Uranium-233

Uranium-233
An ampoule containing solidified pieces of a FLiBe and uranium-233 tetrafluoride mixture Full table
General
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.

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

Fissile material

Molten-Salt Reactor Experiment

Shippingport Atomic Power Station

German THTR-300

In 1946 the public first became informed of U-233 bred from thorium as "a third available source of nuclear energy and atom bombs" (in addition to U-235 and Pu-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 U-233 generates 197.9 MeV = 3.171 × 10⁻¹¹ J, i.e. 19.09 TJ/mol = 81.95 TJ/kg.^[6]

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 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 that the information gathered was of limited value.^[9][10]

• The Soviet Union detonated its first hydrogen bomb the same year, the RDS-37, which contained a fissile core of U-235 and U-233.^[11]
• In 1998, as part of its Pokhran-II tests, India detonated an experimental U-233 device of low-yield (0.2 kt) called Shakti V.^[12][13]

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:

²³²U (α, 68.9 years)

²²⁸Th (α, 1.9 year)

²²⁴Ra (α, 3.6 day, 0.24 MeV)

²²⁰Rn (α, 55 s, 0.54 MeV)

²¹⁶Po (α, 0.15 s)

²¹²Pb (β−, 10.64 h)

²¹²Bi (α, 61 m, 0.78 MeV)

²⁰⁸Tl (β−, 3 m, 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 in the neptunium series.

Uses for uranium-233 include the production of medical isotopes actinium-225 and bismuth-213, 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

• Breeder reactor
• Liquid fluoride thorium reactor

Notes