Uranium-233 (233U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel.[2] It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years.

Uranium-233, 233U
An ampoule containing solidified pieces of a
FLiBe and uranium-233 tetrafluoride mixture
Namesuranium-233, 233U, U-233
Protons (Z)92
Neutrons (N)141
Nuclide data
Half-life (t1/2)160,000 years[1]
Isotope mass233.039 Da
Parent isotopes237Pu (α)
233Np (β+)
233Pa (β)
Decay products229Th
Isotopes of uranium
Complete table of nuclides

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur, to maintain the neutron economy (if it misses the 233U window, the next fissile target is 235U, meaning a total of 4 neutrons needed to trigger fission).

233U usually fissions on neutron absorption, but sometimes retains the neutron, becoming uranium-234. The capture-to-fission ratio of uranium-233 is smaller than those of the other two major fissile fuels, uranium-235 and plutonium-239.[citation needed]

Fissile material

Molten-Salt Reactor Experiment
Shippingport Atomic Power Station
German THTR-300

In 1946, the public first became informed of uranium-233 bred from thorium as "a third available source of nuclear energy and atom bombs" (in addition to uranium-235 and plutonium-239), following a United Nations report and a speech by Glenn T. Seaborg.[3][4]

The United States produced, over the course of the Cold War, approximately 2 metric tons of uranium-233, in varying levels of chemical and isotopic purity.[2] These were produced at the Hanford Site and Savannah River Site in reactors that were designed for the production of plutonium-239.[5]

Nuclear fuel


Uranium-233 has been used as a fuel in several different reactor types, and is proposed as a fuel for several new designs (see thorium fuel cycle), all of which breed it from thorium. Uranium-233 can be bred in either fast reactors or thermal reactors, unlike the uranium-238-based fuel cycles which require the superior neutron economy of a fast reactor in order to breed plutonium, that is, to produce more fissile material than is consumed.

The long-term strategy of the nuclear power program of India, which has substantial thorium reserves, is to move to a nuclear program breeding uranium-233 from thorium feedstock.

Energy released


The fission of one atom of uranium-233 generates 197.9 MeV = 3.171·10−11 J  (i.e. 19.09 TJ/mol = 81.95 TJ/kg = 22764 MWh/kg that is 1.8 million times more than the same mass of diesel).[6]

Source Average energy
released (MeV)
Instantaneously released energy
Kinetic energy of fission fragments 168.2
Kinetic energy of prompt neutrons 004.8
Energy carried by prompt γ-rays 007.7
Energy from decaying fission products
Energy of β particles 005.2
Energy of anti-neutrinos 006.9
Energy of delayed γ-rays 005.0
Sum (excluding escaping anti-neutrinos) 191.0
Energy released when those prompt neutrons which don't (re)produce fission are captured 009.1
Energy converted into heat in an operating thermal nuclear reactor 200.1

Weapon material

The first detonation of a nuclear bomb that included U-233, on 15 April 1955

As a potential weapon material, pure uranium-233 is more similar to plutonium-239 than uranium-235 in terms of source (bred vs natural), half-life and critical mass (both 4–5 kg in beryllium-reflected sphere).[7] Unlike reactor-bred plutonium, it has a very low spontaneous fission rate, which combined with its low critical mass made it initially attractive for compact gun-type weapons, such as small-diameter artillery shells.[8]

A declassified 1966 memo from the US nuclear program stated that uranium-233 has been shown to be highly satisfactory as a weapons material, though it was only superior to plutonium in rare circumstances. It was claimed that if the existing weapons were based on uranium-233 instead of plutonium-239, Livermore would not be interested in switching to plutonium.[9]

The co-presence of uranium-232[10] can complicate the manufacture and use of uranium-233, though the Livermore memo indicates a likelihood that this complication can be worked around.[9]

While it is thus possible to use uranium-233 as the fissile material of a nuclear weapon, speculation[11] aside, there is scant publicly available information on this isotope actually having been weaponized:

The B Reactor and others at the Hanford Site optimized for the production of weapons-grade material have been used to manufacture 233U.[17][18][19][20]

Overall the United States is thought to have produced two tons of 233U, of various levels of purity, some with 232U impurity content as low as 6 ppm.[21]

232U impurity


Production of 233U (through the irradiation of thorium-232) invariably produces small amounts of uranium-232 as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233, or on thorium-232:

232Th (n,γ) → 233Th (β) → 233Pa (β) → 233U (n,2n) → 232U
232Th (n,γ) → 233Th (β) → 233Pa (n,2n) → 232Pa (β)→ 232U
232Th (n,2n) → 231Th (β) → 231Pa (n,γ) → 232Pa (β) → 232U

Another channel involves neutron capture reaction on small amounts of thorium-230, which is a tiny fraction of natural thorium present due to the decay of uranium-238:

230Th (n,γ) → 231Th (β) → 231Pa (n,γ) → 232Pa (β) → 232U

The decay chain of 232U quickly yields strong gamma radiation emitters. Thallium-208 is the strongest of these, at 2.6 MeV:

232U (α, 68.9 y)
228Th (α, 1.9 y)
224Ra (α, 5.44 MeV, 3.6 d, with a γ of 0.24 MeV)
220Rn (α, 6.29 MeV, 56 s, with a γ of 0.54 MeV)
216Po (α, 0.15 s)
212Pb (β, 10.64 h)
212Bi (α, 61 min, 0.78 MeV)
208Tl (β, 1.8 MeV, 3 min, with a γ of 2.6 MeV)
208Pb (stable)

This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, (except possibly in a short period immediately following chemical separation of the uranium from its decay products) and instead requiring complex remote manipulation for fuel fabrication.

The hazards are significant even at 5 parts per million. Implosion nuclear weapons require 232U levels below 50 ppm (above which the 233U is considered "low grade"; cf. "Standard weapon grade plutonium requires a 240Pu content of no more than 6.5%." which is 65,000 ppm, and the analogous 238Pu was produced in levels of 0.5% (5,000 ppm) or less). Gun-type fission weapons additionally need low levels (1 ppm range) of light impurities, to keep the neutron generation low.[10][22]

The production of "clean" 233U, low in 232U, requires a few factors: 1) obtaining a relatively pure 232Th source, low in 230Th (which also transmutes to 232U), 2) moderating the incident neutrons to have an energy not higher that 6 MeV (too-high energy neutrons cause the 232Th (n,2n) → 231Th reaction) and 3) removing the thorium sample from neutron flux before the 233U concentration builds up to a too high level, in order to avoid fissioning the 233U itself (which would produce energetic neutrons).[21][23]

The Molten-Salt Reactor Experiment (MSRE) used 233U, bred in light water reactors such as Indian Point Energy Center, that was about 220 ppm 232U.[24]

Further information


Thorium, from which 233U is bred, is roughly three to four times more abundant in the Earth's crust than uranium.[25][26] The decay chain of 233U itself is part of the neptunium series, the decay chain of its grandparent 237Np.

Uses for uranium-233 include the production of the medical isotopes actinium-225 and bismuth-213 which are among its daughters, low-mass nuclear reactors for space travel applications, use as an isotopic tracer, nuclear weapons research, and reactor fuel research including the thorium fuel cycle.[2]

The radioisotope bismuth-213 is a decay product of uranium-233; it has promise for the treatment of certain types of cancer, including acute myeloid leukemia and cancers of the pancreas, kidneys and other organs.

See also



  1. ^ "Uranium-233 at the Hanford Nuclear Site" (PDF). Washington State Department of Health, Division of Environmental Health, Office of Radiation Protection. December 2002.
  2. ^ a b c C. W. Forsburg; L. C. Lewis (24 September 1999). "Uses For Uranium-233: What Should Be Kept for Future Needs?" (PDF). Ornl-6952. Oak Ridge National Laboratory.
  3. ^ "Atomic Energy 'Secret' Put into Language That Public Can Understand". Pittsburgh Press. United Press. 29 September 1946. Retrieved 18 October 2011.
  4. ^ "Third Nuclear Source Bared". The Tuscaloosa News. United Press. 21 October 1946. Retrieved 18 October 2011.
  5. ^ Orth, D. A. (1 June 1978). "Savannah River Plant Thorium Processing Experience". Nuclear Technology. 43: 63–74. doi:10.13182/NT79-A16175.
  6. ^ "Nuclear fission 4.7.1". kayelaby.npl.co.uk. Retrieved 21 April 2018.
  7. ^ Nuclear proliferation factbook. Committee on Governmental Affairs. Subcommittee on Energy, N. Proliferation., United States. Congress. House. Committee on Foreign Affairs. Subcommittee on International Economic Policy and Trade., United States. Congress. House. Committee on Foreign Affairs. Subcommittee on Arms Control, I. Security. 1985. p. 295. Retrieved 29 November 2019.
  8. ^ Hansen, Chuck (2007). Swords of Armageddon: US Nuclear Weapons Development since 1945, Version 2. Chuckelea Publications. pp. I-262, I-270.
  9. ^ a b Woods, W. K. (10 February 1966). "LRL interest in U-233". United States. DUN-677. doi:10.2172/79078. OSTI 79078.
  10. ^ a b Langford, R. Everett (2004). Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological. Hoboken, New Jersey: John Wiley & Sons. p. 85. ISBN 0471465607. "The US tested a few uranium-233 bombs, but the presence of uranium-232 in the uranium-233 was a problem; the uranium-232 is a copious alpha emitter and tended to 'poison' the uranium-233 bomb by knocking stray neutrons from impurities in the bomb material, leading to possible pre-detonation. Separation of the uranium-232 from the uranium-233 proved to be very difficult and not practical. The uranium-233 bomb was never deployed since plutonium-239 was becoming plentiful."
  11. ^ Agrawal, Jai Prakash (2010). High Energy Materials: Propellants, Explosives and Pyrotechnics. Wiley-VCH. pp. 56–57. ISBN 978-3-527-32610-5. Retrieved 19 March 2012. states briefly that U233 is "thought to be a component of India's weapon program because of the availability of Thorium in abundance in India", and could be elsewhere as well.
  12. ^ "Operation Teapot". Nuclear Weapon Archive. 15 October 1997. Retrieved 9 December 2008.
  13. ^ "Operation Buster-Jangle". Nuclear Weapon Archive. 15 October 1997. Retrieved 18 March 2012.
  14. ^ Stephen F. Ashley. "Thorium and its role in the nuclear fuel cycle". Retrieved 16 April 2014. PDF page 8, citing: D. Holloway, "Soviet Thermonuclear Development", International Security 4:3 (1979–80) 192–197.
  15. ^ Rajat Pandit (28 August 2009). "Forces gung-ho on N-arsenal". The Times of India. Archived from the original on 21 May 2013. Retrieved 20 July 2012.
  16. ^ "India's Nuclear Weapons Program – Operation Shakti: 1998". 30 March 2001. Retrieved 21 July 2012.
  17. ^ "Historical use of thorium at Hanford" (PDF). hanfordchallenge.org. Archived from the original (PDF) on 12 May 2013. Retrieved 21 April 2018.
  18. ^ "Chronology of Important FOIA Documents: Hanford's Semi-Secret Thorium to U-233 Production Campaign" (PDF). hanfordchallenge.org. Archived from the original (PDF) on 15 October 2012. Retrieved 21 April 2018.
  19. ^ "Questions and Answers on Uranium-233 at Hanford" (PDF). radioactivist.org. Retrieved 21 April 2018.
  20. ^ "Hanford Radioactivity in Salmon Spawning Grounds" (PDF). clarku.edu. Retrieved 21 April 2018.
  21. ^ a b Robert Alvarez. "Managing the Uranium-233 Stockpile of the United States" (PDF). Science and Global Security.
  22. ^ Nuclear Materials FAQ
  23. ^ US patent 4393510 
  24. ^ SA LFTR Energy (Pty.) Ltd. "The Superior Design Advantages over All Other Nuclear Reactor Designs of the Liquid Fluoride Thorium Reactor (LFTR), with an Emphasis on Its Anti-Proliferation Features" (PDF). The South Africa Independent LFTR Power Producer Project. p. 10.
  25. ^ "Abundance in Earth's Crust: periodicity". WebElements.com. Archived from the original on 23 May 2008. Retrieved 12 April 2014.
  26. ^ "It's Elemental — The Periodic Table of Elements". Jefferson Lab. Archived from the original on 29 April 2007. Retrieved 14 April 2007.

Uranium-233 is an
isotope of uranium
Decay product of:
plutonium-237 (α)
Decay chain
of uranium-233
Decays to:
thorium-229 (α)