थोरियम ईंधन चक्र

थोरियम ईंधन चक्र ( thorium fuel cycle) एक नाभिकीय ईंधन चक्र है जो थोरियम के समस्थानिक Th-232 का उपयोग उर्वर पदार्थ (फर्टाइल मैटेरियल) के रूप में करता है। रिएक्टर में Th-232 बदलकर यूरेनियम-२३३ बन जाता है जो नाभिकीय ईंधन है।

In the thorium cycle, fuel is formed when साँचा:SimpleNuclide2 captures a neutron (whether in a fast reactor or thermal reactor) to become साँचा:SimpleNuclide2. This normally emits an electron and an anti-neutrino (ν) by β^⁻ decay to become साँचा:SimpleNuclide2. This then emits another electron and anti-neutrino by a second β^⁻ decay to become साँचा:SimpleNuclide2, the fuel:

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

Nuclear fission produces radioactive fission products which can have half-lives from days to greater than 200,000 years. According to some toxicity studies,^[2] the thorium cycle can fully recycle actinide wastes and only emit fission product wastes, and 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.^[3]

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 साँचा:SimpleNuclide2, the transmutations tend to produce useful nuclear fuels rather than transuranic wastes. When साँचा:SimpleNuclide2 absorbs a neutron, it either fissions or becomes साँचा:SimpleNuclide2. The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of साँचा:SimpleNuclide2, therefore, is about 1:12 — which is better than the corresponding capture vs. fission ratios of साँचा:SimpleNuclide2 (about 1:6), or साँचा:SimpleNuclide2 or साँचा:SimpleNuclide2 (both about 1:3).^[4][5] The result is less transuranic waste than in a reactor using the uranium-plutonium fuel cycle. साँचा:Thorium Cycle Transmutationसाँचा:SimpleNuclide2, like most actinides with an even number of neutrons, is not fissile, but neutron capture produces fissile साँचा:SimpleNuclide2. If the fissile isotope fails to fission on neutron capture, it produces साँचा:SimpleNuclide2, साँचा:SimpleNuclide2, साँचा:SimpleNuclide2, and eventually fissile साँचा:SimpleNuclide2 and heavier isotopes of plutonium. The साँचा:SimpleNuclide2 can be removed and stored as waste or retained and transmuted to plutonium, where more of it fissions, while the remainder becomes साँचा:SimpleNuclide2, then americium and curium, which in turn can be removed as waste or returned to reactors for further transmutation and fission.

However, the साँचा:SimpleNuclide2 (with a half-life of 3.27×१०⁴ years) formed via (n,2n) reactions with साँचा:SimpleNuclide2 (yielding साँचा:SimpleNuclide2 that decays to साँचा:SimpleNuclide2), while not a transuranic waste, is a major contributor to the long-term radiotoxicity of spent nuclear fuel.

Uranium-232 is also formed in this process, via (n,2n) reactions between fast neutrons and साँचा:SimpleNuclide2, साँचा:SimpleNuclide2, and साँचा:SimpleNuclide2:

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

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

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

Uranium-232 has a relatively short half-life (68.9 years), and some decay products emit high energy gamma radiation, such as साँचा:SimpleNuclide2, साँचा:SimpleNuclide2 and particularly साँचा:SimpleNuclide2. The full decay chain, along with half-lives and relevant gamma energies, is:

साँचा:SimpleNuclide2 decays to साँचा:SimpleNuclide2 where it joins the [[thorium series|decay chain of साँचा:SimpleNuclide2]]

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

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

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

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

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

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

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

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

Thorium-cycle fuels produce hard gamma emissions, which damage electronics, limiting their use in military bomb triggers. साँचा:SimpleNuclide2 cannot be chemically separated from साँचा:SimpleNuclide2 from used nuclear fuel; however, chemical separation of thorium from uranium removes the decay product साँचा:SimpleNuclide2 and the radiation from the rest of the decay chain, which gradually build up as साँचा:SimpleNuclide2 reaccumulates. The hard gamma emissions also create a radiological hazard which requires remote handling during reprocessing.

• तीन चरणों वाला भारत का परमाणु ऊर्जा कार्यक्रम