# Uranium-233

Uranium-233 ($^{233}$ U 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. 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, to maintain the neutron economy (if it misses the $^{233}$U window, the next fissile target is $^{235}$U, meaning a total of 4 neutrons needed to trigger fission).
$^{233}$U usually fissions on neutron absorption, but sometimes retains the neutron, becoming uranium-234. For both thermal neutrons and fast neutrons, 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

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.
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.

## 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).
| 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

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). 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.
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.
The co-presence of uranium-232 can complicate the manufacture and use of uranium-233, though the Livermore memo indicates a likelihood that this complication can be worked around.
While it is thus possible to use uranium-233 as the fissile material of a nuclear weapon, speculation 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/$^{233}$U composite pit; its design was based on the plutonium/$^{235}$U 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.
* The Soviet Union detonated its first hydrogen bomb the same year, the RDS-37, which contained a fissile core of $^{235}$U and $^{233}$U.
* In 1998, as part of its Pokhran-II tests, India detonated an experimental $^{233}$U device of low-yield (0.2 kt) called Shakti V.
The B Reactor and others at the Hanford Site optimized for the production of weapons-grade material have been used to manufacture $^{233}$U.
Overall the United States is thought to have produced two tons of $^{233}$U, of various levels of purity, some with $^{232}$U impurity content as low as 6 ppm.

### $^{232}$U impurity

Production of $^{233}$U (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:
    $^{232}$Th (n,γ) → $^{233}$Th (β$^−$) → $^{233}$Pa (β$^−$) → $^{233}$U (n,2n) → $^{232}$U
    $^{232}$Th (n,γ) → $^{233}$Th (β$^−$) → $^{233}$Pa (n,2n) → $^{232}$Pa (β$^−$)→ $^{232}$U
    $^{232}$Th (n,2n) → $^{231}$Th (β$^−$) → $^{231}$Pa (n,γ) → $^{232}$Pa (β$^−$) → $^{232}$U
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:
    $^{230}$Th (n,γ) → $^{231}$Th (β$^−$) → $^{231}$Pa (n,γ) → $^{232}$Pa (β$^−$) → $^{232}$U
The decay chain of $^{232}$U quickly yields strong gamma radiation emitters. Thallium-208 is the strongest of these, at 2.6 MeV:
    $^{232}$U (α, 68.9 y)
    $^{228}$Th (α, 1.9 y)
    $^{224}$Ra (α, 5.44 MeV, 3.6 d, with a γ of 0.24 MeV)
    $^{220}$Rn (α, 6.29 MeV, 56 s, with a γ of 0.54 MeV)
    $^{216}$Po (α, 0.15 s)
    $^{212}$Pb (β$^−$, 10.64 h)
    $^{212}$Bi (α, 61 min, 0.78 MeV)
    $^{208}$Tl (β$^−$, 1.8 MeV, 3 min, with a γ of 2.6 MeV)
    $^{208}$Pb (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 $^{232}$U levels below 50 ppm (above which the $^{233}$U is considered "low grade"; cf. "Standard weapon grade plutonium requires a $^{240}$Pu content of no more than 6.5%." which is 65,000 ppm, and the analogous $^{238}$Pu 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.
The production of "clean" $^{233}$U, low in $^{232}$U, requires a few factors: 1) obtaining a relatively pure $^{232}$Th source, low in $^{230}$Th (which also transmutes to $^{232}$U), 2) moderating the incident neutrons to have an energy not higher that 6 MeV (too-high energy neutrons cause the $^{232}$Th (n,2n) → $^{231}$Th reaction) and 3) removing the thorium sample from neutron flux before the $^{233}$U concentration builds up to a too high level, in order to avoid fissioning the $^{233}$U itself (which would produce energetic neutrons).
The Molten-Salt Reactor Experiment (MSRE) used $^{233}$U, bred in light water reactors such as Indian Point Energy Center, that was about 220 ppm $^{232}$U.

## Further information

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