Thorium-232

Thorium-232(²³²
Th
) is the main naturally occurringisotopeofthorium,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 byAlpha decaytoradium-228;itsdecay chainterminates at stablelead-208.

Thorium-232,²³²Th

General
Symbol	232Th
Names	thorium-232, 232Th, Th-232
Protons(Z)	90
Neutrons(N)	142
Nuclide data
Natural abundance	99.98%
Half-life(t1/2)	1.4×1010years
Isotope mass	232.0380536Da
Spin	0+
Parent isotopes	236U(α) 232Ac(β−)
Decay products	228Ra
Decay modes
Decay mode	Decay energy(MeV)
Alpha decay	4.0816
Isotopes of thorium Complete table of nuclides

Thorium-232 is afertile material;it can capture a neutron to form thorium-233, which subsequently undergoes two successivebeta decaystouranium-233,which isfissile.As such, it has been used in thethorium fuel cyclein 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

Thehalf-lifeof thorium-232 (14 billion years) is more than three times theage of the Earth;thorium-232 therefore occurs in nature as aprimordial nuclide.Other thorium isotopes occur in nature in much smaller quantities as intermediate products in thedecay chainsofuranium-238,uranium-235,and thorium-232.

Some minerals that contain thorium includeapatite,sphene,zircon,allanite,monazite,pyrochlore,thorite,andxenotime.

Decay

The 4ndecay chainof²³²Th, commonly called the "thorium series"

Thorium-232 has a half-life of 14 billion years and mainly decays byAlpha decaytoradium-228with adecay energyof 4.0816MeV.The decay chain follows thethorium series,which terminates at stablelead-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.

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	1.4×1010a	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	212Po 208Tl
212Po	α	294.4 ns	8.954	208Pb
208Tl	β−	3.053 min	4.999	208Pb
208Pb	stable	.	.	.

Rare decay modes

Although thorium-232 mainly decays by Alpha decay, it also undergoesspontaneous fission1.1×10⁻⁹% of the time.In addition, it is capable of cluster decay,splitting intoytterbium-182,neon-24,andneon-26;the upper limit for the branching ratio of this decay mode is 2.78×10⁻¹⁰%.Double beta decaytouranium-232is also theoretically possible, but has not been observed.

Use in nuclear power

Main articles:Thorium-based nuclear powerandThorium fuel cycle

Thorium-232 is notfissile;it therefore cannot be used directly as fuel innuclear reactors.However,²³²
Th
isfertile:it can capture a neutron to form²³³
Th
,which undergoesbeta decaywith a half-life of 21.8 minutes to²³³
Pa
.This nuclide subsequently undergoes beta decay with a half-life of 27 days to formfissile²³³
U
.

One potential advantage of a thorium-based nuclear fuel cycle is that thorium is three times more abundant thanuranium,the current fuel for commercial nuclear reactors. It is also more difficult to produce material suitable fornuclear weaponsfrom the thorium fuel cycle compared to the uranium fuel cycle. Some proposed designs for thorium-fueled nuclear reactors include themolten salt reactorand afast 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 asIndiahave actively pursued thorium-based nuclear power.