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Plutonium (94Pu) 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. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states; all have half-lives of less than one second.
The isotopes of plutonium range in atomic weight from 228.0387 u (228Pu) to 247.074 u (247Pu). The primary decay modes before the most stable isotope, 244Pu, are spontaneous fission and alpha emission; the primary mode after is beta emission. The primary decay products before 244Pu are isotopes of uranium and neptunium (not considering fission products), and the primary decay products after are isotopes of americium.
Nuclide [n 1] | Z | N | Isotopic mass (Da) [n 2] [n 3] | Half-life | Decay mode [n 4] | Daughter isotope [n 5] [n 6] | Spin and parity [n 7] [n 8] | Isotopic abundance | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Excitation energy | |||||||||||||||||||
228Pu | 94 | 134 | 228.03874(3) | 1.1+20 −5 s | α | 224U | 0+ | ||||||||||||
β+ (rare) | 228Np | ||||||||||||||||||
229Pu [3] | 94 | 135 | 229.04015(6) | 91(26) s | α (50%) | 225U | 3/2+# | ||||||||||||
β+ (50%) | 229Np | ||||||||||||||||||
230Pu | 94 | 136 | 230.039650(16) | 1.70(17) min | α (>73%) [4] | 226U | 0+ | ||||||||||||
β+ (<27%) | 230Np | ||||||||||||||||||
231Pu [5] | 94 | 137 | 231.041101(28) | 8.6(5) min | β+ (87%) | 231Np | (3/2+) | ||||||||||||
α (13%) | 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) y | α | 232U | 0+ | ||||||||||||
SF (1.37×10−7%) | (various) | ||||||||||||||||||
CD (2×10−12%) | 208Pb 28Mg | ||||||||||||||||||
237Pu | 94 | 143 | 237.0484097(24) | 45.2(1) d | EC | 237Np | 7/2− | ||||||||||||
α (.0042%) | 233U | ||||||||||||||||||
237m1Pu | 145.544(10)2 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) y | α | 234U | 0+ | Trace [n 9] | |||||||||||
SF (1.9×10−7%) | (various) | ||||||||||||||||||
CD (1.4×10−14%) | 206Hg 32Si | ||||||||||||||||||
CD (6×10−15%) | 180Yb 30Mg 28Mg | ||||||||||||||||||
239Pu [n 10] [n 11] | 94 | 145 | 239.0521634(20) | 2.411(3)×104 y | α | 235U | 1/2+ | Trace [n 12] | |||||||||||
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)×103 y | α | 236U | 0+ | Trace [n 13] | |||||||||||
SF (5.7×10−6%) | (various) | ||||||||||||||||||
CD (1.3×10−13%) | 206Hg 34Si | ||||||||||||||||||
241Pu [n 10] | 94 | 147 | 241.0568515(20) | 14.290(6) y | 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 y | α | 238U | 0+ | ||||||||||||
SF (5.5×10−4%) | (various) | ||||||||||||||||||
243Pu [n 10] | 94 | 149 | 243.062003(3) | 4.956(3) h | β− | 243Am | 7/2+ | ||||||||||||
243mPu | 383.6(4) keV | 330(30) ns | (1/2+) | ||||||||||||||||
244Pu | 94 | 150 | 244.064204(5) | 8.00(9)×107 y | α (99.88%) | 240U | 0+ | Trace [n 14] | |||||||||||
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+# | ||||||||||||
This table header & footer: |
CD: | Cluster decay |
EC: | Electron capture |
IT: | Isomeric transition |
SF: | Spontaneous fission |
Actinides [6] by decay chain | Half-life range (a) | Fission products of 235U by yield [7] | ||||||
---|---|---|---|---|---|---|---|---|
4n | 4n + 1 | 4n + 2 | 4n + 3 | 4.5–7% | 0.04–1.25% | <0.001% | ||
228 Ra№ | 4–6 a | 155 Euþ | ||||||
244 Cmƒ | 241 Puƒ | 250 Cf | 227 Ac№ | 10–29 a | 90 Sr | 85 Kr | 113m Cdþ | |
232 Uƒ | 238 Puƒ | 243 Cmƒ | 29–97 a | 137 Cs | 151 Smþ | 121m Sn | ||
248 Bk [8] | 249 Cfƒ | 242m Amƒ | 141–351 a | No fission products have a half-life in the range of 100 a–210 ka ... | ||||
241 Amƒ | 251 Cfƒ [9] | 430–900 a | ||||||
226 Ra№ | 247 Bk | 1.3–1.6 ka | ||||||
240 Pu | 229 Th | 246 Cmƒ | 243 Amƒ | 4.7–7.4 ka | ||||
245 Cmƒ | 250 Cm | 8.3–8.5 ka | ||||||
239 Puƒ | 24.1 ka | |||||||
230 Th№ | 231 Pa№ | 32–76 ka | ||||||
236 Npƒ | 233 Uƒ | 234 U№ | 150–250 ka | 99 Tc₡ | 126 Sn | |||
248 Cm | 242 Pu | 327–375 ka | 79 Se₡ | |||||
1.53 Ma | 93 Zr | |||||||
237 Npƒ | 2.1–6.5 Ma | 135 Cs₡ | 107 Pd | |||||
236 U | 247 Cmƒ | 15–24 Ma | 129 I₡ | |||||
244 Pu | 80 Ma | ... nor beyond 15.7 Ma [10] | ||||||
232 Th№ | 238 U№ | 235 Uƒ№ | 0.7–14.1 Ga | |||||
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239Pu, 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.
240Pu, 241Pu, and 242Pu are produced by further neutron capture. The odd-mass isotopes 239Pu and 241Pu 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 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, 240Pu may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons. 240Pu does have a moderate thermal neutron absorption cross section, so that 241Pu production in a thermal reactor becomes a significant fraction as large as 239Pu production.
241Pu has a half-life of 14 years, and has slightly higher thermal neutron cross sections than 239Pu for both fission and absorption. While nuclear fuel is being used in a reactor, a 241Pu nucleus is much more likely to fission or to capture a neutron than to decay. 241Pu 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 241Pu will beta decay to americium-241, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors.
242Pu 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 241Pu) 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 242Pu again; so the mean number of neutrons absorbed before fission is even higher than 3. Therefore, 242Pu 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, 242Pu's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. 242Pu's half-life is about 15 times as long as 239Pu'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. [14]
243Pu has a half-life of only 5 hours, beta decaying to americium-243. Because 243Pu has little opportunity to capture an additional neutron before decay, the nuclear fuel cycle does not produce the long-lived 244Pu in significant quantity.
238Pu 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 238Pu relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on 239Pu, or by alpha decay of curium-242, which is produced by neutron capture from 241Am. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become 239Pu.
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. 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.
Isotope | Thermal neutron cross section [15] (barns) | Decay Mode | Half-life | |
---|---|---|---|---|
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 |
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.
Element | Isotope | Thermal neutron capture cross section (barn) | Thermal neutron fission Cross section (barn) | decay mode | Half-life |
---|---|---|---|---|---|
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 |
There are small amounts of 238Pu in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a 235U atom captures a neutron, it is converted to an excited state of 236U. Some of the excited 236U nuclei undergo fission, but some decay to the ground state of 236U by emitting gamma radiation. Further neutron capture creates 237U, which has a half-life of 7 days and thus quickly decays to 237Np. Since nearly all neptunium is produced in this way or consists of isotopes that decay quickly, one gets nearly pure 237Np by chemical separation of neptunium. After this chemical separation, 237Np is again irradiated by reactor neutrons to be converted to 238Np, which decays to 238Pu with a half-life of 2 days.
Element | Isotope | Thermal neutron cross section | decay mode | Half-life |
---|---|---|---|---|
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 as a secondary decay mode at a small but significant rate (5.8×10−6%). [1] The presence of 240Pu limits the plutonium's use in a nuclear bomb, because the neutron flux from spontaneous fission initiates the chain reaction prematurely, causing an early release of energy that physically 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 power.
Plutonium consisting of more than about 90% 239Pu is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% 240Pu 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 240Pu is the reason plutonium weapons must use the implosion method. Theoretically, pure 239Pu could be used in a gun-type nuclear weapon, but achieving this level of purity is prohibitively difficult. Plutonium-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 same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone to accidental detonation than are gun-type weapons.
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation.
A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fueled with more-commonly available isotopes of uranium and thorium, such as uranium-238 and thorium-232, as opposed to the rare uranium-235 which is used in conventional reactors. These materials are called fertile materials since they can be bred into fuel by these breeder reactors.
Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of 238U. Thus the ratio of 234
U to 238
U in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of 234U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, 234Th nuclei emit a beta particle to become protactinium-234 (234Pa), or more likely a nuclear isomer denoted 234mPa. Finally, 234Pa or 234mPa nuclei emit another beta particle to become 234U nuclei.
Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years.
Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from 214U to 242U. The standard atomic weight of natural uranium is 238.02891(3).
Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238
U
with neutrons to produce 239
U
, which then underwent beta decay to 239
Np
.
A minor actinide is an actinide, other than uranium or plutonium, found in spent nuclear fuel. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. The most important isotopes of these elements in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples.
Plutonium-240 is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.
Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes, 241Pu is fissile, with a neutron absorption cross section about one-third greater than that of 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.
Uranium-236 (236U) 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.
In nuclear power technology, burnup is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA or %FIFA as well as, preferably, the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/tHM), or similar units.
Plutonium-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years. The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth 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.
Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.
Long-lived fission products (LLFPs) are radioactive materials with a long half-life produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity, it is necessary to isolate them from humans and the biosphere and to confine them in nuclear waste repositories for geological periods of time.
Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.