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Isotopes of thorium
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Isotopes of thorium
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Isotopes of thorium (90Th)
Main isotopes[1] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
227Th trace 18.693 d α 223Ra
228Th trace 1.9125 y α 224Ra
229Th trace 7916 y α 225Ra
230Th 0.02% 75400 y α 226Ra
231Th trace 25.52 h β− 231Pa
232Th 100.0% 1.40×1010 y α 228Ra
233Th trace 21.83 min β− 233Pa
234Th trace 24.11 d β− 234Pa
Standard atomic weight Ar°(Th)

Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is relatively stable, with a half-life of 1.40×1010 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of 230Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being 232Th, 230Th with a half-life of 75,400 years, 229Th with a half-life of 7,916 years, and 228Th with a half-life of 1.91 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy,[4] recently measured to be 8.355733554021(8) eV[5][6] It has been proposed to perform laser spectroscopy of the 229Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.[7][8][9]

The known isotopes of thorium range in mass number from 207[10] to 238.

List of isotopes

[edit]

Nuclide
[n 1] 		Historic
name 		Z N Isotopic mass (Da)[11]
[n 2][n 3] 		Half-life[1]
[n 4] 		Decay
mode[1]
[n 5] 		Daughter
isotope
[n 6] 		Spin and
parity[1]
[n 7][n 8] 		Natural abundance (mole fraction)
Excitation energy Normal proportion[1] Range of variation
207Th[10] 90 117 9.7+46.6
−4.4 ms 		α 203Ra
208Th 90 118 208.017915(34) 2.4(12) ms α 204Ra 0+
209Th 90 119 209.017998(27) 3.1(12) ms α 205Ra 13/2+
210Th 90 120 210.015094(20) 16.0(36) ms α 206Ra 0+
211Th 90 121 211.014897(92) 48(20) ms α 207Ra 5/2−#
212Th 90 122 212.013002(11) 31.7(13) ms α 208Ra 0+
213Th 90 123 213.0130115(99) 144(21) ms α 209Ra 5/2−
213mTh 1180.0(14) keV 1.4(4) μs IT 213Th (13/2)+
214Th 90 124 214.011481(11) 87(10) ms α 210Ra 0+
214mTh 2181.0(27) keV 1.24(12) μs IT 214Th 8+#
215Th 90 125 215.0117246(68) 1.35(14) s α 211Ra (1/2−)
215mTh 1471(50)# keV 770(60) ns IT 215Th 9/2+#
216Th 90 126 216.011056(12) 26.28(16) ms α 212Ra 0+
216m1Th 2041(8) keV 135.4(29) μs IT (97.2%) 216Th 8+
α (2.8%) 212Ra
216m2Th 2648(8) keV 580(26) ns IT 216Th (11−)
216m3Th 3682(8) keV 740(70) ns IT 216Th (14+)
217Th 90 127 217.013103(11) 248(4) μs α 213Ra 9/2+#
217m1Th 673.3(1) keV 141(50) ns IT 217Th (15/2−)
217m2Th 2307(32) keV 71(14) μs IT 217Th (25/2+)
218Th 90 128 218.013276(11) 122(5) ns α 214Ra 0+
219Th 90 129 219.015526(61) 1.023(18) μs α 215Ra 9/2+#
220Th 90 130 220.015770(15) 10.2(3) μs α 216Ra 0+
221Th 90 131 221.0181858(86) 1.75(2) ms α 217Ra 7/2+#
222Th 90 132 222.018468(11) 2.24(3) ms α 218Ra 0+
223Th 90 133 223.0208111(85) 0.60(2) s α 219Ra (5/2)+
224Th 90 134 224.021466(10) 1.04(2) s α[n 9] 220Ra 0+
225Th 90 135 225.0239510(55) 8.75(4) min α (~90%) 221Ra 3/2+
EC (~10%) 225Ac
226Th 90 136 226.0249037(48) 30.70(3) min α 222Ra 0+
CD (<3.2×10−12%) 208Pb
18O
227Th Radioactinium 90 137 227.0277025(22) 18.693(4) d α 223Ra (1/2+) Trace[n 10]
228Th Radiothorium 90 138 228.0287397(19) 1.9125(7) y α 224Ra 0+ Trace[n 11]
CD (1.13×10−11%) 208Pb
20O
229Th 90 139 229.0317614(26) 7916(17) y α 225Ra 5/2+ Trace[n 12]
229mTh 8.355733554021(8) eV[6] 7(1) μs[12] IT[n 13] 229Th+ 3/2+
229mTh+ 8.355733554021(8) eV[6] 29(1) min[13] γ[n 13] 229Th+ 3/2+
230Th[n 14] Ionium 90 140 230.0331323(13) 7.54(3)×104 y α 226Ra 0+ 0.0002(2)[n 15]
CD (5.8×10−11%) 206Hg
24Ne
SF (<4×10−12%) (various)
231Th Uranium Y 90 141 231.0363028(13) 25.52(1) h β− 231Pa 5/2+ Trace[n 10]
232Th[n 16] Thorium 90 142 232.0380536(15) 1.40(1)×1010 y α[n 17] 228Ra 0+ 0.9998(2)
SF (1.1 × 10−9%) (various)
CD (<2.78×10−10%) 208,206Hg
24,26Ne
233Th 90 143 233.0415801(15) 21.83(4) min β− 233Pa 1/2+ Trace[n 18]
234Th Uranium X1 90 144 234.0435998(28) 24.107(24) d β− 234mPa[14] 0+ Trace[n 15]
235Th 90 145 235.047255(14) 7.2(1) min β− 235Pa 1/2+#
236Th 90 146 236.049657(15) 37.3(15) min β− 236Pa 0+
237Th 90 147 237.053629(17) 4.8(5) min β− 237Pa 5/2+#
238Th 90 148 238.05639(30)# 9.4(20) min β− 238Pa 0+
This table header & footer:
  1. ^ mTh – Excited nuclear isomer.
  2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. ^ Bold half-life – nearly stable, half-life longer than age of universe.
  5. ^ Modes of decay:
    EC: Electron capture
    CD: Cluster decay
    IT: Isomeric transition
  6. ^ Bold symbol as daughter – Daughter product is stable.
  7. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  8. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  9. ^ Theorized to also undergo β+β+ decay to 224Ra
  10. ^ a b Intermediate decay product of 235U
  11. ^ Intermediate decay product of 232Th
  12. ^ Intermediate decay product of 237Np
  13. ^ a b Neutral 229mTh decays rapidly by internal conversion, ejecting an electron. There is not enough energy to eject a second electron, so 229mTh+ ions live much longer, decaying by gamma emission. See § Thorium-229m.
  14. ^ Used in uranium–thorium dating
  15. ^ a b Intermediate decay product of 238U
  16. ^ Primordial radionuclide
  17. ^ Theorized to also undergo β−β− decay to 232U
  18. ^ Produced in neutron capture by 232Th

Uses

[edit]

Thorium has been suggested for use in thorium-based nuclear power.

In many countries the use of thorium in consumer products is banned or discouraged because it is radioactive.

It is currently used in cathodes of vacuum tubes, for a combination of physical stability at high temperature and a low work energy required to remove an electron from its surface.

It has, for about a century, been used in mantles of gas and vapor lamps such as gas lights and camping lanterns.

Low dispersion lenses

[edit]

Thorium was also used in certain glass elements of Aero-Ektar lenses made by Kodak during World War II. Thus they are mildly radioactive.[15] Two of the glass elements in the f/2.5 Aero-Ektar lenses are 11% and 13% thorium by weight. The thorium-containing glasses were used because they have a high refractive index with a low dispersion (variation of index with wavelength), a highly desirable property. Many surviving Aero-Ektar lenses have a tea colored tint, possibly due to radiation damage to the glass.

These lenses were used for aerial reconnaissance because the radiation level is not high enough to fog film over a short period. This would indicate the radiation level is reasonably safe. However, when not in use, it would be prudent to store these lenses as far as possible from normally inhabited areas; allowing the inverse square relationship to attenuate the radiation.[16]

Actinides vs. fission products

[edit]
Actinides and fission products by half-life
Actinides[17] by decay chain Half-life
range (a) 		Fission products of 235U by yield[18]
4n
(Thorium) 		4n + 1
(Neptunium) 		4n + 2
(Radium) 		4n + 3
(Actinium) 		4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 a 155Euþ
248Bk[19] > 9 a
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
249Cfƒ 242mAmƒ 141–351 a

No fission products have a half-life
in the range of 100 a–210 ka ...

241Amƒ 251Cfƒ[20] 430–900 a
226Ra№ 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th№ 231Pa№ 32–76 ka
236Npƒ 233Uƒ 234U№ 150–250 ka 99Tc₡ 126Sn
248Cm 242Pu 327–375 ka 79Se₡
1.33 Ma 135Cs₡
237Npƒ 1.61–6.5 Ma 93Zr 107Pd
236U 247Cmƒ 15–24 Ma 129I₡
244Pu 80 Ma

... nor beyond 15.7 Ma[21]

232Th№ 238U№ 235Uƒ№ 0.7–14.1 Ga

Notable isotopes

[edit]

Thorium-228

[edit]

228Th is an isotope of thorium with 138 neutrons. It was once named Radiothorium, due to its occurrence in the disintegration chain of thorium-232. It has a half-life of 1.9125 years. It undergoes alpha decay to 224Ra. Occasionally it decays by the unusual route of cluster decay, emitting a nucleus of 20O and producing stable 208Pb. It is a daughter isotope of 232U and responsible for its radiological hazard.

Together with its decay product 224Ra it is used for alpha particle radiation therapy.[22]

Thorium-229

[edit]

229Th is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7916 years. 229Th is produced by the decay of uranium-233, and its principal use is for the production of the medical isotopes actinium-225 and bismuth-213.[23]

Thorium-229m

[edit]

229Th has a nuclear isomer, 229m
Th, with a remarkably low excitation energy of 8.355733554021(8) eV.[6]

Due to this low energy, the lifetime of 229mTh very much depends on the electronic environment of the nucleus. In neutral 229Th, the isomer decays by internal conversion within a few microseconds.[24][25][12] However, the isomeric energy is not enough to remove a second electron (thorium's second ionization energy is 11.5 eV), so internal conversion is impossible in Th+ ions. Radiative decay occurs with a half-life 8.4 orders of magnitude longer, in excess of 1000 seconds.[25][26] Embedded in ionic crystals, ionization is not quite 100%, so a small amount of internal conversion occurs, leading to a recently measured lifetime of ≈600 s,[5][13] which can be extrapolated to a lifetime for isolated ions of 1740±50 s.[5]

This excitation energy corresponds to a photon frequency of 2020407384335±2 kHz (wavelength 148.3821828827(15) nm).[6][27][5][13] Although in the very high frequency vacuum ultraviolet frequency range, it is possible to build a laser operating at this frequency, giving the only known opportunity for direct laser excitation of a nuclear state,[28] which could have applications like a nuclear clock of very high accuracy[8][9][29][30] or as a qubit for quantum computing.[31]

These applications were for a long time impeded by imprecise measurements of the isomeric energy, as laser excitation's exquisite precision makes it difficult to use to search a wide frequency range. There were many investigations, both theoretical and experimental, trying to determine the transition energy precisely and to specify other properties of the isomeric state of 229Th (such as the lifetime and the magnetic moment) before the frequency was accurately measured in 2024.[5][27][13]

History
[edit]

Early measurements were performed via gamma ray spectroscopy, producing the 29.5855 keV excited state of 229Th, and measuring the difference in emitted gamma ray energies as it decays to either the 229mTh (90%) or 229Th (10%) isomeric states. In 1976, Kroger and Reich sought to understand coriolis force effects in deformed nuclei, and attempted to match thorium's gamma-ray spectrum to theoretical nuclear shape models. To their surprise, the known nuclear states could not be reasonably classified into different total angular momentum quantization levels. They concluded that some states previously identified as 229Th actually arose from a spin-⁠3/2⁠ nuclear isomer, 229mTh, with a remarkably low excitation energy.[32]

At that time the energy was inferred to be below 100 eV, purely based on the non-observation of the isomer's direct decay. However, in 1990, further measurements led to the conclusion that the energy is almost certainly below 10 eV,[33] making it one of the lowest known isomeric excitation energies. In the following years, the energy was further constrained to 3.5±1.0 eV, which was for a long time the accepted energy value.[34]

Improved gamma ray spectroscopy measurements using an advanced high-resolution X-ray microcalorimeter were carried out in 2007, yielding a new value for the transition energy of 7.6±0.5 eV,[35] corrected to 7.8±0.5 eV in 2009.[36] This higher energy has two consequences which had not been considered by earlier attempts to observe emitted photons:

  • Because it is above thorium's 6.08 eV first ionization energy, neutral 229mTh will decay radiatively with an extremely low likelihood, and
  • Because it is above the 6.2 eV vacuum ultraviolet cutoff, the produced photons cannot travel through air.

But even knowing the higher energy, most of the searches in the 2010s for light emitted by the isomeric decay failed to observe any signal,[37][38][39][40] pointing towards a potentially strong non-radiative decay channel. A direct detection of photons emitted in the isomeric decay was claimed in 2012[41] and again in 2018.[42] However, both reports were subject to controversial discussions within the community.[43][44]

A direct detection of electrons being emitted in the internal conversion decay channel of 229mTh was achieved in 2016.[45] However, at the time the isomer's transition energy could only be weakly constrained to between 6.3 and 18.3 eV. Finally, in 2019, non-optical electron spectroscopy of the internal conversion electrons emitted in the isomeric decay allowed for a determination of the isomer's excitation energy to 8.28±0.17 eV.[46] However, this value appeared at odds with the 2018 preprint showing that a similar signal as an 8.4 eV xenon VUV photon can be shown, but with about 1.3+0.2
−0.1 eV less energy and a (retrospectively correct) 1880±170 s lifetime.[42] In that paper, 229Th was embedded in SiO2, possibly resulting in an energy shift and altered lifetime, although the states involved are primarily nuclear, shielding them from electronic interactions.

In another 2018 experiment, it was possible to perform a first laser-spectroscopic characterization of the nuclear properties of 229mTh.[47] In this experiment, laser spectroscopy of the 229Th atomic shell was conducted using a 229Th2+ ion cloud with 2% of the ions in the nuclear excited state. This allowed probing for the hyperfine shift induced by the different nuclear spin states of the ground and the isomeric state. In this way, a first experimental value for the magnetic dipole and the electric quadrupole moment of 229mTh could be inferred.

In 2019, the isomer's excitation energy was constrained to 8.28±0.17 eV based on the direct detection of internal conversion electrons[46] and a secure population of 229mTh from the nuclear ground state was achieved by excitation of the 29 keV nuclear excited state via synchrotron radiation.[48] Additional measurements by a different group in 2020 produced a figure of 8.10±0.17 eV (153.1±3.2 nm wavelength).[49] Combining these measurements, the expected transition energy is 8.12±0.11 eV.[50]

In September 2022, spectroscopy on decaying samples determined the excitation energy to be 8.338±0.024 eV.[51]

In April 2024, two separate groups finally reported precision laser excitation Th4+ cations doped into ionic crystals (of CaF2 and LiSrAlF6 with additional interstitial F− anions for charge compensation), giving a precise (~1 part per million) measurement of the transition energy.[27][7][5][13] A one-part-per-trillion (10−12) measurement soon followed in June 2024,[6][52] and future high-precision lasers will measure the frequency up to the 10−18 accuracy of the best atomic clocks.[6][9][30]

Thorium-230

[edit]

230Th is a radioactive isotope of thorium that can be used to date corals (uranium-thorium dating) and determine ocean current flux. Ionium (symbol Io) was the name given early in the study of radioactive elements to the 230Th isotope produced in the decay chain of 238U before the nature of isotopes was fully realized. The name is still used in ionium–thorium dating, another dating method using this isotope.

Thorium-231

[edit]

231Th has 141 neutrons. It is the decay product of uranium-235. It is found in very small amounts on the earth and has a half-life of 25.52 hours.[53] When it decays, it emits a beta ray and forms protactinium-231, with a decay energy of 0.39 MeV.

Thorium-232

[edit]
Main article: Thorium-232

232Th is the only primordial nuclide of thorium and makes up effectively all of natural thorium, with other isotopes of thorium appearing only in trace amounts as relatively short-lived decay products of uranium and thorium.[54] The isotope decays by alpha decay with a half-life of 1.40×1010 years, over three times the age of the Earth and approximately the age of the universe. Its decay chain is the thorium series, eventually ending in lead-208. The remainder of the chain is quick; the longest half-lives in it are 5.75 years for radium-228 and 1.91 years for thorium-228, with all other half-lives totaling less than a week.

232Th is a fertile material able to absorb a neutron and undergo transmutation into the fissile nuclide uranium-233, which is the basis of the thorium fuel cycle.[55] In the form of Thorotrast, a thorium dioxide suspension, it was used as a contrast medium in early X-ray diagnostics. Thorium-232 is now classified as carcinogenic.[56]

Thorium-233

[edit]

233Th is an isotope of thorium that decays into protactinium-233 through beta decay, then into uranium-233 to join the neptunium series decay chain. It has a half-life of 21.83 minutes. Traces occur in nature as the result of natural neutron activation of 232Th.[57]

Thorium-234

[edit]

234Th is an isotope of thorium whose nuclei contain 144 neutrons. 234Th has a half-life of 24.11 days; it emits a beta particle, transmuting into protactinium-234 with a decay energy around 0.27 MeV. Uranium-238 almost always produces isotope of thorium on decay (although in rare cases it undergoes spontaneous fission, and even more rarely double beta decay).

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Isotopes of thorium

View on Grokipedia
from Grokipedia
Isotopes of thorium are the radioactive nuclides of the element ( 90), distinguished by varying numbers of neutrons in their nuclei. , comprising 142 neutrons, is the only primordial and constitutes essentially all naturally occurring thorium on , with a of 1.405 × 1010 years derived from to radium-228. This longevity exceeds the planet's age, enabling its persistence in the crust at concentrations around 6 parts per million, primarily in minerals like . Of the approximately 30 known thorium isotopes, spanning masses from 210 to 239, only a few exhibit half-lives exceeding years, with the remainder decaying rapidly via alpha, beta, or spontaneous fission pathways. Notable long-lived variants include thorium-230 (half-life 75,380 years), a member of the uranium-238 decay chain, and thorium-229 (half-life about 7,900 years), which features a low-energy nuclear isomer suitable for precision timekeeping applications. Thorium-228, with a half-life of 1.91 years, arises in the thorium-232 decay series and serves as a precursor for generating short-lived medical radioisotopes such as radium-224 and lead-212, used in targeted alpha therapy for cancer treatment. Thorium-232 holds particular significance in as a that, upon , transmutes to protactinium-233 and ultimately fissile , facilitating cycles with lower transuranic waste production compared to uranium-plutonium systems. Experimental thorium-fueled reactors, such as those tested in designs, demonstrate this breeding potential, though commercial deployment remains limited by material corrosion challenges and the need for starters. Trace isotopes like thorium-227 and thorium-234 occur transiently in natural decay sequences but pose radiological hazards due to their alpha emissions and short half-lives (18.7 days and 24.1 days, respectively). Overall, thorium isotopes' properties underpin both geochemical stability and engineered applications, with empirical decay data confirming their roles in actinide evolution and energy production.

Overview

Fundamental properties

All isotopes of thorium (atomic number 90) are radioactive, with no stable nuclides due to the inherent instability of actinide nuclei from electrostatic repulsion overwhelming nuclear binding at high proton counts. Twenty-seven thorium radioisotopes have been characterized, spanning mass numbers from 210 to 236. The longest-lived is the primordial ^{232}Th, with a of 1.405 \times 10^{10} years (approximately 14.05 billion years), decaying predominantly by alpha emission to radium-228, accompanied by gamma rays from daughter excited states. Half-lives among thorium isotopes span over 15 orders of magnitude, from microseconds for neutron-excess light isotopes (e.g., produced in fission or reactions) to the extended duration of ^{232}Th, reflecting shell effects and neutron-proton pairing that enhance stability near N=142. Key longer-lived isotopes beyond ^{232}Th include ^{230}Th ( 7.54 \times 10^4 years, ) and ^{229}Th (7.34 \times 10^3 years, ), both relevant to the thorium-uranium fuel cycle and natural decay series. Shorter-lived examples, such as ^{228}Th (1.912 years, primarily with minor beta minus branch), occur as chain intermediates. Decay modes for thorium isotopes are dominated by alpha emission for proton-rich nuclides near the valley of stability, beta minus decay for neutron-rich ones, and for select heavier variants like ^{236}Th, though fission branching ratios remain low (<10^{-7} for most). These properties underpin thorium's role in natural radioactivity and potential nuclear applications, where low fission cross-sections and high alpha yields influence neutron economy in reactors.

Natural abundance and occurrence

^{232}Th constitutes virtually all naturally occurring thorium, comprising approximately 99.98% of terrestrial thorium deposits, with its primordial abundance persisting due to a half-life of 1.405 \times 10^{10} years. This isotope is present throughout the 's crust at an average concentration of about 10.7 parts per million, primarily in accessory minerals such as monazite ((Ce,La,Nd,Th)PO_4), thorite (ThSiO_4), and thorianite (ThO_2). Trace quantities of other thorium isotopes, including ^{230}Th at roughly 0.02%, occur in nature as intermediate products within the uranium-238 and thorium-232 decay series, achieving secular equilibrium with their longer-lived parents. Isotopes such as ^{228}Th, ^{227}Th, ^{231}Th, and ^{234}Th appear in even smaller amounts, generated in situ from the decay of radium and protactinium precursors in these chains, and are found at low levels in soils, rocks, waters, and biological materials wherever uranium or thorium is present. These short-lived isotopes do not accumulate significantly due to their rapid decay but serve as tracers for geochemical processes, including particle scavenging in marine environments. No other thorium isotopes occur naturally in measurable primordial quantities, as all are either extinct or too short-lived to survive from Earth's formation.

Isotopic composition

Table of isotopes

Mass numberHalf-lifeDecay mode(s)
207Th0.0097 sα
208Th1.7 msα
209Th2.5 msα
210Th1.6(4) msα, EC/β⁺
211Th37 msα, ε
212Th31.7(13) msα, EC/β⁺
213Th144(21) msα
214Th87(10) msα
215Th1.2(2) sα
216Th26.0(2) msα
217Th0.252(4) msα
218Th117(9) nsα
219Th1.05(3) μsα
220Th9.7(6) μsα, EC
221Th1.74(3) msα
222Th2.24(3) msα
223Th0.60(2) sα
224Th1.04(2) sα
225Th8.75(4) minα, EC
226Th30.57(10) minα
227Th18.697(7) dα
228Th1.9116(16) aα
229Th7880(120) aα
230Th7.54(3) × 10⁴ aα, SF
231Th25.57(8) hβ⁻
232Th1.40(1) × 10¹⁰ aα, SF
233Th21.83(4) minβ⁻
234Th24.10(3) dβ⁻
235Th7.2(1) minβ⁻
236Th37.5(2) minβ⁻
237Th4.8(5) minβ⁻
238Th9.4(20) minβ⁻
Data sourced from nuclear databases including NNDC.

Stability and decay characteristics

All isotopes of thorium are radioactive, with no stable nuclides observed among the approximately 30 known variants spanning mass numbers from 209 to 238. Half-lives range from fractions of a second for lighter, neutron-deficient isotopes to 1.405 × 10^{10} years for ^{232}Th, the longest-lived thorium isotope. Alpha decay predominates as the decay mode for neutron-rich thorium isotopes near and above the line of stability, reflecting the even-odd nucleon pairing and high fission barriers in actinides; this process emits an alpha particle (helium-4 nucleus) to form radium daughters, often accompanied by gamma emission for de-excitation. Beta-minus decay occurs in more neutron-deficient isotopes like ^{234}Th, converting a neutron to a proton and yielding protactinium. Spontaneous fission, though theoretically possible due to thorium's high atomic number, has negligible branching ratios (e.g., <10^{-9}% for ^{232}Th), rendering it insignificant compared to alpha decay. The stability increases with proximity to mass number 232, where shell effects enhance resistance to decay; isotopes lighter than ^{232}Th have progressively shorter half-lives due to higher alpha decay probabilities driven by lower fission barriers and Q-values. For instance, the principal naturally occurring or long-lived isotopes exhibit the following characteristics:
IsotopeHalf-lifePrimary decay mode(s)Daughter nuclide
^{227}Th18.72 daysα (99.3%), β⁻ (0.7%)^{223}Ra
^{228}Th1.913 yearsα (primarily)^{224}Ra
^{229}Th7,340 yearsα^{225}Ra
^{230}Th75,400 yearsα (primarily), SF (minor)^{226}Ra
^{232}Th1.405 × 10^{10} yα (primarily), SF (<10^{-9}%)^{228}Ra
^{234}Th24.10 daysβ⁻^{234}Pa
These decay properties underpin thorium's role in natural radioactive series, where alpha emitters contribute to chain progression while beta decays bridge to higher-Z elements.

Decay chains and production

Role in actinium and uranium series

In the uranium-238 decay series (also known as the radium series), thorium-234 is the immediate daughter nuclide produced via alpha decay from uranium-238, with thorium-234 subsequently undergoing beta-minus decay to protactinium-234 (primarily the metastable protactinium-234m isomer). This isotope has a half-life of 24.10 days and contributes to the ingrowth of shorter-lived daughters in natural uranium-bearing materials. Further along the chain, thorium-230 forms through alpha decay of uranium-234 and itself decays by alpha emission to radium-226, with a half-life of 75,400 years; this long-lived thorium isotope accumulates in sediments and ores, influencing geochronology via uranium-thorium dating methods. The actinium series, initiated by uranium-235 decay, incorporates thorium-231 as the direct alpha decay product of uranium-235, which then beta-decays to protactinium-231 with a half-life of 25.52 hours. Thorium-227 appears later, generated by beta-minus decay (74.3% branching ratio) or electron capture (25.7%) from actinium-227, and primarily undergoes alpha decay to radium-223 with a half-life of 18.697 days, releasing significant alpha particle energy (approximately 6.0 MeV total). These thorium isotopes serve as transient intermediaries, facilitating the progression toward stable lead-207 while contributing to the series' overall radioactive equilibrium in natural settings.

Artificial production methods

Artificial thorium isotopes are synthesized mainly through neutron irradiation in nuclear reactors and charged-particle bombardment in particle accelerators. In reactors, the primary route involves neutron capture on or precursor nuclides from decay chains. The reaction ^{232}Th(n,γ)^{233}Th produces thorium-233, an unstable intermediate (half-life 22.3 minutes) that beta-decays to protactinium-233 en route to fissile in thorium fuel cycles; this (n,γ) process has a thermal neutron cross-section of 7.4 barns and has been demonstrated in facilities like the Shippingport Light Water Breeder Reactor and the Molten Salt Reactor Experiment. Further neutron captures on or subsequent isotopes yield neutron-richer species, though most decay rapidly. Thorium-229, used in medical applications, is generated in reactors via neutron transmutation of targets such as radium-226, radium-228, actinium-227, or thorium-228, typically involving three successive neutron captures followed by two beta-minus decays (e.g., ^{226}Ra(n,γ)^{227}Ra → β⁻ → ^{227}Ac(n,γ)^{228}Ac → β⁻ → ^{228}Th(n,γ)^{229}Th). Irradiations at fluxes like those in the Oak Ridge High Flux Isotope Reactor have optimized yields from these pathways. Lighter isotopes, such as thorium-231, arise from fast-neutron reactions on , including (n,2n) processes with thresholds around 6 MeV. Accelerator-based methods employ protons or deuterons on thorium targets to induce (p,xn) or (d,xn) reactions, producing a spectrum of thorium isotopes alongside medical radionuclides like protactinium and actinium species; yields depend on beam energy (e.g., 10-30 MeV) and target thickness. Heavy-ion bombardments, such as with carbon or neon beams, synthesize neutron-deficient thorium isotopes in the mass range 221-224 via multinucleon transfer or incomplete fusion. These techniques enable production of short-lived isotopes not accessible via reactor neutrons but require high-intensity facilities like cyclotrons.

Notable isotopes

Thorium-232

Thorium-232 (²³²Th) is the sole primordial and naturally occurring isotope of thorium, comprising essentially 100% of terrestrial thorium reserves. With an atomic mass of 232.038049 u, it exhibits remarkable stability, possessing a half-life of 1.405 × 10¹⁰ years—approximately three times the age of the Earth—and decays predominantly via alpha particle emission to radium-228 (²²⁸Ra), releasing 4.083 MeV of energy, alongside a minor spontaneous fission branch. This longevity renders it a primordial radionuclide, persisting from the solar system's formation without significant depletion. In the Earth's crust, thorium-232 occurs at concentrations roughly four times those of uranium, averaging 6 parts per million, primarily in accessory minerals like monazite and thorite within granitic rocks, pegmatites, and heavy mineral sands. It is recovered commercially as a byproduct of rare earth element extraction from monazite deposits, with major sources in India, Australia, and Brazil; global identified resources exceed 6 million tonnes. Unlike uranium isotopes, thorium-232 requires no enrichment for natural abundance, as trace isotopes like ²³⁰Th arise solely from uranium decay chains rather than independent primordial stocks. As a fertile rather than fissile material, thorium-232 plays a central role in the thorium-uranium fuel cycle for nuclear reactors. Neutron capture on ²³²Th yields ²³³Th, which beta-decays (half-life 22 minutes) to protactinium-233 (half-life 27 days), followed by another beta decay to fissile uranium-233 (²³³U), enabling breeding in thermal or fast spectrum reactors. This process leverages thorium's higher abundance and lower radiotoxicity in spent fuel compared to uranium-plutonium cycles, though proliferation risks from ²³³U and protactinium separation challenges persist. Experimental reactors, such as India's AHWR and China's TMSR programs, have demonstrated Th-232 utilization since the 1990s, confirming neutron economy advantages in molten salt designs.

Thorium-233

Thorium-233 (²³³Th) is an artificially produced, radioactive isotope of thorium comprising 90 protons and 143 neutrons in its nucleus. It possesses a nuclear spin of 1/2+ in its ground state. This isotope forms primarily through neutron capture on , the dominant natural isotope of thorium: ²³²Th + n → ²³³Th. This reaction occurs in nuclear reactors as the initial step in the thorium-uranium fuel cycle, where the goal is to breed fissile . Thorium-233 decays exclusively via beta minus emission to protactinium-233 (²³³Pa), with a precisely measured half-life of 21.83 ± 0.04 minutes and a total decay energy of 1.245 MeV. The short half-life ensures rapid transformation without significant accumulation, minimizing its direct role beyond serving as a transient precursor to ²³³Pa, which itself beta decays (half-life 26.97 days) to uranium-233. Owing to its fleeting existence and beta decay characteristics, thorium-233 lacks independent applications outside the breeding process and is not observed in natural thorium deposits or decay chains. It can also arise as a decay product from actinium-233 beta decay, though such occurrences are negligible in practical contexts.

Thorium-229

Thorium-229 is a radioactive isotope of thorium that decays primarily by alpha emission to radium-225. Its ground-state half-life has been measured as 7825 ± 87 years. The isotope occurs in trace quantities as part of the neptunium-237 decay series, originating as a granddaughter product of decay, though its natural abundance is negligible due to the rarity of precursor isotopes. For research and applications, thorium-229 is produced artificially, often through the alpha decay of mass-separated or via neutron irradiation of radium-226, radium-228, actinium-227, or thorium-228 targets in reactors such as the Oak Ridge High Flux Isotope Reactor. It is typically supplied in nitrate form in dilute nitric acid solution with high radionuclidic purity (>99%). Thorium-229 is notable for its low-energy nuclear isomer, 229mTh, which lies approximately 8.3 eV above the ground state, corresponding to a vacuum-ultraviolet of about 149 nm. The isomer's excitation energy has been refined through multiple methods, including electron detection and radiative decay measurements, with a precise value of 8.338(24) eV reported in 2020 and further improved to 8.35574(3) eV via laser spectroscopy in 2024. The isomeric state was first conjectured in 1976 through of higher isotopes, with direct detection of its decay achieved in 2016 using ion traps. The half-life of the isomer varies by charge state: around 7-10 seconds for neutral or singly charged thorium atoms due to dominant , but extending to minutes or longer (e.g., ~13-45 minutes) in highly charged ions or doped where electronic conversion is suppressed. This anomalously low transition energy and controllable lifetime position 229mTh as a leading candidate for nuclear clocks, potentially surpassing optical atomic clocks in precision by orders of magnitude while being insensitive to many environmental perturbations. Breakthroughs include the first excitation of the isomer in using a broadband vacuum-ultraviolet source tuned to 148.38 nm, enabling direct optical control and paving the way for standards and tests of fundamental physics. Applications extend to thorium-doped thin films for compact clock prototypes, reducing material needs and radioactivity concerns.

Thorium-230

Thorium-230 (^230Th) is a radioactive of with 230, occurring naturally as an intermediate in the decay series, where it forms via of uranium-234. It possesses a of 75,380 years and decays predominantly by emission to radium-226, accompanied by low-energy gamma rays. The decay energy is approximately 4.687 MeV for the alpha transition. In natural settings, thorium-230 exists in trace amounts within uranium-bearing minerals and ores, as well as in soils and , where its concentration reflects the ingrowth from parent uranium isotopes. Due to its high particle reactivity and low in aqueous environments, thorium-230 rapidly adsorbs onto particulate matter, enabling its use as a tracer for geochemical processes such as marine particle flux, scavenging rates, and deep-sea . In ocean waters, it constitutes up to 0.02% of total dissolved thorium in deep layers, derived from the soluble parent. Historically known as ionium, thorium-230 plays a key role in uranium-series disequilibrium techniques, particularly for calibrating timelines of in corals, stalagmites, and other geological samples spanning 0 to 500 thousand years. This method exploits the lack of initial thorium-230 in uranium-enriched systems, allowing ingrowth measurement relative to stable isotopes for precise age determination. Its long relative to shorter-lived decay products ensures secular equilibrium in aged deposits, influencing assessments in . Artificially, thorium-230 can be produced via on thorium-229 or from proton irradiation of targets, though such methods are limited to contexts.

Thorium-228

Thorium-228 (²²⁸Th) is a radioactive of with an of 228.02873 u. It undergoes primarily to radium-224 (²²⁴Ra), with a of 5.520 MeV and a of 1.9116 years. The specific of thorium-228 is 31.1 TBq/g, reflecting its relatively short compared to thorium-232. In nature, thorium-228 occurs as an intermediate in the thorium-232 decay series, formed via of actinium-228 (²²⁸Ac), which itself arises from radium-228 (²²⁸Ra). Minute traces are also present in decay chains, though its abundance remains low due to rapid ingrowth and decay relative to longer-lived precursors. Artificially, thorium-228 is produced by irradiation of -226 targets, yielding it as a solid for subsequent processing. Alternative methods involve of from natural salts followed by decay to isolate thorium-228. Routine production occurs at facilities like , supporting generator systems for downstream isotopes. Thorium-228 serves as a precursor in the manufacture of radium-224/lead-212 (²¹²Pb) generators for targeted alpha therapy in , including applications against ovarian and other malignancies. Its enables on-demand production of short-lived alpha emitters like ²¹²Pb, which deliver high to tumor cells while minimizing damage to surrounding tissue. Limited non-medical uses include tracing particle dynamics in and sediment analysis via nuclear spectrometry. Health risks from thorium-228 exposure are often assessed alongside radium-228 due to their linked decay and potential in the environment.

Thorium-234

Thorium-234 (^{234}Th) is a short-lived radioactive of , featuring 90 protons and 144 neutrons, with a nuclear spin of 0^+. It undergoes beta-minus decay with a of 24.10 days, primarily (99.8%) to the metastable protactinium-234m , releasing a maximum beta energy of 0.273 MeV. The proceeds rapidly to stable via subsequent beta decay of protactinium-234. In the uranium-238 decay series, thorium-234 forms immediately following the of , which has a of 4.468 billion years. This positions ^{234}Th as the second member in the series, enabling secular equilibrium with its long-lived parent in uranium-bearing minerals and ores, where its activity matches that of U-238 after transient buildup. Artificially, it can be produced via irradiation of or targets, though natural production dominates due to the prevalence of U-238 in (approximately 99% of ). Due to its 24-day and particle-reactive behavior contrasting with the soluble parent U-238, ^{234}Th serves as a geochemical tracer for particle dynamics in aquatic environments. In oceans, it is continuously generated from dissolved U-238 and scavenged onto sinking particles, allowing quantification of export fluxes of carbon, nutrients, and trace metals from surface to deeper waters. Measurements of ^{234}Th deficits relative to U-238 provide rates of particle remineralization and vertical transport, with applications extending to coastal, estuarine, and freshwater systems. Additionally, its presence aids in detecting and assaying U-238 concentrations in geological and environmental samples through radiometric equilibrium analysis.

Nuclear applications

Fuel cycle and reactor potential

The thorium fuel cycle relies on as the primary fertile isotope, which captures a to produce thorium-233. This undergoes beta minus decay to protactinium-233 with a of 22.3 minutes, followed by protactinium-233 decaying via beta minus emission to yield the fissile after approximately 27 days. then undergoes fission in a reactor, releasing an average of about 2.3 s per fission in a neutron spectrum, which supports a positive neutron balance for sustaining the chain reaction and enabling breeding of additional fissile material from thorium-232. This cycle exhibits strong potential for closed-loop operation in thermal neutron reactors, where the eta value (neutrons produced per absorbed in fissile material) for exceeds 2.0, facilitating breeding ratios approaching or exceeding unity under optimized conditions such as low parasitic absorption and efficient fuel management. reactors, like pressurized reactors, leverage their inherently high economy to accommodate fuels, potentially converting up to three times more to compared to light water reactors due to minimized moderation losses. In reactors, the cycle benefits from the ability to dissolve and uranium fluorides directly in the coolant-fuel salt, allowing continuous online processing to extract fission products and manage protactinium-233 separation, thereby enhancing fuel utilization efficiency beyond 90% in theoretical designs. Reactor concepts exploiting this cycle, such as the , demonstrate theoretical breeding ratios of 1.05 to 1.3, depending on spectrum hardening and flux management, which could extend fuel resources significantly given thorium's abundance—estimated at 3 to 4 times that of in the . However, realization requires addressing protactinium-233's cross-section, which can degrade breeding if not isolated promptly, underscoring the need for integrated reprocessing in practical implementations. Overall, the cycle's viability hinges on achieving high conversion factors in pilot-scale demonstrations to validate scalability for baseload power generation.

Advantages over uranium cycles

Thorium-based fuel cycles, primarily utilizing bred into , offer several potential advantages over conventional uranium cycles reliant on enrichment or plutonium breeding. These include greater resource availability, reduced generation of long-lived , enhanced proliferation resistance, and improved sustainability through efficient breeding in thermal-spectrum reactors. Thorium is significantly more abundant in the than , with estimated global resources exceeding 6 million metric tons compared to 's recoverable reserves of about 5.5 million metric tons as of recent assessments. This abundance stems from thorium's occurrence in common minerals like , often as a byproduct of rare-earth , potentially lowering long-term fuel supply costs and reducing geopolitical dependencies on concentrated in fewer regions. In terms of , thorium cycles produce fewer transuranic elements such as , , and , which dominate the long-term radiotoxicity in uranium-fueled spent fuel. Fission of yields a higher proportion of stable or short-lived isotopes, resulting in waste that requires isolation for roughly 300–500 years rather than the tens of thousands of years needed for uranium- cycle residues. This reduction in inventory—potentially by factors of 10 or more—eases repository demands and diminishes environmental risks from and emissions. Proliferation resistance represents another key benefit, as uranium-233 is invariably co-produced with uranium-232 in thorium breeding, introducing intense gamma radiation from thallium-208 (a U-232 daughter) that complicates material handling and enrichment for weapons without specialized shielding. Unlike plutonium-239 from uranium cycles, which can be separated relatively cleanly via reprocessing, thorium-derived fuel lacks a dedicated weapons-grade pathway, and the absence of excess plutonium production further mitigates diversion risks. Thorium's nuclear properties enable higher breeding ratios in thermal or epithermal reactors, where captures s to form protactinium-233, decaying to with a economy that can exceed unity—meaning more generated than consumed—contrasting with the sub-unity ratios typical of light-water reactors without fast breeding. This supports extended utilization, potentially extracting over 200 times more per ton of natural than from unenriched .

Technical challenges and criticisms

One major technical challenge in the thorium fuel cycle is the requirement for advanced reprocessing to separate protactinium-233 and from irradiated , as itself is fertile rather than fissile and does not sustain a without breeding to . This process demands online or frequent fuel handling in reactors like designs to optimize economy, yet no commercial-scale reprocessing facilities for thorium fuels exist, complicating scalability and increasing costs compared to established uranium-plutonium cycles. Fuel fabrication also poses difficulties due to thorium's chemical properties, requiring specialized testing and qualification that have historically lagged behind technologies. Neutron economy in thorium cycles, while theoretically favorable in spectra due to uranium-233's higher fission cross-section and neutron yield (eta ≈ 2.3 versus 2.0 for ), demands precise control to achieve breeding ratios above 1.0, which is marginal without fast spectrum or advanced ; discrepancies in reactivity between fresh and burned further strain power distribution and burnup uniformity. In practice, -based systems often underperform in solid-fuel configurations without fissile starters like , limiting self-sustainability and necessitating hybrid operations that undermine pure thorium advantages. Proliferation risks remain a significant criticism, as is weapons-grade , and while its co-production with introduces gamma-emitting daughters (e.g., thallium-208) that complicate handling and detection, isotopic dilution or chemical separation could still enable diversion, particularly if protactinium-233 is isolated to reduce uranium-232 buildup. Safeguards assessments highlight the need for novel verification technologies, as thorium cycles do not inherently eliminate risks present in uranium cycles and may require more intrusive monitoring due to dispersed fuel processing. Material compatibility issues, especially in molten salt reactor designs often proposed for , include from salts at temperatures exceeding 600°C under high , with no alloys yet proven durable over reactor lifetimes despite decades of unresolved challenges from experiments. Regulatory and economic barriers compound these, as lacks operational data for licensing, costs 2-3 times higher than due to fabrication complexity, and infrastructure investments dwarf returns given abundant, low-cost supplies. Critics argue that 's promotion overlooks these hurdles, framing it as a without addressing that it generates comparable long-lived waste volumes and requires unproven integral testing for viability.

Scientific and other applications

Thorium-229 in nuclear clocks

Thorium-229 features a uniquely low-lying nuclear isomeric state, denoted ^{229m}Th, with an excitation energy of 8.338 ± 0.024 eV above the , placing it in the vacuum ultraviolet range accessible to excitation. This energy level, several orders of magnitude lower than typical nuclear transitions, enables the development of nuclear clocks based on the isomer's transition frequency, potentially offering greater stability than atomic clocks due to the nucleus's smaller size and reduced sensitivity to external electromagnetic perturbations. The suitability of ^{229m}Th for nuclear timekeeping stems from its narrow linewidth and high quality factor, allowing for precision measurements of fundamental constants and searches for their temporal variations, as well as applications in enhanced and . Unlike atomic clocks, which rely on electron transitions, nuclear clocks probe intranuclear dynamics less affected by chemical environments or , with the thorium nucleus's compact charge distribution minimizing quadratic Stark shifts. Significant progress occurred in 2024, when researchers at institutions including and the National Institute of Standards and Technology (NIST) achieved direct laser excitation of the in thorium-doped crystals using ultraviolet light, confirming the transition and enabling with improved resolution. This milestone followed precise energy determinations via methods like magnetic microcalorimetry and resolved prior uncertainties in the 's properties, paving the way for frequency combs in the VUV regime to lock the transition. Challenges persist, including the isomer's short internal conversion lifetime (on the order of microseconds in neutral atoms, extending to seconds in ions), necessitating efficient VUV light sources and low-radioactivity samples for practical deployment. Production of pure ^{229}Th remains limited, often derived from ^{233}U decay chains, and full clock realization requires integrating trapped ions or solid-state hosts with coherent excitation to achieve stabilities rivaling optical lattice clocks. Ongoing efforts, such as thin-film thorium tetrafluoride matrices, aim to reduce handling risks and costs while enhancing scalability. By mid-2025, these advances position ^{229}Th-based clocks as viable for probing subtle effects like influences on nuclear properties.

Medical isotopes and research tools

Thorium-227, with a of 18.7 days, serves as a key alpha-emitting in targeted alpha therapy (TAT) for , where it is conjugated to tumor-targeting antibodies or ligands to deliver cytotoxic radiation selectively to cancer cells while minimizing damage to surrounding healthy tissue. These targeted thorium-227 conjugates (TTCs) exploit the high of alpha particles to induce DNA double-strand breaks in malignant cells, showing preclinical efficacy against disseminated tumors such as , hepatocellular , and CD70-expressing cancers. Clinical trials, including the first-in-human study of BAY2287411 (a PSMA-targeted TTC), have progressed with doses starting at 1.5 MBq, demonstrating feasibility for treating metastatic castration-resistant . Ongoing research highlights TTCs' potential to upregulate danger-associated molecular patterns and trigger immunogenic cell death, enhancing antitumor immune responses. Thorium-228, with a half-life of 1.91 years, functions as a generator precursor for radium-224 (half-life 3.63 days) and lead-212 (half-life 10.64 hours), both alpha emitters employed in TAT for cancers resistant to conventional therapies, such as . Extraction from decay chains enables production of these short-lived daughters for , with U.S. Department of Energy initiatives scaling supply to support and therapies. Strategic agreements, such as those by Thor Medical in 2025, underscore Th-228's role in advancing Pb-212-based precision . Thorium-229, possessing an exceptionally long half-life of approximately 7,340 years, is extracted from stockpiles to yield (half-life 9.92 days), a critical TAT isotope for labeling drugs targeting and other solid tumors. reported extracting over 15 grams of Th-229 by April 2025, facilitating Ac-225 production for clinical trials and commercial therapies like those developed by Isotopes. This chain supports multiple alpha emissions per decay, enhancing therapeutic potency. In research applications beyond direct therapy, isotopes like Th-227 and Th-228 enable of alpha-particle tools and evaluation of chelator stability in development, with studies comparing chelators such as derivatives for optimal conjugation. Th-226 ( 30.6 minutes) has been explored as a TAT via dual-generator systems, though its short lifespan limits routine use. These isotopes also aid in prototyping TAT delivery vectors, informing scalable production from targets irradiated by protons or deuterons.

Non-nuclear uses

Thorium-232 compounds, particularly (thoria), are employed in the production of high-temperature ceramics and refractories, leveraging thoria's exceptionally high melting point of 3300°C for applications in fire bricks and crucibles. Thoria also serves as an additive in rods to enhance arc stability and electrode performance during high-heat processes. In alloys, is alloyed with magnesium to create lightweight, high-strength materials used in components, where it improves creep resistance and heat tolerance. is incorporated into optical glasses for camera and lenses, providing high and dispersion control for precision optics. For lighting, thorium nitrate derived from coats gas lantern mantles, enhancing incandescence and brightness through efficient light emission when heated. Additionally, thorium strengthens filaments in electric bulbs and controls grain size in wire for equipment, improving durability under electrical stress. These applications exploit 's chemical stability and low radioactivity, given its 14 billion-year , though environmental regulations have reduced usage since the mid-20th century due to concerns.

Recent developments

Advances in thorium reactors

China's experimental -based molten salt reactor (MSR), a 2 MW prototype developed under the Molten Salt Reactor (TMSR) program, achieved criticality in October 2023, marking the first operational reactor globally. The reactor reached full power in June 2024, demonstrating stable operation with tetrafluoride dissolved in . In April 2025, engineers successfully refueled the reactor online without shutdown by adding fresh , a milestone enabling continuous operation and reducing downtime compared to traditional solid- reactors. This design incorporates passive safety features, such as a freeze plug for emergency drainage, minimizing meltdown risks. China plans to construct a larger 10 MW thorium MSR, with criticality targeted for 2030, as part of scaling up the for commercial viability. of a dedicated facility in Province, is set to begin in 2025 to support further testing and development. These advances leverage 's substantial thorium reserves, estimated to power the nation for thousands of years, positioning the TMSR as a potential complement to uranium-based reactors amid growing demands. In , the (AHWR), a 300 MWe design using thorium-plutonium oxide fuel, remains in the development phase as part of the three-stage nuclear program aimed at utilizing domestic thorium reserves. A critical facility for AHWR testing has been operational at , validating core physics and safety features, but no prototype construction or operational milestones have been reported beyond design refinements as of 2025. The program prioritizes thorium breeding to for sustained power generation, though deployment timelines extend into the 2030s pending completion of earlier stages. Private sector initiatives include , which since 2014 has advanced modular thorium MSRs for waste-burning and power generation, with plans for a 1 MW test deployment in by 2028. The company secured collaborations, such as with DeepGEO in November 2024 for thorium development and Ocean Power in July 2025 for Norwegian applications, focusing on factory-produced, autonomous units. Similarly, Dutch firm Thorizon is engineering a thorium-fueled reactor targeting 250,000 households' needs through high-temperature steam production, emphasizing safety and efficiency in small modular designs. These efforts highlight growing commercial interest, though they remain pre-commercial without operational prototypes. Global market analyses project the thorium reactor sector to grow from USD 4.56 billion in 2025 to USD 8.97 billion by 2032, driven by research into fuel cycle efficiency and safer designs, though challenges like material corrosion in molten salts persist.

Progress in thorium-229 research

Research on the thorium-229 isomer, denoted ^{229m}Th, has accelerated since its first direct detection in 2016, focusing on precise determination of its excitation energy—initially estimated around 7-8 eV—and development of laser-based manipulation techniques for nuclear clock applications. This isomer's uniquely low-lying first excited state, coupled to the atomic shell via the hyperfine interaction, enables optical access to nuclear transitions, potentially yielding time standards with stability exceeding atomic clocks by orders of magnitude due to insensitivity to external electromagnetic fields. Efforts have emphasized spectroscopy in ion traps, thin films, and crystals to isolate the transition frequency, resolve hyperfine structure, and mitigate environmental decoherence. A pivotal advancement occurred in July 2024, when UCLA researchers reported the most precise measurement of the isomer's energy, yielding 8.355733(45) eV in thin-film samples, narrowing the from prior indirect values and confirming its position in the vacuum ultraviolet range suitable for excitation. Complementing this, synchrotron-based measurements refined the value to 8.338(24) eV, highlighting methodological consistency despite slight variances attributable to calibration differences. These results enabled the first quantum state-resolved of the transition's electric octupole () and (M1) components in September 2024, resolving quadrupolar splittings from field interactions in solid hosts like CaF_2. In April 2024, experimental demonstration of broadband laser excitation of the in ion-trapped ^{229}Th^{3+} confirmed resonant absorption at ~148 nm, marking a step toward coherent control and paving the way for prototype nuclear clocks by quantifying excitation efficiencies and decay pathways. NIST teams advanced metrology in 2024, locking microwave references to the internal conversion signal and achieving sub-kHz precision in the transition , essential for comparing nuclear clocks against optical standards like . By mid-2025, solid-state implementations in doped crystals demonstrated reproducibility at parts-per-billion levels, addressing scalability challenges over ion-based systems. Ongoing work explores ^{229m}Th sensitivity to fundamental constants, with 2025 studies quantifying its coupling to the for detection via clock comparisons, and direct observation of radiative decay confirming the ~8.4 eV lifetime. These developments underscore competitive international efforts, including at PTB and , prioritizing empirical validation over theoretical models to refine estimates (now ~10-60 minutes) and excitation schemes. Challenges persist in suppressing non-radiative quenching and achieving single-ion addressing, but progress positions ^{229}Th-based clocks for applications in redefining traceability and probing variations in nuclear forces.

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