Uranium-233

Uranium-233 ( 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 159,200 years to alpha decay and is a part of the neptunium decay chain.

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 about 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a longer half-life of about 27 days to further decay 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 ²³³U window, the next fissile target is ²³⁵U, meaning a total of 4 neutrons needed to trigger fission).

²³³U 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).

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/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.
• The Soviet Union detonated its first hydrogen bomb the same year, the RDS-37, which contained a fissile core of ²³⁵U and ²³³U.
• In 1998, as part of its Pokhran-II tests, India detonated an experimental ²³³U 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 ²³³U.

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.

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.

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).

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.

Further information

Thorium, from which ²³³U is bred, is roughly three to four times more abundant in the Earth's crust than uranium. 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. The isotope 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. However, producing the medical isotopes from U practically requires separating the long-lived intermediate thorium-229; the stock of that isotope is very limited and there are no plans to increase it.

See also

• Breeder reactor
• Liquid fluoride thorium reactor

Notes