Uranium-233

Uranium-233, ²³³U

General
Symbol	233U
Names	uranium-233, 233U, U-233
Protons (Z)	92
Nuclide data
Half-life (t1/2)	160,000 years
Parent isotopes	237Pu (α) 233Np (β+) 233Pa (β−)
Decay products	229Th
Isotopes of uranium Complete table of nuclides

Uranium-233 is a fissile artificial isotope of uranium, part of the thorium fuel cycle which has been used in a few 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, although the proportion of nonfissions is smaller than for the other common fission fuels, uranium-235, plutonium-239, and plutonium-241. It is slightly smaller at all neutron energies.

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.

Source	Average energy released (MeV)
Instantaneously released energy
Kinetic energy of fission fragments	168.2
Kinetic energy of prompt neutrons	4.9
Energy carried by prompt γ-rays	7.7
Energy from decaying fission products
Energy of β−-particles	5.2
Energy of anti-neutrinos	6.9
Energy of delayed γ-rays	5.0
Sum	197.9
Energy released when those prompt neutrons which don't (re)produce fission are captured	9.1
Energy converted into heat in an operating thermal nuclear reactor	200.1

Breeding uranium-233 from thorium feedstock is the long-term strategy of the nuclear power program of India, which has substantial thorium reserves. Breeding can be done in either fast reactors or thermal reactors, unlike uranium-based fuel cycles which require the superior neutron economy of a fast reactor in order to breed, that is to produce more fissile material than is consumed. Outside of India, interest in the thorium-based fuel cycle is not great, although the world's reserves of thorium are three times those of uranium.

It is also possible to use uranium-233 as the fission fuel of a nuclear weapon, although this has been done only occasionally. The United States first tested U-233 as part of a bomb core in Operation Teapot in 1955. Uranium-233 compares roughly to plutonium-239: its radioactivity is only one seventh (159,200 years half-life versus 24,100 years), but its bare critical mass is 60% higher (16 kg versus 10 kg), and its spontaneous fission rate is twenty times higher (6×10E−9 versus 3×10E−10) — but since the radioactivity is lower, the neutron density is only three times higher. A nuclear explosive device based on uranium-233 is therefore more of a technical challenge than with plutonium, but the technological level involved is roughly the same. The main difference is the co-presence of uranium-232, that makes uranium-233 very dangerous to work on, and quite easy to detect.

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:

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

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

The decay chain of ²³²U quickly yields strong gamma radiation emitters:

²³²U (α, 72 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 s, 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 thorium-228, radium-224, radon-220, and polonium) and instead requiring remote manipulation for fuel fabrication.

The decay chain of ²³³U itself is in the neptunium series. The radioisotope bismuth-213 is a decay product of uranium-233. Bismuth-213 has promise for the treatment of certain types of cancer, including acute myeloid leukemia and cancers of the pancreas, kidneys and other organs.

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. These were produced at the Hanford Site and Savannah River Site in reactors that were designed for the production of plutonium-239. Historical production costs, estimated from the costs of plutonium production, were $2-4 million / kg. There are few reactors remaining in the world with significant capabilities to produce more uranium-233.

Uses

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, use in nuclear weapons and nuclear weapon research, and investigation of the thorium fuel cycle.