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An ampoule containing solidified pieces of a
FLiBe and uranium-233 tetrafluoride mixture

Name, symbol Uranium-233,233U
Neutrons 141
Protons 92
Nuclide data
Half-life 160,000 years[1]
Parent isotopes 237Pu (α)
233Np (β+)
233Pa (β)
Decay products 229Th

Uranium-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 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[why?].

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.

Fissile material

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] Historical production costs, estimated from the costs of plutonium production, were 2–4 million USD/kg. There are few reactors remaining in the world with significant capabilities to produce more uranium-233.

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).[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, though its critical mass is still about 50% larger than for plutonium-239. The main difference is the unavoidable co-presence of uranium-232[7] which can make uranium-233 very dangerous to work on and quite easy to detect.

While it is thus possible to use uranium-233 as the fissile material of a nuclear weapon, speculation[8] aside, there is scant publicly available information on this isotope actually having been weaponized:

  • The United States detonated an experimental device in the 1955 Operation Teapot "MET" test which used a plutonium/U-233 composite pit; its design was based on the plutonium/U-235 pit from the TX-7E, a prototype Mark 7 nuclear bomb design used in the 1951 Operation Buster-Jangle "Easy" test. Although not an outright fizzle, MET's actual yield of 22 kilotons was sufficiently below the predicted 33 kt that the information gathered was of limited value.[9][10]

The B Reactor and others at the Hanford Site optimized for the production of weapons-grade material have been used to manufacture U-233.[14][15][16][17]

U-232 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. The thallium-208 being the strongest at 2.6 MeV.

232U (α, 68.9 years)
228Th (α, 1.9 year)
224Ra (α, 5.44 MeV, 3.6 day, 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 m, 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 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 65000 ppm, and the analogous Pu-238 was produced in levels of 0.5% (5000 ppm) or less). Gun-type fission weapons additionally need low levels (1 ppm range) of light impurities, to keep the neutron generation low.[7][18]

The Molten-Salt Reactor Experiment (MSRE) used U-233, bred in light water reactors such as Indian Point Energy Center, that was about 220 ppm U-232.[19]

Further information

Thorium, from which U-233 is bred, is roughly three to four times more abundant in the earth's crust than uranium.[20][21] 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. ^ http://www.doh.wa.gov/portals/1/Documents/Pubs/320-086_u233han_fs.pdf
  2. ^ a b c C. W. Forsburg and L. C. Lewis (1999-09-24). "Uses For Uranium-233: What Should Be Kept for Future Needs?" (PDF). ORNL-6952. Oak Ridge National Laboratory. 
  3. ^ UP (29 September 1946). "Atomic Energy 'Secret' Put into Language That Public Can Understand". Pittsburgh Press. Retrieved 18 October 2011. 
  4. ^ UP (21 October 1946). "Third Nuclear Source Bared". The Tuscaloosa News. Retrieved 18 October 2011. 
  5. ^ Orth, D.A. (1978-06-01). "Savannah River Plant Thorium Processing Experience". 43. Nuclear Technology: 63. 
  6. ^ http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html
  7. ^ 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. Retrieved 10 October 2012.  "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."
  8. ^ 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.
  9. ^ "Operation Teapot". Nuclear Weapon Archive. 15 October 1997. Retrieved 2008-12-09. 
  10. ^ "Operation Buster-Jangle". Nuclear Weapon Archive. 15 October 1997. Retrieved 2012-03-18. 
  11. ^ 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.
  12. ^ Rajat Pandit (28 Aug 2009). "Forces gung-ho on N-arsenal". The Times Of India. Retrieved 20 July 2012. 
  13. ^ "India's Nuclear Weapons Program - Operation Shakti: 1998". nuclearweaponarchive.org. 30 March 2001. Retrieved 21 July 2012. 
  14. ^ Historical use of thorium at Hanford
  15. ^ Chronology of Important FOIA Documents: Hanford’s Semi-Secret Thorium to U-233 Production Campaign
  16. ^ Questions and Answers on Uranium-233 at Hanford
  17. ^ Hanford Radioactivity in Salmon Spawning Grounds
  18. ^ Nuclear Materials FAQ
  19. ^ [1] (see PDF page 10)
  20. ^ "Abundance in Earth's Crust: periodicity". WebElements.com. Retrieved 2014-04-12. 
  21. ^ "It's Elemental — The Periodic Table of Elements". Jefferson Lab. Archived from the original on 29 April 2007. Retrieved 2007-04-14. 

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