Thorium-232

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Thorium-232, 232Th
General
Decay mode
Decay energy (MeV)
alpha decay4.0816[3]
Isotopes of thorium
Complete table of nuclides

Thorium-232 (232
Th
) is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay chain terminates at stable lead-208.

Thorium-232 is a fertile material; it can capture a neutron to form thorium-233, which subsequently undergoes two successive beta decays to uranium-233, which is fissile. As such, it has been used in the thorium fuel cycle in nuclear reactors; various prototype thorium-fueled reactors have been designed; however, as of 2024, thorium has not been used for commercial-scale nuclear power.

Natural occurrence

The

age of the Earth; thorium-232 therefore occurs in nature as a primordial nuclide. Other thorium isotopes occur in nature in much smaller quantities as intermediate products in the decay chains of uranium-238, uranium-235, and thorium-232.[4]

Some minerals that contain thorium include

Decay

Ball-and-arrow presentation of the thorium decay series
The 4n decay chain of 232Th, commonly called the "thorium series"

Thorium-232 has a half-life of 14 billion years and mainly decays by

thorium series, which terminates at stable lead-208. The intermediates in the thorium-232 decay chain are all relatively short-lived; the longest-lived intermediate decay products are radium-228 and thorium-228, with half lives of 5.75 years and 1.91 years, respectively. All other intermediate decay products have half lives of less than four days.[5]

The following table lists the intermediate decay products in the thorium-232 decay chain:

nuclide decay mode half-life
(a=year)
energy released, MeV product of decay
232Th α 1.4×1010 a 4.081
228Ra
228Ra β 5.75 a 0.046
228Ac
228Ac β 6.15 h 2.134
228Th
228Th α 1.9116 a 5.520
224Ra
224Ra α 3.6319 d 5.789
220Rn
220Rn α 55.6 s 6.405
216Po
216Po α 0.145 s 6.906
212Pb
212Pb β 10.64 h 0.569
212Bi
212Bi β 64.06%
α 35.94%
60.55 min 2.252
6.207
208Tl
212Po α 294.4 ns[1] 8.954 [3]
208Pb
208Tl β 3.053 min 4.999 [3] 208Pb
208Pb stable . . .

Rare decay modes

Although thorium-232 mainly decays by alpha decay, it also undergoes spontaneous fission 1.1×10−9% of the time.[3] In addition, it is capable of

neon-26; the upper limit for the branching ratio of this decay mode is 2.78×10−10%. Double beta decay to uranium-232 is also theoretically possible, but has not been observed.[1]

Use in nuclear power

Thorium-232 is not fissile; it therefore cannot be used directly as fuel in nuclear reactors. However, 232
Th
is fertile: it can capture a neutron to form 233
Th
, which undergoes beta decay with a half-life of 21.8 minutes to 233
Pa
. This nuclide subsequently undergoes beta decay with a half-life of 27 days to form fissile 233
U
.[4]

One potential advantage of a thorium-based nuclear fuel cycle is that thorium is more abundant in nature than

fast neutron reactor, among others. Although thorium-based nuclear reactors have been proposed since the 1960s and several prototype reactors have been built, there has been relatively little research on the thorium fuel cycle compared to the more established uranium fuel cycle; thorium-based nuclear power has not seen large-scale commercial use as of 2024. Nevertheless, some countries such as India have actively pursued thorium-based nuclear power.[4]

References


Lighter:
thorium-231
Thorium-232 is an
isotope of thorium
Heavier:
thorium-233
actinium-232
Decay chain
of thorium-232
radium-228