Uranium-233

Nuclear fuel

Uranium-233 has been used as fuel in several reactor types and has been proposed for use in new designs. 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.

In 1968, the Molten-Salt Reactor Experiment at Oak Ridge became the first reactor to operate on uranium-233.

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 average energy released per fission of uranium-233 is about 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.

Chuck Hansen wrote in Swords of Armageddon that ²³³U was evaluated by the United States for its ability to substitute for supergrade plutonium in nuclear weapons. As it was deemed equivalent to plutonium rather than superior to it, military interest in ²³³U-based weapons waned in the late 1960s. However, several ²³³U-based nuclear weapons were tested by the US between 1955 and 1968. The Rocky Flats Plant, which refined fissile materials for weapons production, was refining ²³³U until the early 1980s.

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 United States conducted several additional weapons tests with ²³³U for unknown purposes.
• 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. The United States Atomic Energy Commission used the weapons production reactors at Hanford to breed significant quantities of ²³³U from thorium for use in both nuclear weapons and for civilian research. 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 that contained 222 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. Bismuth-213 has been investigated for use in cancer treatment. However, producing the medical isotopes from ²³³U practically requires separating the long-lived intermediate thorium-229. At Oak Ridge, thorium-229 is recovered from stored uranium-233 and supplied to TerraPower Isotopes for the production of actinium-225 used in cancer-treatment research.