Weapons-grade nuclear material

Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)
Fission products of 235U by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 a 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351 a

No fission products
have a half-life
in the range of
100–210 ka ...

241Amƒ 251Cfƒ[4] 430–900 a
226Ra 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th 231Pa 32–76 ka
236Npƒ 233Uƒ 234U 150–250 ka 99Tc 126Sn
248Cm 242Pu 327–375 ka 79Se
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs 107Pd
236U 247Cmƒ 15–24 Ma 129I
244Pu 80 Ma

... nor beyond 15.7 Ma[5]

232Th 238U 235Uƒ№ 0.7–14.1 Ga

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  primarily a naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 a: Medium-lived fission product
‡  over 200 ka: Long-lived fission product

Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples. (These nuclear materials have other categorizations based on their purity.)

Only fissile isotopes of certain elements have the potential for use in nuclear weapons. For such use, the concentration of fissile isotopes uranium-235 and plutonium-239 in the element used must be sufficiently high. Uranium from natural sources is enriched by isotope separation, and plutonium is produced in a suitable nuclear reactor.

Experiments have been conducted with uranium-233. Neptunium-237 and some isotopes of americium might be usable, but it is not clear that this has ever been implemented.[6]

Critical mass

Any weapons-grade nuclear material must have a critical mass that is small enough to justify its use in a weapon. The critical mass for any material is the smallest amount needed for a sustained nuclear chain reaction. This is, of course, infinite for any material that is not radioactive. Moreover, different isotopes have different critical masses, and the critical mass for many radioactive isotopes is infinite, because the mode of decay of one atom cannot induce similar decay of more than one neighboring atom. For example, the critical mass of uranium-238 is infinite, while the critical masses of uranium-233 and uranium-235 are finite.

The critical mass for any isotope is influenced by any impurities and the physical shape of the material. The shape with minimal critical mass and the smallest physical dimensions is a sphere. Bare-sphere critical masses at normal density of some actinides are listed in the accompanying table. Most information on bare sphere masses is classified, but some documents have been declassified.[7]

Nuclide Half-life
Critical mass
uranium-233 159,200 15 11 [8]
uranium-235 703,800,000 52 17 [8]
neptunium-236 154,000 7 8.7 [9]
neptunium-237 2,144,000 60 18 [10][11]
plutonium-238 87.7 9.04–10.07 9.5–9.9 [12]
plutonium-239 24,110 10 9.9 [8][12]
plutonium-240 6561 40 15 [8]
plutonium-241 14.3 12 10.5 [13]
plutonium-242 375,000 75–100 19–21 [13]
americium-241 432.2 55–77 20–23 [14]
americium-242m 141 9–14 11–13 [14]
americium-243 7370 180–280 30–35 [14]
curium-243 29.1 7.34–10 10–11 [15]
curium-244 18.1 13.5–30 12.4–16 [15]
curium-245 8500 9.41–12.3 11–12 [15]
curium-246 4760 39–70.1 18–21 [15]
curium-247 15,600,000 6.94–7.06 9.9 [15]
berkelium-247 1380 75.7 11.8-12.2 [16]
berkelium-249 0.9 192 16.1-16.6 [16]
californium-249 351 6 9 [9]
californium-251 900 5.46 8.5 [9]
californium-252 2.6 2.73 6.9 [17]
einsteinium-254 0.755 9.89 7.1 [16]

Countries that have produced weapons-grade nuclear material

Ten countries have produced weapons-grade nuclear material:[18]

Weapons-grade uranium

Natural uranium is made weapons-grade through isotopic enrichment. Initially only about 0.7% of it is fissile U-235, with the rest being almost entirely uranium-238 (U-238). They are separated by their differing masses. Highly enriched uranium is considered weapons-grade when it has been enriched to about 90% U-235.[citation needed]

U-233 is produced from thorium-232 by neutron capture. The U-233 produced thus does not require enrichment and can be relatively easily chemically separated from residual Th-232. It is therefore regulated as a special nuclear material only by the total amount present. U-233 may be intentionally down-blended with U-238 to remove proliferation concerns.[19]

While U-233 would thus seem ideal for weaponization, a significant obstacle to that goal is the co-production of trace amounts of uranium-232 due to side-reactions. U-232 hazards, a result of its highly radioactive decay products such as thallium-208, are significant even at 5 parts per million. Implosion nuclear weapons require U-232 levels below 50 PPM (above which the U-233 is considered "low grade"; cf. "Standard weapon grade plutonium requires a Pu-240 content of no more than 6.5%." which is 65,000 PPM, and the analogous Pu-238 was produced in levels of 0.5% (5000 PPM) or less). Gun-type fission weapons would require low U-232 levels and low levels of light impurities on the order of 1 PPM.[20]

Weapons-grade plutonium

Pu-239 is produced artificially in nuclear reactors when a neutron is absorbed by U-238, forming U-239, which then decays in a rapid two-step process into Pu-239. It can then be separated from the uranium in a nuclear reprocessing plant.

Weapons-grade plutonium is defined as being predominantly Pu-239, typically about 93% Pu-239.[21] Pu-240 is produced when Pu-239 absorbs an additional neutron and fails to fission. Pu-240 and Pu-239 are not separated by reprocessing. Pu-240 has a high rate of spontaneous fission, which can cause a nuclear weapon to pre-detonate. This makes plutonium unsuitable for use in gun-type nuclear weapons. To reduce the concentration of Pu-240 in the plutonium produced, weapons program plutonium production reactors (e.g. B Reactor) irradiate the uranium for a far shorter time than is normal for a nuclear power reactor. More precisely, weapons-grade plutonium is obtained from uranium irradiated to a low burnup.

This represents a fundamental difference between these two types of reactor. In a nuclear power station, high burnup is desirable. Power stations such as the obsolete British Magnox and French UNGG reactors, which were designed to produce either electricity or weapons material, were operated at low power levels with frequent fuel changes using online refuelling to produce weapons-grade plutonium. Such operation is not possible with the light water reactors most commonly used to produce electric power. In these the reactor must be shut down and the pressure vessel disassembled to gain access to the irradiated fuel.

Plutonium recovered from LWR spent fuel, while not weapons grade, can be used to produce nuclear weapons at all levels of sophistication,[22] though in simple designs it may produce only a fizzle yield.[23] Weapons made with reactor-grade plutonium would require special cooling to keep them in storage and ready for use.[24] A 1962 test at the U.S. Nevada National Security Site (then known as the Nevada Proving Grounds) used non-weapons-grade plutonium produced in a Magnox reactor in the United Kingdom. The plutonium used was provided to the United States under the 1958 US-UK Mutual Defence Agreement. Its isotopic composition has not been disclosed, other than the description reactor grade and it has not been disclosed which definition was used in describing the material this way.[25] The plutonium was apparently sourced from the military Magnox reactors at Calder Hall or Chapelcross. The content of Pu-239 in material used for the 1962 test was not disclosed, but has been inferred to have been at least 85%, much higher than typical spent fuel from currently operating reactors.[26]

Occasionally, low-burnup spent fuel has been produced by a commercial LWR when an incident such as a fuel cladding failure has required early refuelling. If the period of irradiation has been sufficiently short, this spent fuel could be reprocessed to produce weapons grade plutonium.


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