Plutonium (Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight 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 plutonium-238 in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years; plutonium-242 with a half-life of 373,300 years; and plutonium-239 with a half-life of 24,110 years; and plutonium-240 with a half-life of 6,560 years. This element also has eight meta states; all have half-lives of less than one second.
Isotopes of plutonium range in atomic weight from 228.0387 u (Pu) to 247.074 u (Pu). The primary decay modes before the most stable isotope, Pu, are spontaneous fission and alpha decay; the primary mode after is beta emission. The primary decay products before Pu are isotopes of uranium and neptunium (not considering fission products), and the primary decay products after are isotopes of americium.
|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 134| rowspan=2|228.03874(3)| rowspan=2|| α | U| rowspan=2|0+| rowspan=2||-| β (rare)| Np|-| rowspan=2|Pu[1] | rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 135| rowspan=2|229.04015(6)| rowspan=2|91(26) s| α (50%)| U| rowspan=2|3/2+#| rowspan=2||-| β (50%)| Np|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 136| rowspan=2|230.039650(16)| rowspan=2|1.70(17) min| α (>73%)[2] | U| rowspan=2|0+| rowspan=2||-| β (<27%)| Np|-| rowspan=2|Pu[3] | rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 137| rowspan=2|231.041101(28)| rowspan=2|8.6(5) min| β (87%)| Np| rowspan=2|(3/2+)| rowspan=2||-| α (13%)| U|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 138| rowspan=2|232.041187(19)| rowspan=2|33.7(5) min| EC (89%)| Np| rowspan=2|0+| rowspan=2||-| α (11%)| U|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 139| rowspan=2|233.04300(5)| rowspan=2|20.9(4) min| β (99.88%)| Np| rowspan=2|5/2+#| rowspan=2||-| α (.12%)| U|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 140| rowspan=2|234.043317(7)| rowspan=2|8.8(1) h| EC (94%)| Np| rowspan=2|0+| rowspan=2||-| α (6%)| U|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 141| rowspan=2|235.045286(22)| rowspan=2|25.3(5) min| β (99.99%)| Np| rowspan=2|(5/2+)| rowspan=2||-| α (.0027%)| U|-| rowspan=3|Pu| rowspan=3 style="text-align:right" | 94| rowspan=3 style="text-align:right" | 142| rowspan=3|236.0460580(24)| rowspan=3|2.858(8) y| α| U| rowspan=3|0+| rowspan=3||-| SF (1.37×10%)| (various)|-| CD (2×10%)| Pb
Mg|-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 143| rowspan=2|237.0484097(24)| rowspan=2|45.2(1) d| EC| Np| rowspan=2|7/2−| rowspan=2||-| α (.0042%)| U|-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 145.544(10)2 keV| 180(20) ms| IT| Pu| 1/2+||-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 2900(250) keV| 1.1(1) μs|||||-| rowspan=4|Pu| rowspan=4 style="text-align:right" | 94| rowspan=4 style="text-align:right" | 144| rowspan=4|238.0495599(20)| rowspan=4|87.7(1) y| α| U| rowspan=4|0+| rowspan=4|Trace[4] |-| SF (1.9×10%)| (various)|-| CD (1.4×10%)| Hg
Si|-| CD (6×10%)| Yb
Mg
Mg|-| rowspan=2|Pu[5] [6] | rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 145| rowspan=2|239.0521634(20)| rowspan=2|2.411(3)×104 y| α| U| rowspan=2|1/2+| rowspan=2|Trace[7] |-| SF (3.1×10%)| (various)|-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 391.584(3) keV| 193(4) ns||| 7/2−||-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 3100(200) keV| 7.5(10) μs||| (5/2+)||-| rowspan=3|Pu| rowspan=3 style="text-align:right" | 94| rowspan=3 style="text-align:right" | 146| rowspan=3|240.0538135(20)| rowspan=3|6.561(7)×10 y| α| U| rowspan=3|0+| rowspan=3|Trace[8] |-| SF (5.7×10%)| (various)|-| CD (1.3×10%)| Hg
Si|-| rowspan=3|Pu| rowspan=3 style="text-align:right" | 94| rowspan=3 style="text-align:right" | 147| rowspan=3|241.0568515(20)| rowspan=3|14.290(6) y| β (99.99%)| Am| rowspan=3|5/2+| rowspan=3||-| α (.00245%)| U|-| SF (2.4×10%)| (various)|-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 161.6(1) keV| 0.88(5) μs||| 1/2+||-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 2200(200) keV| 21(3) μs|||||-| rowspan=2|Pu| rowspan=2 style="text-align:right" | 94| rowspan=2 style="text-align:right" | 148| rowspan=2|242.0587426(20)| rowspan=2|3.75(2)×10 y| α| U| rowspan=2|0+| rowspan=2||-| SF (5.5×10%)| (various)|-| Pu| style="text-align:right" | 94| style="text-align:right" | 149| 243.062003(3)| 4.956(3) h| β| Am| 7/2+||-| style="text-indent:1em" | Pu| colspan="3" style="text-indent:2em" | 383.6(4) keV| 330(30) ns||| (1/2+)||-| rowspan=3|Pu| rowspan=3 style="text-align:right" | 94| rowspan=3 style="text-align:right" | 150| rowspan=3|244.064204(5)| rowspan=3|8.00(9)×10 y| α (99.88%)| U| rowspan=3|0+| rowspan=3|Trace[9] |-| SF (.123%)| (various)|-| ββ (<7.3×10%)| Cm|-| Pu| style="text-align:right" | 94| style="text-align:right" | 151| 245.067747(15)| 10.5(1) h| β| Am| (9/2−)||-| Pu| style="text-align:right" | 94| style="text-align:right" | 152| 246.070205(16)| 10.84(2) d| β| Am| 0+||-| Pu| style="text-align:right" | 94| style="text-align:right" | 153| 247.07407(32)#| 2.27(23) d| β| Am| 1/2+#|
Pu, a fissile isotope that is the second most used nuclear fuel in nuclear reactors after uranium-235, and the most used fuel in the fission portion of nuclear weapons, is produced from uranium-238 by neutron capture followed by two beta decays.
Pu, Pu, and Pu are produced by further neutron capture. The odd-mass isotopes Pu and Pu 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 next heavier isotope. The even-mass isotopes are fertile but not fissile and also have a lower 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 may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons. Pu does have a moderate thermal neutron absorption cross section, so that Pu production in a thermal reactor becomes a significant fraction as large as Pu production.
Pu has a half-life of 14 years, and has slightly higher thermal neutron cross sections than Pu for both fission and absorption. While nuclear fuel is being used in a reactor, a Pu nucleus is much more likely to fission or to capture a neutron than to decay. Pu accounts for a significant portion 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 will beta decay to americium-241, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors.
Pu has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (either curium-245 or Pu) and fission. Even then, there is a chance either of those two fissile isotopes will fail to fission but instead absorb a 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 again; so the mean number of neutrons absorbed before fission is even higher than 3. Therefore, Pu 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's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. Pu's half-life is about 15 times as long as Pu's half-life; therefore, it is 1/15 as radioactive and not one of the larger contributors to nuclear waste radioactivity.Pu's gamma ray emissions are also weaker than those of the other isotopes.[12]
Pu has a half-life of only 5 hours, beta decaying to americium-243. Because Pu has little opportunity to capture an additional neutron before decay, the nuclear fuel cycle does not produce the long-lived Pu in significant quantity.
Pu 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 relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on Pu, or by alpha decay of curium-242, which is produced by neutron capture of Am. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become Pu.
The fission cross section for Pu 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. For high burnup used fuel, the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel that is reprocessed to obtain weapons grade plutonium.
| Thermal neutron cross section[13] (barns) ! | Decay Mode | Half-life | |||
|---|---|---|---|---|---|
| Capture | Fission | ||||
| 2.683 | 0.000 | α | 4.468 x 109 years | ||
| 20.57 | 14.11 | β− | 23.45 minutes | ||
| 77.03 | – | β− | 2.356 days | ||
| 270.7 | 747.9 | α | 24,110 years | ||
| 287.5 | 0.064 | α | 6,561 years | ||
| 363.0 | 1012 | β− | 14.325 years | ||
| 19.16 | 0.001 | α | 373,300 years |
See 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. Pu is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons. Uranium-238 is present in quantity in most reactor fuel; hence Pu is continuously made in these reactors. Since Pu can itself be split by neutrons to release energy, Pu provides a portion of the energy generation in a nuclear reactor.
| U | 238 | 2.68 | 5·10 | α | 4.47 x 10 years | |
|---|---|---|---|---|---|---|
| U | 239 | 22 | 15 | β | 23 minutes | |
| Np | 239 | 30 | 1 | β | 2.36 days | |
| Pu | 239 | 271 | 750 | α | 24,110 years |
See main article: Plutonium-238. There are small amounts of Pu in the plutonium from usual reactors. However, isotopic separation would be quite expensive compared to another method: when U captures a neutron, it is converted to an excited state of U. Some of the excited U nuclei undergo fission, but some decay to the ground state of U by emitting gamma radiation. Further neutron capture creates U; which, with a half-life of 7 days, decays to Np. Since nearly all neptunium is produced in this way or consists of isotopes that decay quickly, one gets nearly pure Np. After chemical separation of neptunium, Np is again irradiated by reactor neutrons to be converted to Np, which decays to Pu with a half-life of 2 days.
| 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 |
Plutonium-240 undergoes spontaneous fission at a small but significant rate (%). The presence of Pu limits the plutonium's use in a nuclear bomb, because a neutron from spontaneous fission starts the chain reaction prematurely, causing an early release of energy that disperses the core before full implosion is reached. This prevents most of the core from participation in the chain reaction and reduces the bomb's yield.
Plutonium consisting of more than about 90% Pu is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% Pu 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.
Contamination due to Pu is the reason plutonium weapons must use the implosion method. Theoretically, pure Pu could be used in a gun-type bomb, but achieving this level of purity is prohibitively difficult. Plutonium-240 contamination has proven a mixed blessing. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those same difficulties are a barrier to nuclear proliferation. Implosion bombs are also inherently more efficient and less prone to accidental detonation than are gun-type bombs.