Uranium-236

Uranium-236 Full table
General
Name, symbol	Uranium-236,²³⁶U
Neutrons	144
Protons	92
Nuclide data
Natural abundance	< 10⁻¹¹
Half-life	2.348 x10⁷ years
Parent isotopes	²³⁶Pa ²³⁶Np ²⁴⁰Pu
Decay products	²³²Th
Isotope mass	236.045568(2) u
Spin	0+
Binding energy	1790415.042 ± 1.974 keV
Decay mode	Decay energy
Alpha	4.572 MeV

Uranium-236 is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Actinides and fission products by half-life
Actinides^[1] by decay chain				Half-life range (y)	Fission products of ²³⁵U by yield^[2]
4n	4n+1	4n+2	4n+3		Fission products of ²³⁵U by yield^[2]
4n	4n+1	4n+2	4n+3			4.5–7%	0.04–1.25%	<0.001%
²²⁸Ra^№				4–6	†		¹⁵⁵Eu^þ
²⁴⁴Cm	²⁴¹Pu^ƒ	²⁵⁰Cf	²²⁷Ac^№	10–29		⁹⁰Sr	⁸⁵Kr	^113mCd^þ
²³²U^ƒ		²³⁸Pu^№	²⁴³Cm^ƒ	29–97		¹³⁷Cs	¹⁵¹Sm^þ	^121mSn
²⁴⁸Bk^[3]	²⁴⁹Cf^ƒ	^242mAm^ƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	²⁴¹Am^ƒ		²⁵¹Cf^ƒ^[4]	430–900
		²²⁶Ra^№	²⁴⁷Bk	1.3 k – 1.6 k
²⁴⁰Pu	²²⁹Th^№	²⁴⁶Cm	²⁴³Am^ƒ	4.7 k – 7.4 k
	²⁴⁵Cm^ƒ	²⁵⁰Cm		8.3 k – 8.5 k
			²³⁹Pu^ƒ№	24.1 k
		²³⁰Th^№	²³¹Pa^№	32 k – 76 k
²³⁶Np^ƒ	²³³U^ƒ№	²³⁴U^№		150 k – 250 k	‡	⁹⁹Tc^₡	¹²⁶Sn
²⁴⁸Cm		²⁴²Pu		327 k – 375 k			⁷⁹Se^₡
				1.53 M		⁹³Zr
	²³⁷Np^№			2.1 M – 6.5 M		¹³⁵Cs^₡	¹⁰⁷Pd
²³⁶U^№			²⁴⁷Cm^ƒ	15 M – 24 M			¹²⁹I^₡
²⁴⁴Pu^№				80 M	... nor beyond 15.7 M years^[5]
²³²Th^№		²³⁸U^№	²³⁵U^ƒ№	0.7 G – 14.1 G	... nor beyond 15.7 M years^[5]
Legend for superscript symbols ₡ has thermal neutron capture cross section in the range of 8–50 barns ƒ fissile m metastable isomer № naturally occurring radioactive material (NORM) þ neutron poison (thermal neutron capture cross section greater than 3k barns) † range 4–97 y: Medium-lived fission product ‡ over 200,000 y: Long-lived fission product

Creation and yield

The fissile isotope uranium-235 fuels most nuclear reactors. U-235 that absorbs a thermal neutron may go one of two ways. About 82% of the time, it will fission. About 18% of the time it will not fission, instead emitting gamma radiation and yielding U-236. Thus, the yield of U-236 per 100 U-235+n reactions is about 18%, and the yield per 100 fissions is about 22%. In comparison, the yields of the most abundant individual fission products like Cs-137, Sr-90, Tc-99 are between 6% and 7% per 100 fissions, and the combined yield of medium-lived (10 years and up) and long-lived fission products is about 32%, or a few percent less as some are destroyed by neutron capture.

The second most used fissile isotope plutonium-239 can also fission or not fission on absorbing a thermal neutron. The product plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). Pu-240 decays with a half-life of 6561 years into U-236. In a closed nuclear fuel cycle, most Pu-240 will be fissioned (possibly after more than one neutron capture) before it decays, but Pu-240 discarded as nuclear waste will decay over thousands of years.

While the largest part of uranium-236 has been produced by neutron capture in nuclear power reactors, it is for the most part stored in nuclear reactors and waste repositories. The most significant contribution to uranium-236 abundance in the environment is the ²³⁸U(n,3n)²³⁶U reaction by fast neutrons in thermonuclear weapons. The bomb-testing of the 1950s and 1960s has raised the environmental abundance levels significantly above the expected natural levels.^[6]

Destruction and decay

²³⁶U, on absorption of a thermal neutron, does not undergo fission, but becomes ²³⁷U, which quickly beta decays to ²³⁷Np. However, the neutron capture cross section of ²³⁶U is low, and this process does not happen quickly in a thermal reactor. Spent nuclear fuel typically contains about 0.4% U-236. With a much greater cross-section, ²³⁷Np may eventually absorb another neutron, becoming ²³⁸Np, which quickly beta decays to plutonium-238.

²³⁶U and most other actinides are fissionable by fast neutrons in a nuclear bomb or a fast neutron reactor. A small number of fast reactors have been in research use for decades, but widespread use for power production is still in the future.

Uranium-236 alpha decays with a half-life of 23.420 million years to thorium-232. It is longer-lived than any other artificial actinides or fission products produced in the nuclear fuel cycle. (Plutonium-244 which has a half-life of 80 million years is not produced in significant quantity by the nuclear fuel cycle, and the longer-lived U-235, U-238, and thorium-232 occur in nature.)

Difficulty of separation

Unlike plutonium, minor actinides, fission products, or activation products, chemical processes cannot separate U-236 from U-238, U-235, U-232 or other uranium isotopes. It is even difficult to remove with isotopic separation, as low enrichment will concentrate not only the desirable U-235 and U-233 but the undesirable U-236, U-234 and U-232. On the other hand, U-236 in the environment cannot separate from U-238 and concentrate separately, which limits its radiation hazard in any one place.

Contribution to radioactivity of reprocessed uranium

U-238's halflife is about 190 times as long as U-236; therefore U-236 should have about 190 times as much specific activity. That is, in reprocessed uranium with 0.5% U-236, the U-236 and U-238 will produce about the same level of radioactivity. (U-235 contributes only a few percent.)

The ratio is less than 190 when the decay products of each are included. U-238's decay chain to uranium-234 and eventually lead-206 involves emission of eight alpha particles in a time (hundreds of thousands of years) short compared to the halflife of U-238, so that a sample of U-238 in equilibrium with its decay products (as in natural uranium ore) will have eight times the alpha activity of U-238 alone. Even purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of U-234 and therefore about twice the alpha activity of pure U-238. Enrichment to increase U-235 content will increase U-234 to an even greater degree, and roughly half of this U-234 will survive in the spent fuel. On the other hand, U-236 decays to thorium-232 which has a halflife of 14 billion years, equivalent to a decay rate only 31.4% as great as that of U-238.

Depleted uranium

Depleted uranium used in kinetic energy penetrators, etc. is supposed to be made from uranium enrichment tailings that have never been irradiated in a nuclear reactor, not reprocessed uranium. However, there have been claims that some depleted uranium has contained small amounts of U-236.^[7]

Lighter: uranium-235	Uranium-236 is an isotope of uranium	Heavier: uranium-237
Decay product of: protactinium-236 neptunium-236 plutonium-240	Decay chain of uranium-236	Decays to: thorium-232

References

↑ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
↑ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
↑ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics 71 (2): 299. doi:10.1016/0029-5582(65)90719-4.
"The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk²⁴⁸ with a half-life greater than 9 y. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
↑ This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
↑ Excluding those "classically stable" isotopes with half-lives significantly in excess of ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd is nearly eight quadrillion years.
↑ Winkler, Stephan; Peter Steier; Jessica Carilli (2012). "Bomb fall-out 236U as a global oceanic tracer using an annually resolved coral core". Earth and Planet. Sci. Lett. 359-360: 124–130. doi:10.1016/j.epsl.2012.10.004.
↑ http://www.un.org/News/Press/docs/2001/unep81.doc.htm

External links

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