Uranium-238

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For the German submarine, see German submarine U-238.
Uranium-238
Uranium03.jpg

10 gram sample

General
Name, symbol Uranium-238,238U
Neutrons 146
Protons 92
Nuclide data
Natural abundance 99.284%
Half-life 4.468 billion years
Parent isotopes 242Pu (α)
238Pa (β)
Decay products 234Th
Isotope mass 238.05078826 u
Decay mode Decay energy
Alpha decay 4.267 MeV

Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239. 238U is fissionable by fast neutrons, but cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of U-238's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

Around 99.284% of natural uranium is uranium-238, which has a half-life of 1.41×1017 seconds (4.468×109 years, or 4.468 billion years).[1] Depleted uranium has an even higher concentration of the 238U isotope, and even low-enriched uranium (LEU), while having a higher proportion of the uranium-235 isotope (in comparison to depleted uranium), is still mostly 238U. Reprocessed uranium is also mainly 238U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236, and much smaller amounts of other isotopes of uranium such as uranium-234, uranium-233, and uranium-232.[2]

Nuclear energy applications[edit]

In a fission nuclear reactor, uranium-238 can be used to breed 239Pu, which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power does come from the fission of 239Pu, which is not supplied as a fuel to the reactor, but rather, produced from 238U.

Breeder reactors[edit]

238U is not usable directly as nuclear fuel, though it can produce energy via "fast" fission. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of 238U to split in two. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the about 1.7 neutrons produced in each fission have enough speed to continue a chain reaction.

238U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope 238U into fissile Pu-239. It has been estimated that there is anywhere from 10,000 to five billion years worth of 238U for use in these power plants.[3] Breeder technology has been used in several experimental nuclear reactors.[4]

By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned to be started, having been shut down since 1995, and both China and India have announced plans to build nuclear breeder reactors.

The breeder reactor as its name implies creates even larger quantities of Pu-239 than the fission nuclear reactor.

The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to control delayed neutrons, will potentially be able to burn 238U as fuel once the reactor is started with LEU fuel. This design is still in the early stages of development.

Radiation shielding[edit]

238U is also used as a radiation shield — its alpha radiation is easily stopped by the non-radioactive casing of the shielding and the uranium's high atomic weight and high number of electrons are highly effective in absorbing gamma rays and x-rays. It is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium and depleted uranium dioxide are used for radiation shielding. Uranium is about five times better as a gamma ray shield than lead, so a shield with the same effectiveness can be packed into a thinner layer.

DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storage systems to store radioactive waste.

Downblending[edit]

The opposite of enriching is downblending. Surplus highly enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low enriched uranium suitable for use in commercial nuclear fuel.

238U from depleted uranium and natural uranium is also used with recycled Pu-239 from nuclear weapons stockpiles for making mixed oxide fuel (MOX), which is now being redirected to become fuel for nuclear reactors. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the very expensive and complex chemical separation of uranium and plutonium process before assembling a weapon.

Nuclear weapons[edit]

Most modern nuclear weapons utilize 238U as a "tamper" material (see nuclear weapon design). A tamper which surrounds a fissile core works to reflect neutrons and to add inertia to the compression of the Pu-239 charge. As such, it increases the efficiency of the weapon and reduces the critical mass required. In the case of a thermonuclear weapon 238U can be used to encase the fusion fuel, the high flux of very energetic neutrons from the resulting fusion reaction causes 238U nuclei to split and adds more energy to the "yield" of the weapon. Such weapons are referred to as fission-fusion-fission weapons after the three consecutive stages of the explosion. An example of such a weapon is Castle Bravo.

The larger portion of the total explosive yield in this design comes from the final fission stage fueled by 238U, producing enormous amounts of radioactive fission products. For example, an estimated 77% of the 10.4-megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper. Because depleted uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The Soviet Union's test of the "Tsar Bomba" in 1961 produced "only" 60 megatons of explosive power, over 90% of which came from fusion, because the 238U final stage had been replaced with lead. Had 238U been used instead, the yield of the "Tsar Bomba" could have been well-above 100 megatons, and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time.

Radioactivity and decay[edit]

238U radiates alpha-particles and decays (by way of thorium-234 and protactinium-234) into uranium-234. 234U has a half-life of 245,500 years. The relation between 238U and 234U gives an indication of the age of sediments that are between 100,000 years and 1,200,000 years in age.[5]

238U occasionally decays by spontaneous fission or double beta decay with probabilities of 5×10−7 and 2×10−12 per alpha decay, respectively.[6]

Radium series (or uranium series)[edit]

The 4n+2 chain of 238U is commonly called the "radium series" (sometimes "uranium series"). Beginning with naturally occurring uranium-238, this series includes the following elements: astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium. All are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral.

parent nuclide historic name (short)
[citation needed]
historic name (long) atomic mass [RS 1] decay mode [RS 2] branch chance [RS 2] half life [RS 2] energy released, MeV [RS 2] daughter nuclide [RS 2] Subtotal MeV
238U UI Uranium I 238.051 α 100 % 9.654.468·109 a 4.26975 234Th 4.2698
234Th UX1 Uranium X1 234.044 β- 100 % -1.1824.10 d 0.273088 234mPa 4.5428
234mPa UX2 Uranium X2, Brevium 234.043 IT 0.16 % -5.661.159 min 0.07392 234Pa 4.6168
234mPa UX2 Uranium X2, Brevium 234.043 β- 99.84 % -5.661.159 min 2.268205 234U 6.8110
234Pa UZ Uranium Z 234.043 β- 100 % -3.126.70 h 2.194285 234U 6.8110
234U UII Uranium II 234.041 α 100 % 5.392.455·105 a 4.8598 230Th 11.6708
230Th Io Ionium 230.033 α 100 % 4.887.54·104 a 4.76975 226Ra 16.4406
226Ra Ra Radium 226.025 α 100 % 3.201600 a 4.87062 222Rn 21.3112
222Rn Rn Radon, Radium Emanation 222.018 α 100 % -1.983.8235 d 5.59031 218Po 26.9015
218Po RaA Radium A 218.009 β- 0.020 % -5.233.098 min 0.259913 218At 27.1614
218Po RaA Radium A 218.009 α 99.980 % -5.233.098 min 6.11468 214Pb 33.0162
218At 218.009 β- 0.1 % -7.321.5 s 2.881314 218Rn 30.0428
218At 218.009 α 99.9 % -7.321.5 s 6.874 214Bi 34.0354
218Rn 218.006 α 100 % -8.9635 ms 7.26254 214Po 37.3053
214Pb RaB Radium B 214.000 β- 100 % -4.2926.8 min 1.019237 214Bi 34.0354
214Bi RaC Radium C 213.999 β- 99.979 % -4.4219.9 min 3.269857 214Po 37.3053
214Bi RaC Radium C 213.999 α 0.021 % -4.4219.9 min 5.62119 210Tl 39.6566
214Po RaC' Radium C' 213.995 α 100 % -11.28164.3 μs 7.83346 210Pb 45.1388
210Tl RaC" Radium C" 209.990 β- 100 % -5.611.30 min 5.48213 210Pb 45.1388
210Pb RaD Radium D 209.984 β- 100 % 1.3522.20 a 0.063487 210Bi 45.2022
210Pb RaD Radium D 209.984 α 0.0000019%1.9·10-6 % 1.3522.20 a 3.7923 206Hg 48.9311
210Bi RaE Radium E 209.984 β- 100 % -1.865.012 d 1.161234 210Po 46.3635
210Bi RaE Radium E 209.984 α 0.000133%13.2·10-5 % -1.865.012 d 5.03647 206Tl 50.2387
210Po RaF Radium F 209.983 α 100 % -0.42138.376 d 5.40745 206Pb 51.7709
206Hg 205.978 β- 100 % -4.808.32 min 1.307649 206Tl 50.2387
206Tl RaE" Radium E" 205.976 β- 100 % -5.104.202 min 1.532221 206Pb 51.7709
206Pb RaG Radium G 205.974 stable - 999- - - 51.7709
  1. ^ "The Risk Assessment Information System: Radionuclide Decay Chain". The University of Tennessee. 
  2. ^ a b c d e "Evaluated Nuclear Structure Data File". National Nuclear Data Center. 

The mean lifetime of 238U is 1.41×1017 seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2×1017 seconds, so 1 mole of 238U emits 3×106 alpha particles per second, producing the same number of thorium-234 (Th-234) atoms. In a closed system an equilibrium would be reached, with all amounts except for lead-206 and 238U in fixed ratios, in slowly decreasing amounts. The amount of Pb-206 will increase accordingly while that of 238U decreases; all steps in the decay chain have this same rate of 3×106 decayed particles per second per mole 238U.

Thorium-234 has a mean lifetime of 3×106 seconds, so there is equilibrium if one mole of 238U contains 9×1012 atoms of thorium-234, which is 1.5×10−11 mole (the ratio of the two half-lives). Similarly, in an equilibrium in a closed system the amount of each decay product, except the end product lead, is proportional to its half-life.

As already touched upon above, when starting with pure 238U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of 238U, 3×106 times per second one alpha and two beta particles and gamma ray are produced, together 6.7 MeV, a rate of 3 µW. Extrapolated over 2×1017 seconds this is 600 gigajoules, the total energy released in the first three steps in the decay chain.


Lighter:
uranium-237
Uranium-238 is an
isotope of uranium
Heavier:
uranium-239
Decay product of:
plutonium-242 (α)
protactinium-238 (β-)
Decay chain
of uranium-238
Decays to:
thorium-234 (α)

References[edit]

  1. ^ Mcclain, D.E.; A.C. Miller, J.F. Kalinich (December 20, 2007). "Status of Health Concerns about Military Use of Depleted Uranium and Surrogate Metals in Armor-Penetrating Munitions" (pdf). NATO. Retrieved November 14, 2010. 
  2. ^ Nuclear France: Materials and sites. "Uranium from reprocessing". 
  3. ^ Facts from Cohen. Formal.stanford.edu (2007-01-26). Retrieved on 2010-10-24.
  4. ^ Advanced Nuclear Power Reactors | Generation III+ Nuclear Reactors. World-nuclear.org. Retrieved on 2010-10-24.
  5. ^ Encyclopædia Britannica (14 November 2007). "uranium-234–uranium-238 dating". 
  6. ^ [1]

External links[edit]

See also[edit]