access-date=24 July 2023}}</ref> or "25" (a combination of the last digit of the atomic number of uranium-235, which is 92, and the last digit of its mass number, which is 235); and <sup>239</sup>Pu, also known as plutonium-239, or "49" (from "94" and "239").<ref>{{cite book |last=Rhodes |first=Richard |author-link=Richard Rhodes |title=[[The Making of the Atomic Bomb]] |location=New York |publisher=Simon & Schuster |year=1986 |isbn=0-671-44133-7|oclc=13793436|page=563}}</ref>

Uranium's most common [[isotope]], <sup>238</sup>U, is fissionable but not fissile, meaning that it cannot sustain a chain reaction because its daughter fission neutrons are not (on average) energetic enough to cause follow-on <sup>238</sup>U fissions. However, the neutrons released by fusion of the heavy hydrogen isotopes [[deuterium]] and [[tritium]] will fission <sup>238</sup>U. This <sup>238</sup>U fission reaction in the outer jacket of the secondary assembly of a two-stage thermonuclear bomb produces by far the greatest fraction of the bomb's energy yield, as well as most of its radioactive debris.

For national powers engaged in a nuclear arms race, this fact of <sup>238</sup>U's ability to fast-fission from thermonuclear neutron bombardment is of central importance. The plenitude and cheapness of both bulk dry fusion fuel (lithium deuteride) and <sup>238</sup>U (a byproduct of uranium enrichment) permit the economical production of very large nuclear arsenals, in comparison to pure fission weapons requiring the expensive <sup>235</sup>U or <sup>239</sup>Pu fuels.