Isotopes of plutonium

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Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)
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 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

Plutonium (Pu) is an artificial element, except for trace quantities of primordial 244Pu, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are Pu-244, with a half-life of 80.8 million years, Pu-242, with a half-life of 373,300 years, and Pu-239, with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states, though none is very stable; all meta states have half-lives of less than one second.

The isotopes of plutonium range in atomic weight from 228.0387 u (Pu-228) to 247.074 u (Pu-247). The primary decay modes before the most stable isotope, Pu-244, are spontaneous fission and alpha emission; the initial mode after is beta emission. The primary decay products before Pu-244 are isotopes of uranium and neptunium (neglecting the wide range of daughter nuclei created by fission processes), and the primary products after are isotopes of americium.

Notable Isotopes[edit]

Production and uses[edit]

A pellet of plutonium-238, glowing from its own heat, used for radioisotope thermoelectric generators.
Transmutation flow between 238Pu and 244Cm in LWR.[7]
Transmutation speed not shown and varies greatly by nuclide.
245Cm–248Cm are long-lived with negligible decay.

Pu-239, a fissile isotope which is the second most used nuclear fuel in nuclear reactors after U-235, and the most used fuel in the fission portion of nuclear weapons, is produced from U-238 by neutron capture followed by two beta decays.

Pu-240, Pu-241, Pu-242 are produced by further neutron capture. The odd-mass isotopes Pu-239 and Pu-241 have about a 3/4 chance of undergoing fission on capture of a thermal neutron and about a 1/4 chance of retaining the neutron and becoming the following isotope. The even-mass isotopes are fertile material but not fissile and also have a lower overall probability (cross section) of neutron capture; therefore, they tend to accumulate in nuclear fuel used in a thermal reactor, the design of nearly all nuclear power plants today. In plutonium that has been used a second time in thermal reactors in MOX fuel, Pu-240 may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons. Pu-240 does have a moderate thermal neutron absorption cross section, so that Pu-241 production in a thermal reactor becomes a significant fraction as large as Pu-239 production.

Pu-241 has a half-life of 14 years, and has slightly higher thermal neutron cross sections than Pu-239 for both fission and absorption. While nuclear fuel is being used in a reactor, a Pu-241 nucleus is much more likely to fission or to capture a neutron than to decay. Pu-241 accounts for a significant proportion of fissions in thermal reactor fuel that has been used for some time. However, in spent nuclear fuel that does not quickly undergo nuclear reprocessing but instead is cooled for years after use, much or most of the Pu-241 will beta decay to americium-241, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors.

Pu-242 has a particularly low cross section for thermal neutron capture; and it takes four neutron absorptions to become another fissile isotope (either curium-245 or Pu-241) and fission. Even then, there is a chance either of those two fissile isotopes will fail to fission but instead absorb the fourth neutron, becoming curium-246 (on the way to even heavier actinides like californium, which is a neutron emitter by spontaneous fission and difficult to handle) or becoming Pu-242 again; so the mean number of neutrons absorbed before fission is even higher than 4. Therefore Pu-242 is particularly unsuited to recycling in a thermal reactor and would be better used in a fast reactor where it can be fissioned directly. However, Pu-242's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. Pu-242's half-life is about 15 times as long as Pu-239's half-life; therefore it is 1/15 as radioactive and not one of the larger contributors to nuclear waste radioactivity. 242Pu's gamma ray emissions are also weaker than those of the other isotopes.[8]

Pu-243 has a half-life of only 5 hours, beta decaying to americium-243. Because Pu-243 has little opportunity to capture an additional neutron before decay, the nuclear fuel cycle does not produce the extremely long-lived Pu-244 in significant quantity.

Pu-238 is not normally produced in as large quantity by the nuclear fuel cycle, but some is produced from neptunium-237 by neutron capture (this reaction can also be used with purified neptunium to produce Pu-238 relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on Pu-239, or by alpha decay of curium-242 which is produced by neutron capture from Am-241. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become Pu-239.

Manufacture[edit]

Pu-240, Pu-241 and Pu-242[edit]

The fission cross section for 239Pu is 747.9 barns for thermal neutrons, while the activation cross section is 270.7 barns (the ratio approximates to 11 fissions for every 4 neutron captures). The higher plutonium isotopes are created when the uranium fuel is used for a long time. It is the case that for high burnup used fuel that the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel which is reprocessed to obtain weapons grade plutonium.

The formation of 240Pu, 241Pu and 242Pu from 238U
Isotope Thermal neutron
cross section[9]
(barns)
Decay
Mode
Halflife
Capture Fission
238U 2.683 0.000 α 4.468 x 109 years
239U 20.57 14.11 β 23.45 minutes
239Np 77.03 β 2.356 days
239Pu 270.7 747.9 α 24,110 years
240Pu 287.5 0.064 α 6,561 years
241Pu 363.0 1012 β 14.325 years
242Pu 19.16 0.001 α 373,300 years

Pu-239[edit]

Main article: Plutonium-239

Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor.

A ring of weapons-grade electrorefined plutonium, with 99.96% purity. This 5.3 kg ring is enough plutonium for use in an efficient nuclear weapon. The ring shape is needed to depart from a spherical shape and avoid criticality.
The formation of 239Pu from 238U[10]
Element Isotope Thermal neutron capture
cross section (barn)
Thermal neutron fission
Cross section (barn)
decay mode halflife
U 238 2.68 5·10−6 α 4.47 x 109 years
U 239 22 15 β 23 minutes
Np 239 30 1 β 2.36 days
Pu 239 271 750 α 24,110 years

Pu-238[edit]

Main article: Plutonium-238

There are small amounts of Pu-238 in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a U-235 atom captures a neutron, it is converted to an excited state of U-236. Some of the excited U-236 nuclei undergo fission, but some decay to the ground state of U-236 by emitting gamma radiation. Further neutron capture creates U-237 which has a half-life of 7 days and thus quickly decays to Np-237. Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure Np-237 by chemical separation of neptunium. After this chemical separation, Np-237 is again irradiated by reactor neutrons to be converted to Np-238 which decays to Pu-238 with a half-life of 2 days.

The formation of 238Pu from 235U
Element Isotope Thermal neutron
cross section
decay mode halflife
U 235 99 α 703,800,000 years
U 236 5.3 α 23,420,000 years
U 237 - β 6.75 days
Np 237 165 (capture) α 2,144,000 years
Np 238 - β 2.11 days
Pu 238 - α 87.7 years

Pu-240 as obstacle to nuclear weapons[edit]

Pu-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of Pu-240 limits the plutonium's nuclear bomb potential because the neutron flux from spontaneous fission, initiates the chain reaction prematurely and reduces the bomb's power by exploding the core before full implosion is reached. Plutonium consisting of more than about 90% Pu-239 is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% Pu-240 and is called reactor-grade plutonium. However, modern nuclear weapons use fusion boosting which mitigates the predetonation problem; if the pit can generate a nuclear weapon yield of even a fraction of a kiloton, which is enough to start deuterium-tritium fusion, the resulting burst of neutrons will fission enough plutonium to ensure a yield of tens of kilotons.

Pu-240 contamination is the reason plutonium weapons must use the implosion method. Theoretically, pure Pu-239 could be used in a gun-type nuclear weapon, but achieving this level of purity is prohibitively difficult. Pu-240 contamination has proven a mixed blessing to nuclear weapons design. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those very same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone toward accidental detonation than are gun-type weapons.

Table[edit]

nuclide
symbol
Z(p) N(n)  
isotopic mass (u)
 
half-life decay
mode(s)[11][n 1]
daughter
isotope(s)[n 2]
nuclear
spin
representative
isotopic
composition
(mole fraction)
range of natural
variation
(mole fraction)
excitation energy
228Pu 94 134 228.03874(3) 1.1(+20-5) s α (99.9%) 224U 0+
β+ (.1%) 228Np
229Pu 94 135 229.04015(6) 120(50) s α 225U 3/2+#
230Pu 94 136 230.039650(16) 1.70(17) min α 226U 0+
β+ (rare) 230Np
231Pu 94 137 231.041101(28) 8.6(5) min β+ 231Np 3/2+#
α (rare) 227U
232Pu 94 138 232.041187(19) 33.7(5) min EC (89%) 232Np 0+
α (11%) 228U
233Pu 94 139 233.04300(5) 20.9(4) min β+ (99.88%) 233Np 5/2+#
α (.12%) 229U
234Pu 94 140 234.043317(7) 8.8(1) h EC (94%) 234Np 0+
α (6%) 230U
235Pu 94 141 235.045286(22) 25.3(5) min β+ (99.99%) 235Np (5/2+)
α (.0027%) 231U
236Pu 94 142 236.0460580(24) 2.858(8) a α 232U 0+
SF (1.37×10−7%) (various)
CD (2×10−12%) 208Pb
28Mg
β+β+ (rare) 236U
237Pu 94 143 237.0484097(24) 45.2(1) d EC 237Np 7/2-
α (.0042%) 233U
237m1Pu 145.544(10) keV 180(20) ms IT 237Pu 1/2+
237m2Pu 2900(250) keV 1.1(1) µs
238Pu 94 144 238.0495599(20) 87.7(1) a α 234U 0+
SF (1.9×10−7%) (various)
CD (1.4×10−14%) 206Hg
32Si
CD (6×10−15%) 180Yb
30Mg
28Mg
239Pu[n 3][n 4] 94 145 239.0521634(20) 2.411(3)×104 a α 235U 1/2+
SF (3.1×10−10%) (various)
239m1Pu 391.584(3) keV 193(4) ns 7/2-
239m2Pu 3100(200) keV 7.5(10) µs (5/2+)
240Pu 94 146 240.0538135(20) 6,561(7) a α 236U 0+
SF (5.7×10−6%) (various)
CD (1.3×10−13%) 206Hg
34Si
241Pu[n 3] 94 147 241.0568515(20) 14.290(6) a β- (99.99%) 241Am 5/2+
α (.00245%) 237U
SF (2.4×10−14%) (various)
241m1Pu 161.6(1) keV 0.88(5) µs 1/2+
241m2Pu 2200(200) keV 21(3) µs
242Pu 94 148 242.0587426(20) 3.75(2)×105 a α 238U 0+
SF (5.5×10−4%) (various)
243Pu[n 3] 94 149 243.062003(3) 4.956(3) h β- 243Am 7/2+
243mPu 383.6(4) keV 330(30) ns (1/2+)
244Pu[n 5] 94 150 244.064204(5) 8.00(9)×107 a α (99.88%) 240U 0+ Trace
SF (.123%) (various)
β-β- (7.3×10−9%) 244Cm
245Pu 94 151 245.067747(15) 10.5(1) h β- 245Am (9/2-)
246Pu 94 152 246.070205(16) 10.84(2) d β- 246mAm 0+
247Pu 94 153 247.07407(32)# 2.27(23) d β- 247Am 1/2+#
  1. ^ Abbreviations:
    CD: Cluster decay
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  2. ^ Bold for stable isotopes
  3. ^ a b c Fissile nuclide
  4. ^ Most useful isotope for nuclear weapons
  5. ^ Primordial radionuclide

Notes[edit]

  • 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. ^ ieer.org
  7. ^ 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. 
  8. ^ "PLUTONIUM ISOTOPIC RESULTS OF KNOWN SAMPLES USING THE SNAP GAMMA SPECTROSCOPY ANALYSIS CODE AND THE ROBWIN SPECTRUM FITTING ROUTINE" (PDF). 
  9. ^ National Nuclear Data Center Interactive Chart of Nuclides
  10. ^ Miner 1968, p. 541
  11. ^ http://www.nucleonica.net/unc.aspx
Isotopes of neptunium Isotopes of plutonium Isotopes of americium
Table of nuclides