Isotopes of uranium

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Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)
Fission products by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 155Euþ
244Cm 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Pu 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210k years…

241Am 251Cfƒ[4] 430–900
226Ra 247Bk 1.3k–1.6k
240Pu 229Th 246Cm 243Am 4.7k–7.4k
245Cmƒ 250Cm 8.3k–8.5k
239Puƒ 24.1k
230Th 231Pa 32k–76k
236Npƒ 233Uƒ 234U 150k–250k 99Tc 126Sn
248Cm 242Pu 327k–375k 79Se
1.53M 93Zr
237Np 2.1M–6.5M 135Cs 107Pd
236U 247Cmƒ 15M–24M 129I
244Pu 80M

...nor beyond 15.7M[5]

232Th 238U 235Uƒ№ 0.7G–14.1G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4a–97a: Medium-lived fission product
‡  over 200ka: Long-lived fission product

Uranium (U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes (uranium-238 and uranium-235) that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The average atomic mass of natural uranium is 238.02891(3) u. Other isotopes such as uranium-232 have been produced in breeder reactors.

Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.2739 - 99.2752% natural abundance), uranium-235 (0.7198 - 0.7202%), and uranium-234 (0.0050 - 0.0059%).[6] All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683×109 years (close to the age of the Earth).

Uranium-238 is an α emitter, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members that ends in lead-207. The constant rates of decay in these series makes comparison of the ratios of parent to daughter elements useful in radiometric dating. Uranium-233 is made from thorium-232 by neutron bombardment.

The isotope uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons. The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.

Uranium-232[edit]

Isotopes of uranium
General
Name, symbol U-232,232U
Neutrons 140
Protons 92
Nuclide data
Half-life 68.9 years
Parent isotopes 236Pu (α)
232Np (β+)
232Pa (β)
Decay products 228Th

Uranium 232 (232
92
U
140
, 232U, U-232) is an isotope of uranium. It has a half-life of 68.9 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamma radiation of 232U's decay products makes the 233U contaminated with it more difficult to handle.

Production of 233U (through the neutron irradiation of 232Th) invariably produces small amounts of 232U as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233:

232Th (n,γ) 233Th (β−) 233Pa (β−) 233U (n,2n) 232U
232Th (n,γ) 233Th (β−) 233Pa (n,2n) 232Pa (β−) 232U

The decay chain of 232U quickly yields strong gamma radiation emitters:

232U (α, 68.9 years)
228Th (α, 1.9 year)
224Ra (α, 3.6 day, 0.24 MeV) (at this point, the decay chain is identical to that of 232Th)
220Rn (α, 55 s, 0.54 MeV)
216Po (α, 0.15 s)
212Pb (β−, 10.64 h)
212Bi (α, 61 m, 0.78 MeV)
208Tl (β−, 3 m, 2.6 MeV) (35.94% branching ratio)
208Pb (stable)

This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, (except possibly in a short period immediately following chemical separation of the uranium from thorium-228, radium-224, radon-220, and polonium) and instead requiring remote manipulation for fuel fabrication.

Unusually for an isotope with even mass number, 232U has a significant neutron absorption cross section for fission (thermal neutrons 75 barns (b), resonance integral 380 b) as well as for neutron capture (thermal 73 b, resonance integral 280 b).


Lighter:
uranium-231
Isotopes of uranium is an
isotope of uranium
Heavier:
uranium-233
Decay product of:
plutonium-236 (α)
neptunium-232 (β+)
protactinium-232 (β)
Decay chain
of isotopes of uranium
Decays to:
thorium-228 (α)

Uranium-233[edit]

Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel; however, it was never deployed in nuclear weapons or used commercially as a nuclear fuel.[1] It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 159,200 years.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.

Uranium-233 usually fissions on neutron absorption but sometimes retains the neutron, becoming uranium-234. The capture-to-fission ratio is smaller than the other two major fissile fuels uranium-235 and plutonium-239; it is also lower than that of short-lived plutonium-241, but bested by very difficult-to-produce neptunium-236.

Uranium-234[edit]

Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, U-234 occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of U-238. The path of production of U-234 via nuclear decay is as follows: U-238 nuclei emit an alpha particle to become thorium-234 (Th-234). Next, with a short half-life, Th-234 nuclei emit a beta particle to become protactinium-234 (Pa-234). Finally, Pa-234 nuclei emit another beta particle to become U-234 nuclei.

U-234 nuclei usually last for hundreds of thousands of years, but then they decay by alpha emission to thorium-230, except for the small percentage of nuclei which undergo spontaneous fission.

Extraction of rather small amounts of U-234 from natural uranium would be feasible using isotope separation, similar to that used for regular uranium-enrichment. However there is no real demand in chemistry, physics, or engineering for isolating U-234. Very small pure samples of U-234 can be extracted via the chemical ion-exchange process - from samples of plutonium-238 that have been aged somewhat to allow some decay to U-234 via alpha emission.

Enriched uranium contains more U-234 than natural uranium as a byproduct of the uranium enrichment process aimed at obtaining U-235, which concentrates lighter isotopes even more strongly than it does U-235. The increased percentage of U-234 in enriched natural uranium is acceptable in current nuclear reactors, but (re-enriched) reprocessed uranium might contain even higher fractions of U-234, which is undesirable. This is because U-234 is not fissile, and tends to absorb slow neutrons in a nuclear reactor - becoming U-235.

U-234 has a neutron-capture cross-section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral - the average over neutrons having various intermediate energies. In a nuclear reactor non-fissile isotopes capture a neutron breeding fissile isotopes. U-234 is converted to U-235 more easily and therefore at a greater rate than U-238 is to Pu-239 (via neptunium-239) because U-238 has a much smaller neutron-capture cross-section of just 2.7 barns.

Uranium-235[edit]

Uranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.

Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its (fission) nuclear cross section[disambiguation needed] for slow thermal neutron is about 504.81 barns. For fast neutrons it is on the order of 1 barn. Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.

Uranium-236[edit]

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.

Uranium-237[edit]

Uranium-237 is an isotope of uranium. It has a half live of about 6.75(1) days. It decays into neptunium-237 by beta decay.

Uranium-238[edit]

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. Uranium-238 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). Depleted uranium has an even higher concentration of the U-238 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 U-238, 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

Uranium-239[edit]

Isotopes of uranium
General
Name, symbol U-239,239U
Neutrons 147
Protons 92
Nuclide data
Half-life 23.45 mins
Decay products 239Np
Decay mode Decay energy
Beta decay 20% 1.28 MeV
Beta decay 80% 1.21 MeV

Uranium-239 is an isotope of uranium. It is usually produced by exposing 238U to neutron radiation in a nuclear reactor. 239U has a half-life of about 23.45 minutes and decays into neptunium-239 through beta decay, with a total decay energy of about 1.29 MeV.[7] The most common gamma decay at 74.660 keV accounts for the difference in the two major channels of beta emission energy, at 1.28 and 1.21 MeV.[8]

239Np further decays to plutonium-239, in a second important step that ultimately produces fissile 239Pu (used in weapons and for nuclear power), from 238U in reactors.


Lighter:
uranium-238
Isotopes of uranium is an
isotope of uranium
Heavier:
uranium-240
Decay product of:
protactinium-239 (β-)
Decay chain
of isotopes of uranium
Decays to:
neptunium-239 (β-)


Table[edit]

nuclide
symbol
historic
name
Z(p) N(n)  
isotopic mass (u)
 
half-life decay
mode(s)[9][n 1]
daughter
isotope(s)[n 2]
nuclear
spin
representative
isotopic
composition
(mole fraction)
range of natural
variation
(mole fraction)
excitation energy
217U 92 125 217.02437(9) 26(14) ms
[16(+21-6) ms]
1/2-#
218U 92 126 218.02354(3) 6(5) ms α 214Th 0+
219U 92 127 219.02492(6) 55(25) ms
[42(+34-13) ms]
α 215Th 9/2+#
220U 92 128 220.02472(22)# 60# ns α 216Th 0+
β+ (rare) 220Pa
221U 92 129 221.02640(11)# 700# ns α 217Th 9/2+#
β+ (rare) 221Pa
222U 92 130 222.02609(11)# 1.4(7) us
[1.0(+10-4) us]
α 218Th 0+
β+ (10−6%) 222Pa
223U 92 131 223.02774(8) 21(8) us
[18(+10-5) us]
α 219Th 7/2+#
224U 92 132 224.027605(27) 940(270) us α 220Th 0+
225U 92 133 225.02939# 61(4) ms α 221Th (5/2+)#
226U 92 134 226.029339(14) 269(6) ms α 222Th 0+
227U 92 135 227.031156(18) 1.1(1) min α 223Th (3/2+)
β+ (.001%) 227Pa
228U 92 136 228.031374(16) 9.1(2) min α (95%) 224Th 0+
EC (5%) 228Pa
229U 92 137 229.033506(6) 58(3) min β+ (80%) 229Pa (3/2+)
α (20%) 225Th
230U 92 138 230.033940(5) 20.8 d α 226Th 0+
SF (1.4×10−10%) (various)
β+β+ (rare) 230Th
231U 92 139 231.036294(3) 4.2(1) d EC 231Pa (5/2)(+#)
α (.004%) 227Th
232U 92 140 232.0371562(24) 68.9(4) y α 228Th 0+
CD (8.9×10−10%) 208Pb
24Ne
CD (5×10−12%) 204Hg
28Mg
SF (10−12%) (various)
233U 92 141 233.0396352(29) 1.592(2)×105 y α 229Th 5/2+
SF (6×10−9%) (various)
CD (7.2×10−11%) 209Pb
24Ne
CD (1.3×10−13%) 205Hg
28Mg
234U[n 3][n 4] Uranium II 92 142 234.0409521(20) 2.455(6)×105 y α 230Th 0+ [0.000054(5)][n 5] 0.000050-
0.000059
SF (1.73×10−9%) (various)
CD (1.4×10−11%) 206Hg
28Mg
CD (9×10−12%) 184Hf
26Ne
24Ne
234mU 1421.32(10) keV 33.5(20) ms 6-
235U[n 6][n 7][n 8] Actin Uranium
Actino-Uranium
92 143 235.0439299(20) 7.04(1)×108 y α 231Th 7/2- [0.007204(6)] 0.007198-
0.007207
SF (7×10−9%) (various)
CD (8×10−10%) 186Hf
25Ne
24Ne
235mU 0.0765(4) keV ~26 min IT 235U 1/2+
236U 92 144 236.045568(2) 2.342(3)×107 y α 232Th 0+
SF (9.6×10−8%) (various)
236m1U 1052.89(19) keV 100(4) ns (4)-
236m2U 2750(10) keV 120(2) ns (0+)
237U 92 145 237.0487302(20) 6.75(1) d β- 237Np 1/2+
238U[n 4][n 6][n 7] Uranium I 92 146 238.0507882(20) 4.468(3)×109 y α 234Th 0+ [0.992742(10)] 0.992739-
0.992752
SF (5.45×10−5%) (various)
β-β- (2.19×10−10%) 238Pu
238mU 2557.9(5) keV 280(6) ns 0+
239U 92 147 239.0542933(21) 23.45(2) min β- 239Np 5/2+
239m1U 20(20)# keV >250 ns (5/2+)
239m2U 133.7990(10) keV 780(40) ns 1/2+
240U 92 148 240.056592(6) 14.1(1) h β- 240Np 0+
α (10−10%) 236Th
241U 92 149 241.06033(32)# 5# min β- 241Np 7/2+#
242U 92 150 242.06293(22)# 16.8(5) min β- 242Np 0+
  1. ^ Abbreviations:
    CD: Cluster decay
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  2. ^ Bold for stable isotopes, bold italics for nearly-stable isotopes (half-life longer than the age of the universe)
  3. ^ Used in uranium-thorium dating
  4. ^ a b Used in uranium-uranium dating
  5. ^ Intermediate decay product of 238U
  6. ^ a b Primordial radionuclide
  7. ^ a b Used in Uranium-lead dating
  8. ^ Important in nuclear reactors

Notes[edit]

  • Evaluated isotopic composition is for most but not all commercial samples.
  • The precision of the isotope abundances and atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
  • Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
  • Commercially available materials may have been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations from the given mass and composition can occur.
  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References[edit]

  1. ^ 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 a notable 1600 years, thus merits the element's inclusion here.
  2. ^ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  3. ^ 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.  edit
    "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."
  4. ^ This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
  5. ^ 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.
  6. ^ Uranium Isotopes, retrieved 14 March 2012 
  7. ^ CRC Handbook, 57th Ed. p. B-345
  8. ^ CRC Handbook, 57th Ed. p. B-423
  9. ^ http://www.nucleonica.net/unc.aspx
Isotopes of protactinium Isotopes of uranium Isotopes of neptunium
Table of nuclides