Editing Thorium (section)

==Potential use for nuclear energy==
{{Main|Thorium-based nuclear power|Thorium fuel cycle}}
The main nuclear power source in a reactor is the neutron-induced fission of a nuclide; the synthetic fissile{{efn|name="fissionable"}} nuclei <sup>233</sup>U and <sup>239</sup>Pu can be [[breeder reactor|bred]] from neutron capture by the naturally occurring quantity nuclides <sup>232</sup>Th and <sup>238</sup>U. <sup>235</sup>U occurs naturally in significant amounts and is also fissile.<ref>{{cite journal |last1=Ronen |first1=Yigal |title=A Rule for Determining Fissile Isotopes |journal=Nuclear Science and Engineering |date=March 2006 |volume=152 |issue=3 |pages=334–335 |doi=10.13182/nse06-a2588 |bibcode=2006NSE...152..334R |s2cid=116039197 }}</ref><ref name="anucene">{{Cite journal |last1= Ronen |first1= Y. |title= Some remarks on the fissile isotopes |doi= 10.1016/j.anucene.2010.07.006 |journal= Annals of Nuclear Energy |volume= 37 |issue= 12 |pages= 1783–1784 |year= 2010 |bibcode= 2010AnNuE..37.1783R }}</ref>{{efn|The thirteen fissile actinide isotopes with half-lives over a year are <sup>229</sup>Th, <sup>233</sup>U, <sup>235</sup>U, [[neptunium-236|<sup>236</sup>Np]], <sup>239</sup>Pu, [[plutonium-241|<sup>241</sup>Pu]], [[americium-242m|<sup>242m</sup>Am]], [[curium-243|<sup>243</sup>Cm]], [[curium-245|<sup>245</sup>Cm]], [[curium-247|<sup>247</sup>Cm]], [[californium-249|<sup>249</sup>Cf]], [[californium-251|<sup>251</sup>Cf]], and [[einsteinium-252|<sup>252</sup>Es]]. Of these, only <sup>235</sup>U have significant amounts in nature, and only <sup>233</sup>U and <sup>239</sup>Pu can be bred from naturally occurring nuclei with a single neutron capture.<ref name="anucene" />}} In the thorium fuel cycle, the fertile isotope <sup>232</sup>Th is bombarded by [[slow neutron]]s, undergoing neutron capture to become <sup>233</sup>Th, which undergoes two consecutive beta decays to become first [[protactinium-233|<sup>233</sup>Pa]] and then the fissile <sup>233</sup>U:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}

:<chem>{^{232}_{90}Th} ->[\text{(n,}\gamma\text{)}] {^{233}_{90}Th}->[\beta^-][\text{21.8 min}] {^{233}_{91}Pa} ->[\beta^-][\text{27 days}] {^{233}_{92}U} \ (->[\alpha][1.60 \times 10^5\text{years}]) </chem>

{{Thorium Cycle Transmutation}}
<sup>233</sup>U is fissile and can be used as a nuclear fuel in the same way as <sup>235</sup>U or [[plutonium-239|<sup>239</sup>Pu]]. When <sup>233</sup>U undergoes nuclear fission, the neutrons emitted can strike further <sup>232</sup>Th nuclei, continuing the cycle.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} This parallels the uranium fuel cycle in [[fast breeder reactor]]s where <sup>238</sup>U undergoes neutron capture to become <sup>239</sup>U, beta decaying to first <sup>239</sup>Np and then fissile <sup>239</sup>Pu.<ref>{{cite web|url=http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/plutonium.aspx|title=Plutonium|year=2017|publisher=World Nuclear Association|access-date=29 September 2017|archive-date=5 October 2017|archive-url=https://web.archive.org/web/20171005061205/http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/plutonium.aspx|url-status=live}}</ref>

The fission of {{Nuclide|Uranium|233}} produces 2.48 neutrons on average.<ref>{{cite web|url=https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/uranium/uranium-233/uranium-233-fission/|title=Uranium 233 Fission|year=2023|publisher=Nuclear Power|access-date=28 April 2023}}</ref>
One neutron is needed to keep the fission reaction going. For a self-contained continuous breeding cycle, one more neutron is needed to breed a new {{Nuclide|Uranium|233}} atom from the fertile  {{Nuclide|Thorium|232}}. This leaves a margin of 0.45 neutrons (or 18% of the neutron flux) for losses.

===Advantages===
Thorium is more abundant than uranium, and can satisfy world energy demands for longer.{{sfn|Greenwood|Earnshaw|1997|p=1259}} It is particularly suitable for being used as fertile material in [[molten salt reactor]]s.

<sup>232</sup>Th absorbs neutrons more readily than <sup>238</sup>U, and <sup>233</sup>U has a higher probability of fission upon neutron capture (92.0%) than <sup>235</sup>U (85.5%) or <sup>239</sup>Pu (73.5%).<ref>{{cite web |url=http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf |title=Interactive Chart of Nuclides |publisher=[[Brookhaven National Laboratory]] |access-date=12 August 2013 |archive-date=24 January 2017 |archive-url=https://web.archive.org/web/20170124175936/http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf }}</ref> It also releases more neutrons upon fission on average.{{sfn|Greenwood|Earnshaw|1997|p=1259}} A single neutron capture by <sup>238</sup>U produces transuranic waste along with the fissile <sup>239</sup>Pu, but <sup>232</sup>Th only produces this waste after five captures, forming <sup>237</sup>Np. This number of captures does not happen for 98–99% of the <sup>232</sup>Th nuclei because the intermediate products <sup>233</sup>U or <sup>235</sup>U undergo fission, and fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in [[MOX fuel|mixed oxide fuels]] to minimise the generation of transuranics and maximise the destruction of [[plutonium]].<ref name="wnn-20130621">{{cite news |url=http://www.world-nuclear-news.org/ENF_Thorium_test_begins_2106131.html |title=Thorium test begins |publisher=World Nuclear News |year=2013 |access-date=21 July 2013 |archive-date=19 July 2013 |archive-url=https://web.archive.org/web/20130719084439/http://www.world-nuclear-news.org/ENF_Thorium_test_begins_2106131.html |url-status=live }}</ref>

Thorium fuels result in a safer and better-performing [[reactor core]]{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} because thorium dioxide has a higher melting point, higher [[thermal conductivity]], and a lower [[coefficient of thermal expansion]]. It is more stable chemically than the now-common fuel uranium dioxide, because the latter oxidises to [[triuranium octoxide]] ({{chem2|U3O8}}), becoming substantially less dense.<ref name="Thorium Fuel Cycle – Potential Benefits and Challenges">{{cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|publisher=International Atomic Energy Agency|title=IAEA-TECDOC-1450 Thorium Fuel Cycle – Potential Benefits and Challenges|date=2005|access-date=23 March 2009|archive-date=4 August 2016|archive-url=https://web.archive.org/web/20160804054758/http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|url-status=live}}</ref>

===Disadvantages===
The used fuel is difficult and dangerous to reprocess because many of the daughters of <sup>232</sup>Th and <sup>233</sup>U are strong gamma emitters.{{sfn|Greenwood|Earnshaw|1997|p=1259}} All <sup>233</sup>U production methods result in impurities of [[uranium-232|<sup>232</sup>U]], either from parasitic knock-out (n,2n) reactions on <sup>232</sup>Th, <sup>233</sup>Pa, or <sup>233</sup>U that result in the loss of a neutron, or from double neutron capture of <sup>230</sup>Th, an impurity in natural <sup>232</sup>Th:<ref name="Intro2WMD" />
:{{nuclide|Th|230}} + n → {{nuclide|Th|231}} + {{math|γ}} {{overunderset|→|''β''<sup>−</sup>|25.5 h}} {{nuclide|Pa|231}} ( {{overunderset|→|''α''|3.28 × 10{{su|p=4}} y}} {{nuclide|Ac|227}} )
:{{nuclide|Pa|231}} + n → {{nuclide|Pa|232}} + {{math|γ}} {{overunderset|→|''β''<sup>−</sup>|1.3 d}} {{nuclide|U|232}} {{overunderset|→|''α''|69 y}}

<sup>232</sup>U by itself is not particularly harmful, but quickly decays to produce the strong gamma emitter [[Isotopes of thallium|<sup>208</sup>Tl]]. (<sup>232</sup>Th follows the same decay chain, but its much longer half-life means that the quantities of <sup>208</sup>Tl produced are negligible.){{sfn|Stoll|2005|p=30}} These impurities of <sup>232</sup>U make <sup>233</sup>U easy to detect and dangerous to work on, and the impracticality of their separation limits the possibilities of [[nuclear proliferation]] using <sup>233</sup>U as the fissile material.<ref name="Intro2WMD">{{cite book |title= Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological |last= Langford |first= R. E. |year= 2004 |publisher= John Wiley & Sons |isbn=978-0-471-46560-7 |page=85 }}</ref> <sup>233</sup>Pa has a relatively long half-life of 27&nbsp;days and a high [[cross section (physics)|cross section]] for neutron capture. Thus it is a [[neutron poison]]: instead of rapidly decaying to the useful <sup>233</sup>U, a significant amount of <sup>233</sup>Pa converts to <sup>234</sup>U and consumes neutrons, degrading [[neutron economy|the reactor efficiency]]. To avoid this, <sup>233</sup>Pa is extracted from the active zone of thorium [[molten salt reactor]]s during their operation, so that it does not have a chance to capture a neutron and will only decay to <sup>233</sup>U.<ref name="NakajimaGroult2005">{{cite book|last1=Nakajima|first1=Ts.|last2=Groult|first2=H.|title=Fluorinated Materials for Energy Conversion|year=2005|publisher=Elsevier|isbn=978-0-08-044472-7|pages=562–565}}</ref>

The irradiation of <sup>232</sup>Th with neutrons, followed by its processing, need to be mastered before these advantages can be realised, and this requires more advanced technology than the uranium and plutonium fuel cycle;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} research continues in this area. Others cite the low commercial viability of the thorium fuel cycle:<ref>{{cite news|url=https://www.theguardian.com/environment/2011/jun/23/thorium-nuclear-uranium|title=Don't believe the spin on thorium being a greener nuclear option|last=Rees|first=E.|year=2011|newspaper=[[The Guardian]]|access-date=29 September 2017|archive-date=27 September 2017|archive-url=https://web.archive.org/web/20170927111531/https://www.theguardian.com/environment/2011/jun/23/thorium-nuclear-uranium|url-status=live}}</ref><ref name="SovacoolValentine2012">{{cite book|last1=Sovacool |first1=B. K. |last2=Valentine |first2=S. V. |title=The National Politics of Nuclear Power: Economics, Security, and Governance|date=2012|publisher=[[Routledge]]|isbn=978-1-136-29437-2|page=226}}</ref><ref>{{cite web |url=http://www.ne.anl.gov/pdfs/NuclearEnergyFAQ.pdf |title=Nuclear Energy FAQs |publisher=[[Argonne National Laboratory]] |year=2014 |access-date=13 January 2018 |archive-date=7 October 2014 |archive-url=https://web.archive.org/web/20141007005609/http://www.ne.anl.gov/pdfs/NuclearEnergyFAQ.pdf |url-status=live }}</ref> the international [[Nuclear Energy Agency]] predicts that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which may persist "in the coming decades".<ref name="FindlayFindlay2011">{{cite book|last=Findlay |first=T. |author-link=Trevor Findlay |title=Nuclear Energy and Global Governance: Ensuring Safety, Security and Non-proliferation|date=2011|publisher=Routledge|isbn=978-1-136-84993-0|page=9}}</ref> The isotopes produced in the thorium fuel cycle are mostly not transuranic, but some of them are still very dangerous, such as <sup>231</sup>Pa, which has a half-life of 32,760&nbsp;years and is a major contributor to the long-term [[radiotoxic]]ity of spent nuclear fuel.<ref name="NakajimaGroult2005" />