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Transmutation ﬂow between ²³⁸Pu and ²⁴⁴Cm in LWR.^[1]
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
²⁴⁵Cm–²⁴⁸Cm are long-lived with negligible decay.

The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed the major actinides. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium.^[2] The most important isotopes in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.

Plutonium and the minor actinides will be responsible for the bulk of the radiotoxicity and heat generation of used nuclear fuel in the medium term (300 to 20,000 years in the future).^[3]

The plutonium from a power reactor tends to have a greater amount of Pu-241 than the plutonium generated by the lower burnup operations designed to create weapons-grade plutonium. Because the reactor-grade plutonium contains so much Pu-241 the presence of americium-241 makes the plutonium less suitable for making a nuclear weapon. The ingrowth of americium in plutonium is one of the methods for identifying the origin of an unknown sample of plutonium and the time since it was last separated chemically from the americium.

Americium is commonly used in industry as both an alpha particle and low photon energy gamma radiation source. For instance it is used in many smoke detectors. Americium can be formed by neutron capture of Pu-239 and Pu-240 forming Pu-241 which then decays by beta decay to Am-241.^[4] In general, as the energy of the neutrons increases, the ratio of the fission cross section to the neutron capture cross section changes in favour of fission. Hence if MOX is used in a thermal reactor such as a boiling water reactor (BWR) or pressurized water reactor (PWR) then more americium can be expected in the used fuel than that from a fast neutron reactor.^[5]

Some of them have been found in fallout from bomb tests. See Actinides in the environment for details of the actinides in the environment.

Transuranics in LWR spent fuel (burnup 55 GWd_th/T) and mean neutron consumption to fission ^[6]
Isotope	Fraction	D_LWR	D_fast	D_superthermal
Np-237	0.0539	1.12	-0.59	-0.46
Pu-238	0.0364	0.17	-1.36	-0.13
Pu-239	0.451	-0.67	-1.46	-1.07
Pu-240	0.206	0.44	-0.96	0.14
Pu-241	0.121	-0.56	-1.24	-0.86
Pu-242	0.0813	1.76	-0.44	1.12
Am-241	0.0242	1.12	-0.62	-0.54
Am-242m	0.000088	0.15	-1.36	-1.53
Am-243	0.0179	0.82	-0.60	0.21
Cm-243	0.00011	-1.90	-2.13	-1.63
Cm-244	0.00765	-0.15	-1.39	-0.48
Cm-245	0.000638	-1.48	-2.51	-1.37
Weighted sum		-0.03	-1.16	-0.51

References

^ Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of Nuclear Science and Technology. 41 (4): 448–456. doi:10.3327/jnst.41.448.
^ Moyer, Bruce A. (2009). Ion Exchange and Solvent Extraction: A Series of Advances, Volume 19. CRC Press. p. 120. ISBN 9781420059700.
^ Stacey, Weston M. (2007). Nuclear Reactor Physics. John Wiley & Sons. p. 240. ISBN 9783527406791.
^ Raj, Gurdeep (2008). Advanced Inorganic Chemistry Vol-1, 31st ed. Krishna Prakashan Media. p. 356. ISBN 9788187224037.
^ Berthou, V.; et al. (2003). "Transmutation characteristics in thermal and fast neutron spectra: application to americium" (PDF). Journal of Nuclear Materials. 320: 156–162. doi:10.1016/S0022-3115(03)00183-1.
^ Etienne Parent (2003). "Nuclear Fuel Cycles for Mid-Century Deployment" (PDF). MIT. p. 104.

[1] Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of Nuclear Science and Technology. 41 (4): 448–456. doi:10.3327/jnst.41.448.

[2] Moyer, Bruce A. (2009). Ion Exchange and Solvent Extraction: A Series of Advances, Volume 19. CRC Press. p. 120. ISBN 9781420059700.

[3] Stacey, Weston M. (2007). Nuclear Reactor Physics. John Wiley & Sons. p. 240. ISBN 9783527406791.

[4] Raj, Gurdeep (2008). Advanced Inorganic Chemistry Vol-1, 31st ed. Krishna Prakashan Media. p. 356. ISBN 9788187224037.

[5] Berthou, V.; et al. (2003). "Transmutation characteristics in thermal and fast neutron spectra: application to americium" (PDF). Journal of Nuclear Materials. 320: 156–162. doi:10.1016/S0022-3115(03)00183-1.

[6] Etienne Parent (2003). "Nuclear Fuel Cycles for Mid-Century Deployment" (PDF). MIT. p. 104.

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@@ Line 2: / Line 2: @@
 [[Image:Sasahara.svg|thumb|375px|Transmutation ﬂow between <sup>238</sup>Pu and <sup>244</sup>Cm in LWR.<ref>{{cite journal|url=http://www.jstage.jst.go.jp/article/jnst/41/4/448/_pdf|title=Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels|journal=Journal of Nuclear Science and Technology|volume=41|issue=4|pages=448–456|date=April 2004|doi=10.3327/jnst.41.448|author=Sasahara, Akihiro|last2=Matsumura|first2=Tetsuo|last3=Nicolaou|first3=Giorgos|last4=Papaioannou|first4=Dimitri}}</ref><br>Fission percentage is 100 minus shown percentages.<br>Total rate of transmutation varies greatly by nuclide.<br><sup>245</sup>Cm&ndash;<sup>248</sup>Cm are long-lived with negligible decay.]]
+{| style="float:right; padding:2px; margin:0 0 0.25em 0.5em;"
+|{{periodic table (micro)|mark=Np,Am,Cm,Bk,Cf,Es,Fm|title=Minor actinides in the periodic table}}
+|-
+|{{periodic table (micro)|mark=U,Pu|title=Major actinides in the periodic table}}
+|}
 The '''minor actinides''' are the [[actinide]] elements in used [[nuclear fuel]] other than [[uranium]] and [[plutonium]], which are termed the [[major actinide]]s. The minor actinides include [[neptunium]], [[americium]], [[curium]], [[berkelium]], [[californium]], [[einsteinium]], and [[fermium]].<ref>{{cite book|last=Moyer|first=Bruce A.|title=Ion Exchange and Solvent Extraction: A Series of Advances, Volume 19|year=2009|publisher=CRC Press|isbn=9781420059700|pages=120|url=http://books.google.com/books?id=NTgjUaLZiDsC&pg=PA120#v=onepage&q&f=false}}</ref> The most important isotopes in [[spent nuclear fuel]] are [[neptunium-237]], [[americium-241]], [[americium-243]], [[curium]]-242 through -248, and [[californium]]-249 through -252.