 10-gram sample | |
| General | |
|---|---|
| Symbol | 238U |
| Names | uranium-238 |
| Protons(Z) | 92 |
| Neutrons(N) | 146 |
| Nuclide data | |
| Natural abundance | 99.274% |
| Half-life(t1/2) | 4.463×109 years[1] |
| Isotope mass | 238.050787[2]Da |
| Spin | 0 |
| Parent isotopes | 242Pu (α) 238Pa (β−) |
| Decay products | 234Th |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| alpha decay | 4.270[3] |
| Isotopes of uranium Complete table of nuclides | |
Uranium-238 (238
U orU-238) is the most commonisotope of uranium found in nature, with a relative abundance above 99%. Unlikeuranium-235, it is non-fissile, which means it cannot sustain achain reaction in athermal-neutron reactor. However, it isfissionable byfast neutrons, and isfertile, meaning it can betransmuted to fissileplutonium-239.238U cannot support a chain reaction becauseinelastic scattering reducesneutron energy below the range wherefast fission of one or more next-generation nuclei is probable.Doppler broadening of238U'sneutron absorptionresonances, increasing absorption as fuel temperature increases, is also an essentialnegative feedback mechanism for reactor control.
The isotope has a half-life of 4.463 billion years (1.408×1017 s). Due to its abundance and half-life relative rate of decay to otherradioactive elements,238U is responsible for about 40% of the radioactive heat produced within the Earth.[4] The238Udecay chain contributes sixelectron anti-neutrinos per238U nucleus (one perbeta decay), resulting in a large detectablegeoneutrino signal when decays occur within the Earth.[5] The decay of238U to daughter isotopes is extensively used inradiometric dating, particularly for material older than approximately 1 million years.
Depleted uranium has an even higher concentration of the238U isotope, and evenlow-enriched uranium (LEU), while having a higher proportion of theuranium-235 isotope (in comparison to depleted uranium), is still mostly238U.Reprocessed uranium is also mainly238U, with about as muchuranium-235 as natural uranium, a comparable proportion ofuranium-236, and much smaller amounts of otherisotopes of uranium such asuranium-234,uranium-233, anduranium-232.[6]
In a fissionnuclear reactor, uranium-238 can be used to generateplutonium-239, which itself can be used in anuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power comes from the fission of239Pu, which is not supplied as a fuel to the reactor, but rather,produced from238U.[7] A certain amount of production of239
Pu from238
U is unavoidable wherever it is exposed toneutron radiation. Depending onburnup andneutron temperature, different shares of the239
Pu are converted to240
Pu, which determines the "grade" of produced plutonium, ranging fromweapons grade, throughreactor grade, to plutonium so high in240
Pu that it cannot be used in current reactors operating with a thermal neutron spectrum. The latter usually involves used "recycled"MOX fuel which entered the reactor containing significant amounts of plutonium[citation needed].
238U 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 of238U to split. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the average 2.5 neutrons[8] produced in each fission have enough speed to continue a chain reaction (this is why natural uranium will not work in a bomb).
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 oftransmutation to convert thefertile isotope238U into fissile239Pu. It has been estimated that there is anywhere from 10,000 to five billion years worth of238U for use in thesepower plants.[9] Breeder technology has been used in several experimental nuclear reactors.[10]
By December 2005, the only breeder reactor producing power was the 600-megawattBN-600 reactor at theBeloyarsk Nuclear Power Station in Russia. Russia later built another unit,BN-800, at the Beloyarsk Nuclear Power Station which became fully operational in November 2016. Also, Japan'sMonju breeder reactor, which has been inoperative for most of the time since it was originally built in 1986, was ordered for decommissioning in 2016, after safety and design hazards were uncovered, with a completion date set for 2047. Both China and India have announced plans to build nuclear breeder reactors.[citation needed]
The breeder reactor as its name implies creates larger quantities of239Pu or233U (thefissile isotopes) than it consumes.[citation needed]
TheClean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to controldelayed neutrons, will potentially be able to use238U as fuel once the reactor is started withLow-enriched uranium (LEU) fuel. This design is still in the early stages of development.[citation needed]
Natural uranium, with 0.72%235
U, is usable asnuclear fuel in reactors designed specifically for this, such as the heavy-waterCANDU reactor. By making use of non-enriched uranium, such reactor designs give a nation access to nuclear power for the purpose of electricity production without necessitating the development of fuel enrichment capabilities, which are often seen as a prelude to weapons production[citation needed].
238U is also used as aradiation shield – itsalpha radiation is easily stopped by the non-radioactive casing of the shielding and the uranium's highatomic weight and high number ofelectrons are highly effective in absorbinggamma rays andX-rays. It is not as effective as ordinary water for stoppingfast neutrons. Both metallicdepleted uranium and depleteduranium dioxide are used for radiation shielding. Uranium is about five times[dubious –discuss] better as a gamma ray shield thanlead, so a shield with the same effectiveness can be packed into a thinner layer.[citation needed]
DUCRETE, a concrete made with uranium dioxideaggregate instead of gravel, is being investigated as a material fordry cask storage systems to storeradioactive waste.[citation needed]
The opposite of enriching isdownblending. Surplushighly 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 recycled239Pu from nuclear weapons stockpiles for makingmixed 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.[citation needed]
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Most modernnuclear weapons utilize238U as a "tamper" material (seenuclear weapon design). A tamper which surrounds a fissile core works toreflect neutrons and to addinertia to the compression of the239Pu charge. As such, it increases the efficiency of the weapon and reduces thecritical mass required. In the case of athermonuclear weapon,238Ucan be used to encase the fusion fuel, the high flux of very energeticneutrons from the resultingfusion reaction causes238U nuclei to split and adds more energy to the "yield" of the weapon. Such weapons are referred to asfission-fusion-fission weapons after the order in which each reaction takes place. An example of such a weapon isCastle Bravo.
The larger portion of the total explosive yield in this design comes from the final fission stage fueled by238U, producing enormous amounts of radioactivefission products. For example, an estimated 77% of the 10.4-megaton yield of theIvy Mike thermonuclear test in 1952 came from fast fission of the depleted uraniumtamper. Because depleted (or natural) uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. TheSoviet Union's test of theTsar Bomba in 1961 produced "only" 50 megatons of explosive power, over 90% of which came from fusion because the238U final stage had been replaced with lead. Had238U been used instead, the yield of the Tsar Bomba could have been well above 100 megatons, and it would have producednuclear fallout equivalent to one third of the global total that had been produced up to that time.
Uranium-238 is an alpha emitter, producingthorium-234 which is a beta emitter, etc. This leads to adecay chain, commonly called theradium series oruranium series. Beginning with naturally occurring uranium-238, this series includes isotopes ofastatine,bismuth,lead,polonium,protactinium,radium,radon,thallium,thorium anduranium, all of which are present in natural uranium sources. The decay proceeds as (only main decay branches shown):[11]
Or in tabular form, including minor branches:
| Nuclide | Decay mode | Half-life (a = years) | Energy released MeV | Decay product |
|---|---|---|---|---|
| 238U | α | 4.463×109 a | 4.270 | 234Th |
| 234Th | β− | 24.11 d | 0.195 | 234mPa |
| 234mPa | IT 0.16% β− 99.84% | 1.16 min | 0.079 2.273 | 234Pa 234U |
| 234Pa | β− | 6.70 h | 2.194 | 234U |
| 234U | α | 2.455×105 a | 4.858 | 230Th |
| 230Th | α | 7.54×104 a | 4.770 | 226Ra |
| 226Ra | α | 1600 a | 4.871 | 222Rn |
| 222Rn | α | 3.8215 d | 5.590 | 218Po |
| 218Po | α 99.98% β− 0.02% | 3.097 min | 6.115 0.257 | 214Pb 218At |
| 218At | α 100% β− | 1.28 s | 6.876 2.883 | 214Bi 218Rn |
| 218Rn | α | 33.75 ms | 7.262 | 214Po |
| 214Pb | β− | 27.06 min | 1.018 | 214Bi |
| 214Bi | β− 99.979% α 0.021% | 19.9 min | 3.269 5.621 | 214Po 210Tl |
| 214Po | α | 163.5 μs | 7.833 | 210Pb |
| 210Tl | β− β−n 0.009% | 1.30 min | 5.481 0.296 | 210Pb 209Pb (inneptunium series) |
| 210Pb | β− α 1.9×10−6% | 22.2 a | 0.0635 3.793 | 210Bi 206Hg |
| 210Bi | β− α 1.32×10−4% | 5.012 d | 1.161 5.035 | 210Po 206Tl |
| 210Po | α | 138.376 d | 5.407 | 206Pb |
| 206Hg | β− | 8.32 min | 1.307 | 206Tl |
| 206Tl | β− | 4.20 min | 1.532 | 206Pb |
| 206Pb | stable |
Themean lifetime of238U (or any nuclide) is the half-life divided byln(2) ≈ 0.693 (or multiplied by 1/ln(2) ≈ 1.443), which is about 2×1017 seconds, so 1mole of238U emits 3×106 alpha particles per second, producing the same number of thorium-234atoms. In a closed system an equilibrium would be reached in which all members except the stable end-product have fixed ratios to one another, but in slowly decreasing amount. The amount of206Pb will increase accordingly while that of238U decreases; all steps in the decay chain have this same rate of 3×106 decayed particles per second per mole238U.
While238U is minimally radioactive, its decay products, thorium-234 and protactinium-234, arebeta particle emitters withhalf-lives of about 20 days and one minute respectively. Protactinium-234 decays to uranium-234, which has a half-life of hundreds of millennia, and thisisotope does not reach an equilibrium concentration for a very long time. When the two first isotopes in the decay chain reach their relatively small equilibrium concentrations, a sample of initially pure238U will emit three times the radiation due to238U itself, and most of this radiation is beta particles.
As already touched upon above, when starting with pure238U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of238U, 3×106 times per second one alpha and two beta particles and a gamma ray are produced, together 6.7 MeV, for a rate of 3 μW.[12][13]
The238U atom is itself a gamma emitter at 49.55 keV with probability 0.084%, but that is a very weak gamma line, so activity is measured through its daughter nuclides in its decay series.[14][15]
238U abundance and its decay to daughter isotopes comprises multiple uranium dating techniques and is one of the most common radioactive isotopes used inradiometric dating. The most common dating method isuranium-lead dating, which is used to date rocks older than 1 million years old and has provided ages for the oldest rocks on Earth at 4.4 billion years old.[16]
The relation between238U and234U gives an indication of the age ofsediments and seawater that are between 100,000 years and 1,200,000 years in age.[17]
The238U daughter product,206Pb, is an integral part oflead–lead dating, which is most famous for the determination of theage of the Earth.[18]
TheVoyager program spacecraft carry small amounts of initially pure238U on the covers of theirgolden records to facilitate dating in the same manner.[19]
Uranium emitsalpha radiation, so external exposure has limited effect. Significant internal exposure to tiny particles of uranium or its decay products, such as thorium-230,radium-226 andradon-222, can cause severe health effects, such as cancer of the bone or liver.
Uranium is also chemically toxic, meaning that ingestion of uranium can cause kidney damage from its chemical properties much sooner than its radioactive properties would cause cancers of the bone or liver.[20][21]
| Lighter: uranium-237 | Uranium-238 is an isotope ofuranium | Heavier: uranium-239 |
| Decay product of: plutonium-242 (α) protactinium-238 (β−) | Decay chain of uranium-238 | Decays to: thorium-234 (α) |