Uranium-233

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]

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]

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 United States detonated an experimental device in the 1955 Operation Teapot "MET" test which used a plutonium/²³³U composite pit; its design was based on the plutonium/²³⁵U 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.^[12][13]
• The Soviet Union detonated its first hydrogen bomb the same year, the RDS-37, which contained a fissile core of ²³⁵U and ²³³U.^[14]
• In 1998, as part of its Pokhran-II tests, India detonated an experimental ²³³U device of low-yield (0.2 kt) called Shakti V.^[15][16]

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]

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.

• Breeder reactor
• Liquid fluoride thorium reactor