Thorium fuel cycle

The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, ²³²
Th
, as the fertile material. In the reactor ²³²
Th
is transmuted into the fissile artificial uranium isotope ²³³
U
which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as ²³¹
Th
), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source are necessary to initiate the fuel cycle. In a thorium-fueled reactor, ²³²
Th
absorbs neutrons eventually to produce ²³³
U
. This parallels the process in uranium reactors whereby fertile ²³⁸
U
absorbs neutrons to form fissile ²³⁹
Pu
. Depending on the design of the reactor and fuel cycle, the ²³³
U
generated either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including thorium's greater abundance, superior physical and nuclear properties, enhanced proliferation resistance, and reduced plutonium and actinide production.

History

Concerns about the limits of worldwide uranium resources motivated initial interest in the thorium fuel cycle. It was envisioned that as uranium reserves were depleted, thorium would supplement uranium as a fertile material. However, for most countries, uranium was relatively abundant, and research in thorium fuel cycles waned. A notable exception was India's three stage nuclear power programme. In the twenty-first century thorium's potential for improving proliferation resistance and waste characteristics led to renewed interest in the mineral.

At Oak Ridge National Laboratory in the 1960s, the Molten-Salt Reactor Experiment used ²³³
U
as the fissile fuel as an experiment to demonstrate a part of the Molten Salt Breeder Reactor that was designed to operate on the thorium fuel cycle. Molten Salt Reactor (MSR) experiments assessed thorium's feasibility, using thorium(IV) fluoride dissolved in a molten salt fluid which eliminated the need to fabricate fuel elements. The MSR program was defunded in 1976.

In 2006 Carlo Rubbia proposed the concept of an energy amplifier or "accelerator driven system" (ADS), which he saw as a novel and safe way to produce nuclear energy that exploited existing accelerator technologies. Rubbia's proposal offered the potential to incinerate high-activity nuclear waste and produce energy from natural thorium and depleted uranium.

Nuclear reactions with thorium

Actinides and fission products by half-life
Actinides by decay chain				Half-life range (a)	Fission products of 235U by yield
4n	4n + 1	4n + 2	4n + 3		4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a		155Euþ
248Bk				> 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	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
232Th№		238U№	235Uƒ№	0.7–14.1 Ga
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  fissile №,  primarily a naturally occurring radioactive material (NORM) þ,  neutron poison (thermal neutron capture cross section greater than 3k barns)

In the thorium cycle, fuel is formed when ²³²
Th
captures a neutron (whether in a fast reactor or thermal reactor) to become ²³³
Th
. This normally emits an electron and an anti-neutrino (
ν
) by
β⁻
decay to become ²³³
Pa
. This then emits another electron and anti-neutrino by a second
β⁻
decay to become ²³³
U
, the fuel:

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{233}\mathrm {Th} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{233}\mathrm {Pa} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{233}\mathrm {U}$

Fission product wastes

Nuclear fission produces radioactive fission products which can have half-lives from days to greater than 200,000 years. According to some toxicity studies the thorium cycle can fully recycle actinide wastes and only emit fission product wastes, after a few hundred years the waste from a thorium reactor can be less toxic than the uranium ore that would have been used to produce low enriched uranium fuel for a light water reactor of the same power. Other studies assume some actinide losses and find that actinide wastes dominate thorium cycle waste radioactivity at some future periods.

Actinide wastes

In a reactor, when a neutron hits a fissile atom (such as certain isotopes of uranium), it either splits the nucleus or is captured and transmutes the atom. In the case of ²³³
U
, the transmutations tend to produce useful nuclear fuels rather than transuranic wastes. When ²³³
U
absorbs a neutron, it either fissions or becomes ²³⁴
U
. The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of ²³³
U
, therefore, is about 1:10 - which is better than the corresponding capture vs. fission ratios of ²³⁵
U
(about 1:6), or ²³⁹
Pu
(about 1:2), or ²⁴¹
Pu
(about 1:4). The result is shorter-lived transuranic waste than in a reactor using the uranium-plutonium fuel cycle.

Transmutations in the thorium fuel cycle
														237Np
														↑
		231U	←	232U	↔	233U	↔	234U	↔	235U	↔	236U	→	237U
		↓		↑		↑		↑
		231Pa	→	232Pa	←	233Pa	→	234Pa
		↑				↑
230Th	→	231Th	←	232Th	→	233Th
Nuclides with a yellow background in italic have half-lives under 30 days Nuclides in bold have half-lives over 1,000,000 years Nuclides in red frames are fissile

²³⁴
U
, like most actinides with an even number of neutrons, is not fissile, but neutron capture produces fissile ²³⁵
U
. If the fissile isotope fails to fission on neutron capture, it produces ²³⁶
U
, ²³⁷
Np
, ²³⁸
Pu
, and eventually fissile ²³⁹
Pu
and heavier isotopes of plutonium. The ²³⁷
Np
can be removed and stored as waste or retained and transmuted to plutonium, where more of it fissions, while the remainder becomes ²⁴²
Pu
then americium and curium, which in turn can be removed as waste or returned to reactors for further transmutation and fission.

However, the ²³¹
Pa
(with a half-life of 3.27×10⁴ years) formed via (n,2n) reactions with ²³²
Th
(yielding ²³¹
Th
that decays to ²³¹
Pa
), while not a transuranic waste, is a major contributor to the long term radiotoxicity of spent nuclear fuel.

Uranium-232 contamination

Uranium-232 is also formed in this process, via (n,2n) reactions between fast neutrons and ²³³
U
, ²³³
Pa
, and ²³²
Th
:

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{233}\mathrm {Th} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{233}\mathrm {Pa} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{233}\mathrm {U} +\mathrm {n} \rightarrow {}_{\ 92}^{232}\mathrm {U} +2\mathrm {n}$

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{233}\mathrm {Th} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{233}\mathrm {Pa} +\mathrm {n} \rightarrow {}_{\ 91}^{232}\mathrm {Pa} +2\mathrm {n} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{232}\mathrm {U}$

$\mathrm {n} +{}_{\ 90}^{232}\mathrm {Th} \rightarrow {}_{\ 90}^{231}\mathrm {Th} +2\mathrm {n} {\xrightarrow {\beta ^{-}}}{}_{\ 91}^{231}\mathrm {Pa} +\mathrm {n} \rightarrow {}_{\ 91}^{232}\mathrm {Pa} {\xrightarrow {\beta ^{-}}}{}_{\ 92}^{232}\mathrm {U}$

Uranium-232 has a relatively short half-life (68.9 years), and some decay products emit high energy gamma radiation, such as ²²⁴
Rn
, ²¹²
Bi
and particularly ²⁰⁸
Tl
. The full decay chain, along with half-lives and relevant gamma energies, is:

²³²
U
decays to ²²⁸
Th
where it joins decay chain of ²³²
Th

${}_{\ 92}^{232}\mathrm {U} {\xrightarrow {\ \alpha \ }}{}_{\ 90}^{228}\mathrm {Th} \ \mathrm {(68.9\ a)}$

${}_{\ 90}^{228}\mathrm {Th} {\xrightarrow {\ \alpha \ }}{}_{\ 88}^{224}\mathrm {Ra} \ \mathrm {(1.9\ a)}$

${}_{\ 88}^{224}\mathrm {Ra} {\xrightarrow {\ \alpha \ }}{}_{\ 86}^{220}\mathrm {Rn} \ \mathrm {(3.6\ d,\ 0.24\ MeV)}$

${}_{\ 86}^{220}\mathrm {Rn} {\xrightarrow {\ \alpha \ }}{}_{\ 84}^{216}\mathrm {Po} \ \mathrm {(55\ s,\ 0.54\ MeV)}$

${}_{\ 84}^{216}\mathrm {Po} {\xrightarrow {\ \alpha \ }}{}_{\ 82}^{212}\mathrm {Pb} \ \mathrm {(0.15\ s)}$

${}_{\ 82}^{212}\mathrm {Pb} {\xrightarrow {\beta ^{-}\ }}{}_{\ 83}^{212}\mathrm {Bi} \ \mathrm {(10.64\ h)}$

${}_{\ 83}^{212}\mathrm {Bi} {\xrightarrow {\ \alpha \ }}{}_{\ 81}^{208}\mathrm {Tl} \ \mathrm {(61\ m,\ 0.78\ MeV)}$

${}_{\ 81}^{208}\mathrm {Tl} {\xrightarrow {\beta ^{-}\ }}{}_{\ 82}^{208}\mathrm {Pb} \ \mathrm {(3\ m,\ 2.6\ MeV)}$

Thorium-cycle fuels produce hard gamma emissions, which damage electronics, limiting their use in military bomb triggers. ²³²
U
cannot be chemically separated from ²³³
U
from used nuclear fuel; however, chemical separation of thorium from uranium removes the decay product ²²⁸
Th
and the radiation from the rest of the decay chain, which gradually build up as ²²⁸
Th
reaccumulates. The hard gamma emissions also create a radiological hazard which requires remote handling during reprocessing.

Advantages as a nuclear fuel

Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust, although present knowledge of reserves is limited. Current demand for thorium has been satisfied as a by-product of rare-earth extraction from monazite sands. Also, unlike uranium, mined thorium consists of a single isotope (²³²
Th
). Consequently, it is useful in thermal reactors without the need for isotope separation.

Thorium-based fuels exhibit several attractive properties relative to uranium-based fuels. The thermal neutron absorption cross section (^σ
_a) and resonance integral (average of neutron cross sections over intermediate neutron energies) for ²³²
Th
are about three times and one third of the respective values for ²³⁸
U
; consequently, fertile conversion of thorium is more efficient in a thermal reactor. Also, although the thermal neutron fission cross section (^σ
_f) of the resulting ²³³
U
is comparable to ²³⁵
U
and ²³⁹
Pu
, it has a much lower capture cross section (^σ
_γ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy. Finally, the ratio of neutrons released per neutron absorbed (η) in ²³³
U
is greater than two over a wide range of energies, including the thermal spectrum; as a result, thorium-based fuels can be the basis for a thermal breeder reactor.

Thorium-based fuels also display favorable physical and chemical properties which improve reactor and repository performance. Compared to the predominant reactor fuel, uranium dioxide (UO
₂), thorium dioxide (ThO
₂) has a higher melting point, higher thermal conductivity, and lower coefficient of thermal expansion. Thorium dioxide also exhibits greater chemical stability and, unlike uranium dioxide, does not further oxidize.

Because the ²³³
U
produced in thorium fuels is inevitably contaminated with ²³²
U
, thorium-based used nuclear fuel possesses inherent proliferation resistance. ²³²
U
can not be chemically separated from ²³³
U
and has several decay products which emit high energy gamma radiation. These high energy photons are a radiological hazard that necessitate the use of remote handling of separated uranium and aid in the passive detection of such materials.

The long term (on the order of roughly 10³ to 10⁶ years) radiological hazard of conventional uranium-based used nuclear fuel is dominated by plutonium and other minor actinides, after which long-lived fission products become significant contributors again. A single neutron capture in ²³⁸
U
is sufficient to produce transuranic elements, whereas six captures are generally necessary to do so from ²³²
Th
. 98–99% of thorium-cycle fuel nuclei would fission at either ²³³
U
or ²³⁵
U
, so fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide (MOX) fuels to minimize the generation of transuranics and maximize the destruction of plutonium.

Disadvantages as nuclear fuel

There are several challenges to the application of thorium as a nuclear fuel, particularly for solid fuel reactors.

Unlike uranium, natural thorium contains no fissile isotopes; fissile material, generally ²³³
U
, ²³⁵
U
, or plutonium, must be added to achieve criticality. This, along with the high sintering temperature necessary to make thorium-dioxide fuel, complicates fuel fabrication. Oak Ridge National Laboratory experimented with thorium tetrafluoride as fuel in a molten salt reactor from 1964–1969, which was far easier to both process and separate from contaminants that slow or stop the chain reaction.

In an open fuel cycle (i.e. utilizing ²³³
U
in situ), higher burnup is necessary to achieve a favorable neutron economy. Although thorium dioxide performed well at burnups of 170,000 MWd/t and 150,000 MWd/t at Fort St. Vrain Generating Station and AVR respectively, challenges complicate achieving this in light water reactors (LWR), which compose the vast majority of existing power reactors.

Another challenge associated with a once-through thorium fuel cycle is the comparatively long interval over which ²³²
Th
breeds to ²³³
U
. The half-life of ²³³
Pa
is about 27 days, which is an order of magnitude longer than the half-life of ²³⁹
Np
. As a result, substantial ²³³
Pa
develops in thorium-based fuels. ²³³
Pa
is a significant neutron absorber, and although it eventually breeds into fissile ²³⁵
U
, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.

Alternatively, if solid thorium is used in a closed fuel cycle in which ²³³
U
is recycled, remote handling is necessary for fuel fabrication because of the high radiation levels resulting from the decay products of ²³²
U
. This is also true of recycled thorium because of the presence of ²²⁸
Th
, which is part of the ²³²
U
decay sequence. Further, unlike proven uranium fuel recycling technology (e.g. PUREX), recycling technology for thorium (e.g., THOREX) is only under development.

Although the presence of ²³²
U
complicates matters, ²³³
U
has occasionally been used to produce fission weapons. The United States first tested ²³³
U
as part of a bomb core in Operation Teapot in 1955. However, unlike plutonium, ²³³
U
can be easily denatured by mixing it with natural or depleted uranium. Another option is to mix thorium fuels with small amounts of natural or depleted uranium during fabrication to ensure that ²³³
U
concentrations at cycle end are acceptably low.

Though thorium-based fuels produce far less long-lived transuranics than uranium-based fuels, some long-lived actinide products constitute a long term radiological impact, especially ²³¹
Pa
.

Advocates for liquid core and molten salt reactors claim that these technologies negate thorium's disadvantages. Since only one liquid core reactor using thorium has been built, it is hard to validate the exact benefits. The lack of relevance to the nuclear weapon industry can be seen as a disadvantage to the development of Thorium usage in power generation, but a worldwide resurgence of nuclear power use could provide enough incentives and funding to negate this disadvantage.

Reactors

Thorium fuels have fueled several different reactor types, including light water reactors, heavy water reactors, high temperature gas reactors, sodium-cooled fast reactors, and molten salt reactors.

List of thorium-fueled reactors (Most of this list appear to be Uranium or Plutonium fueled, rather than Thorium)

Name	Country	Type	Power	Fuel	Operation period
AVR	Germany	HTGR, Experimental (Pebble bed reactor)	015000 15 MW(e)	Th+235 U Driver Fuel, Coated fuel particles, Oxide & dicarbides	1967–1988
THTR-300	Germany	HTGR, Power (Pebble Type)	300000 300 MW(e)	Th+235 U , Driver Fuel, Coated fuel particles, Oxide & dicarbides	1985–1989
Lingen	Germany	BWR Irradiation-testing	060000 60 MW(e)	Test Fuel (Th,Pu)O2 pellets	1968; terminated in 1973
Dragon (OECD-Euratom)	UK (also Sweden, Norway & Switzerland)	HTGR, Experimental (Pin-in-Block Design)	020000 20 MWt	Th+235 U Driver Fuel, Coated fuel particles, Oxide & Dicarbides	1966–1973
Peach Bottom	USA	HTGR, Experimental (Prismatic Block)	040000 40 MW(e)	Th+235 U Driver Fuel, Coated fuel particles, Oxide & dicarbides	1966–1972
Fort St Vrain	USA	HTGR, Power (Prismatic Block)	330000 330 MW(e)	Th+235 U Driver Fuel, Coated fuel particles, Dicarbide	1976–1989
MSRE ORNL	USA	MSBR	007500 7.5 MWt	233 U Molten Fluorides	1964–1969
Shippingport	USA	LWBR PWR, (Pin Assemblies)	100000 100 MW(e)	Th+233 U Driver Fuel, Oxide Pellets	1977–1982
Indian Point 1	USA	LWBR PWR, (Pin Assemblies)	285000 285 MW(e)	Th+233 U Driver Fuel, Oxide Pellets	1962–1980
SUSPOP/KSTR KEMA	Netherlands	Aqueous Homogenous Suspension (Pin Assemblies)	001000 1 MWt	Th+HEU, Oxide Pellets	1974–1977
NRX & NRU	Canada	MTR (Pin Assemblies)	020000 20MW; 200MW (see)	Th+235 U , Test Fuel	1947 (NRX) + 1957 (NRU); Irradiation–testing of few fuel elements
CIRUS; DHRUVA; & KAMINI	India	MTR Thermal	040000 40 MWt; 100 MWt; 30 kWt (low power, research)	Al+233 U Driver Fuel, ‘J’ rod of Th & ThO2, ‘J’ rod of ThO2	1960-2010 (CIRUS); others in operation
KAPS 1 &2; KGS 1 & 2; RAPS 2, 3 & 4	India	PHWR, (Pin Assemblies)	220000 220 MW(e)	ThO2 Pellets (For neutron flux flattening of initial core after start-up)	1980 (RAPS 2) +; continuing in all new PHWRs
FBTR	India	LMFBR, (Pin Assemblies)	040000 40 MWt	ThO2 blanket	1985; in operation

(IAEA TECDOC-1450 "Thorium Fuel Cycle - Potential Benefits and Challenges", Table 1. Thorium utilization in different experimental and power reactors.)

References and links

See also

• Thorium
• Nuclear fuel cycle
• Nuclear power
• Nuclear fission
• Radioactive waste
• World energy resources and consumption
• Uranium depletion
• Peak uranium
• Fuji MSR
• Energy amplifier
• Alvin Radkowsky
• Molten salt reactor

External links

• FactSheet on Thorium, World Nuclear Association.
• Thorium fuel cycle — Potential benefits and challenges, International Atomic Energy Agency, May 2005.
• Thorium Fuel Links
• The Use of Thorium as Nuclear Fuel American Nuclear Society, Position Statement, November 2006
• Revisiting the thorium-uranium nuclear fuel cycle, © European Physical Society, EDP Sciences 2007.
• Thorium Energy Advocacy Organization
• Web site devoted to the discussion of thorium as a future energy resource
• Annotated bibliography for the thorium fuel cycle from the Alsos Digital Library for Nuclear Issues

Recent interest in the thorium fuel cycle

• Article by Michael Anissimov advocating adopting Thorium reactors
• Thorium information page by World Nuclear Association WNA
• New Age Nuclear: article on thorium reactors, Cosmos Magazine
• Thorium as a Secure Nuclear Fuel Alternative
• UK Independent: Is thorium the answer to our energy crisis?
• Thorium Energy Blog, discussion forum and document repository
• Another thorium information portal
• Wired - Uranium Is So Last Century — Enter Thorium, the New Green Nuke
• International Thorium Energy Organisation