URANIUM-236

'Uranium-236'
Full table
'General'
Name, symbol	Uranium-236, 236U
Neutrons	144
Protons	92
'Nuclide Data'
Natural abundance	0%
Half-life	15·106 years
Decay products	Thorium-232
Isotope mass	236.045568(2) u
Spin	0+
Excess energy	? ± ? keV
Binding energy	1783870.285 ± 1.996 keV
'Decay mode'	'Decay energy'
SF	n/a MeV
Alpha decay	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 the reprocessed uranium made from it.

Contents

Creation and yield

Destruction and decay

Difficulty of separation

Contribution to radioactivity of reprocessed uranium

Depleted uranium

See also

External links

Creation and yield

The fissile isotope uranium-235 which fuels most nuclear reactors will fission after absorbing a thermal neutron about 6/7 of the time. About 1/7 of the time, it merely emits gamma radiation and remains U-236. Thus, the yield of U-236 per 100 fissions is about 16%. In comparison, the yields of the most abundant individual fission products like Cs-137, Sr-90, Tc-99 are between 6% and 7%.

Destruction and decay

²³⁶U, on absorption of a neutron, does not 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.
²³⁶U and all other actinides are fissionable by fast neutrons in a nuclear bomb or a fast neutron reactor; however, the latter are not yet in industrial use.
Uranium-236 alpha decays to Thorium-232 with a half-life of 23.42 million years. It is longer-lived than any other actinides other than the naturally occurring Th-232, U-238, and U-235, or Plutonium-244 which has a half-life of 80 million years, but is not produced in significant quantity by the nuclear fuel cycle.

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 U-234 and eventually Lead-206 involves emission of 8 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 8 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 most 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 weapons, 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 DU has contained small amounts of U-236. [1]

See also

★ Depleted uranium

★ Uranium market

★ Nuclear reprocessing

★ United States Enrichment Corporation

★ Nuclear fuel cycle

★ Nuclear power

External links

★ Uranium | Radiation Protection Program | US EPA

★ NLM Hazardous Substances Databank - Uranium, Radioactive