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Thorium fuel cycle

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 is India's three stage nuclear power programme. Recently there has been renewed interest in thorium-based fuels for improving proliferation resistance and waste characteristics of used nuclear fuel.

Thorium fuels have been used in several power and research reactors. One of the earliest efforts to use a thorium fuel cycle took place at Oak Ridge National Laboratory in the 1960s. An experimental Molten Salt Reactor technology to study the feasibility of such an approach, used thorium(IV) fluoride dissolved in a molten salt kept hot enough to be liquid, thus eliminating the need for fabricating fuel elements. This effort culminated in the Molten-Salt Reactor Experiment that used 232Th as the fertile material and 233U as the fissile fuel. Due to a lack of funding, the MSR program was discontinued in 1976.

Nuclear reactions with thorium

Actinides

Half-life

Fission products

244Cm

241Pu f

250Cf

243Cmf

1030 y

137Cs

90Sr

85Kr

232U f

238Pu

f is for

fissile

6990 y

151Sm nc

249Cf f

242Amf

141351

No fission product

has half-life 102

to 2105 years

241Am

251Cf f

431898

240Pu

229Th

246Cm

243Am

57 ky

245Cmf

250Cm

239Pu f

824 ky

233U f

230Th

231Pa

32160

4n+1

234U

4n+3

211290

99Tc

126Sn

79Se

248Cm

242Pu

340373

Long-lived fission products

237Np

4n+2

12 my

93Zr

135Cs nc

236U

4n+1

247Cmf

623

107Pd

129I

244Pu

80 my

>7%

>5%

>1%

>.1%

232Th

238U

235U f

0.712by

fission product yield

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

Fission product wastes

Nuclear fission produces radioactive fission products which can have half-lives from less than 100 years to greater than 200,000 years. According to some toxicity studies which assume that the thorium cycle can 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 time periods in the future.

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 233U, the transmutations tend to produce useful nuclear fuels rather than transuranic wastes. When 233U absorbs a neutron, it either fissions or becomes 234U. The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of 233U, therefore, is about 1:10 - which is better than the corresponding capture vs. fission ratios of 235U (about 1:6), or 239Pu (about 1:2), or 241Pu (about 1:4). The result is less long-lived, hazardous transuranic waste than in a reactor using the uranium-plutonium fuel cycle.

231Th

232Th

233Th

(White actinides: t<27d)

231Pa

232Pa

233Pa

234Pa

(Colored : t>68y)

231U

232U

233U

234U

235U

236U

237U

(Fission products with t<90y or t>200ky)

237Np

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

However, the 231Pa (with a half life of 3.27 104 years) formed via (n,2n) reactions with 232Th (yielding 231Th that decays to 231Pa), 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 and233U, 233Pa, and 232Th:

Uranium-232 has a relatively short half-life (73.6 years), and some decay products emit high energy gamma radiation, such as 224Rn, 212Bi and particularly 208Tl. The full decay chain, along with half-lives and relevant gamma energies, is:

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

The hard gamma emissions damage electronics, and make the use of Thorium-cycle fuels difficult in military bomb triggers. 232U cannot be chemically separated from 233U from used nuclear fuel; however, chemical separation of thorium from uranium will remove the decay product 228Th and the radiation from the rest of the decay chain, which will gradually build up again as 228Th reaccumulates. The hard gamma emissions also create a radiological hazard which requires remote handling during reprocessing. Of course, a sufficiently well-funded, determined organization could overcome these obstacles, but Plutonium production is a less-risky development path for nuclear weapons.

Advantages of thorium as a nuclear fuel

There are several potential advantages to thorium-based fuels.

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, naturally occurring thorium consists of only a single isotope (232Th) in significant quantities. Consequently, all mined thorium is useful in thermal reactors without the need for separation of thorium isotopes.

Thorium-based fuels, which produce fissile isotopes in a reactor, exhibit several attractive nuclear properties relative to uranium-based fuels. The thermal neutron absorption cross section (a) and resonance integral (average of neutron cross sections over intermediate neutron energies) for 232Th are about three times and one third of the respective values for 238U; consequently, fertile conversion of thorium is more efficient in a thermal reactor. Also, although the thermal neutron fission cross section (f) of the the resulting 233U is comparable to 235U and 239Pu, it has a much lower capture cross section () 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 () in 233U 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 (UO2), thorium dioxide (ThO2) 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 233U produced in thorium fuels is inevitably contaminated with 232U, thorium-based used nuclear fuel possesses inherent proliferation resistance. Uranium-232 can not be chemically separated from 233U 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[citation needed], after which long-lived fission products become significant contributors again. A single neutron capture in 238U is sufficient to produce transuranic elements, whereas six captures are generally necessary to do so from 232Th. 9899% of thorium-cycle fuel nuclei would fission at either 233U or 235U, 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 of thorium as nuclear fuel

There are several challenges to the application of thorium as a nuclear fuel.

Unlike uranium, natural thorium contains no fissile isotopes; fissile material, generally 233U, 235U, or plutonium, must be supplemented to achieve criticality. This, along with the high sintering temperature necessary to make thorium-dioxide fuel, complicates the fuel fabrication process. 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 fuel poisons (contaminants that slow or stop the chain reaction.)

If thorium is used in an open fuel cycle (i.e. utilizing 233U in-situ), higher burnup is necessary to achieve a favorable neutron economy. Although thorium dioxide has performed well at burnups of 170,000 MWd/t and 150,000 MWd/t at Fort St. Vrain Generating Station and AVR respectively, there are challenges associated with achieving this burnup 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 time scale over which 232Th breeds to 233U. The half-life of 233Pa is about 27 days, which is an order of magnitude longer than the half-life of 239Np. As a result, substantial 233Pa builds into thorium-based fuels. Protactinium-233 is a significant neutron absorber, and although it eventually breeds into fissile 235U, this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.

Alternately, if thorium is used in a closed fuel cycle in which 233U is recycled, remote handling is necessary for fuel fabrication because of the high radiation dose resulting from the decay products of 232U. This is also true of recycled thorium because of the presence of 228Th, which is part of the 232U decay sequence. Further, although there is substantial worldwide experience recycling uranium fuels (e.g. PUREX), similar technology for thorium (e.g. THOREX) is still under development.

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

Despite the fact that thorium-based fuels produce far less long-lived transuranics than uranium-based fuels, there are some long-lived actinides produced that constitute a long term radiological impact, especially 231Pa.

Reactors

Thorium fuels have been demonstrated in 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

Name and Country

Type

Power

Fuel

Operation period

AVR, Germany

HTGR, Experimental (Pebble bed reactor)

15 MW(e)

Th+235U Driver Fuel, Coated fuel particles, Oxide & dicarbides

1967 1988

THTR-300, Germany

HTGR, Power (Pebble Type)

300 MW(e)

Th+235U, Driver Fuel, Coated fuel particles, Oxide & dicarbides

1985 1989

Lingen, Germany

BWR Irradiation-testing

60 MW(e)

Test Fuel (Th,Pu)O2 pellets

Terminated in 1973

Dragon, UK OECD-Euratom also Sweden, Norway & Switzerland

HTGR, Experimental (Pin-in-Block Design)

20 MWt

Th+235U Driver Fuel, Coated fuel particles, Oxide & Dicarbides

1966 - 1973

Peach Bottom, USA

HTGR, Experimental (Prismatic Block)

40 MW(e)

Th+235U Driver Fuel, Coated fuel particles, Oxide & dicarbides

1966 1972

Fort St Vrain, USA

HTGR, Power (Prismatic Block)

330 MW(e)

Th+235U Driver Fuel, Coated fuel particles, Dicarbide

1976 - 1989

MSRE ORNL, USA

MSBR

7.5 MWt

233U Molten Fluorides

1964 - 1969

Shippingport & Indian Point 1, USA

LWBR PWR, (Pin Assemblies)

100 MW(e), 285 MW(e)

Th+233U Driver Fuel, Oxide Pellets

1977 1982, 1962 1980

SUSPOP/KSTR KEMA, Netherlands

Aqueous Homogenous Suspension (Pin Assemblies)

1 MWt

Th+HEU, Oxide Pellets

1974 - 1977

NRU & NRX, Canada

MTR (Pin Assemblies)

Th+235U, Test Fuel

Irradiationesting of few fuel elements

KAMINI; CIRUS; & DHRUVA, India

MTR Thermal

30 kWt; 40 MWt; 100 MWt

Al+233U Driver Fuel, rod of Th & ThO2, rod of ThO2

All three research reactors in operation

KAPS 1 &2; KGS 1 & 2; RAPS 2, 3 & 4, India

PHWR, (Pin Assemblies)

220 MW(e)

ThO2 Pellets (For neutron flux flattening of initial core after start-up)

Continuing in all new PHWRs

FBTR, India

LMFBR, (Pin Assemblies)

40 MWt

ThO2 blanket

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

References

^ a b c d e f "IAEA-TECDOC-1450 Thorium Fuel Cycle-Potential Benefits and Challenges". International Atomic Energy Agency. May 2005. http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf. Retrieved 2009-03-23.

^ "IAEA-TECDOC-1349 Potential of thorium-based fuel cycles to constrain plutonium and to reduce the long-lived waste toxicity". International Atomic Energy Agency. 2002. http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/tdi33008.pdf. Retrieved 2009-03-24.

^ Scientist Urges Switch to Thorium:

^ Wired Magazineecember 2009ranium Is So Last Centurynter Thorium, the New Green Nuke

^ a b Le Brun, C., mpact of the MSBR concept technology on long-lived radio-toxicity and proliferation resistance, Technical Meeting on Fissile Material Management Strategies for Sustainable Nuclear Energy, Vienna 2005

^ a b "Nuclear Energy With (Almost) No Radioactive Waste?". July 2001. http://lpsc.in2p3.fr/gpr/english/NEWNRW/NEWNRW.html#foot284. "according to computer simulations done at ISN, this Protactinium dominates the residual toxicity of losses at 10 000 years"

^ "The Use of Thorium as Nuclear Fuel". American Nuclear Society. November 2006. http://www.ans.org/pi/ps/docs/ps78.pdf. Retrieved 2009-03-24.

^ "Operation Teapot". Nuclear Weapon Archive. 15 October 1997. http://nuclearweaponarchive.org/Usa/Tests/Teapot.html. Retrieved 2008-12-09.

^ "IAEA-TECDOC-1319 Thorium Fuel Utilization: Options and trends". International Atomic Energy Agency. November 2002. http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/te_1319_f.pdf. Retrieved 2009-03-24.

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