Uranium-238

Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U 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 238U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

Uranium-238, 238U
10 gram sample
General
Symbol238U
Namesuranium-238, 238U, U-238
Protons (Z)92
Neutrons (N)146
Nuclide data
Natural abundance99.2745%
Half-life (t1/2)4.468×109 years
Isotope mass238.05078826 Da
Spin0
Parent isotopes242Pu (α)
238Pa (β−)
Decay products234Th
Decay modes
Decay modeDecay energy (MeV)
alpha decay4.267
Isotopes of uranium
Complete table of nuclides

Around 99.284% of natural uranium's mass is uranium-238, which has a half-life of 1.41×1017 seconds (4.468×109 years, or 4.468 billion years).[1] Due to its natural abundance and half-life relative to other radioactive elements, 238U produces ~40% of the radioactive heat produced within the Earth.[2] The 238U decay chain contributes six electron anti-neutrinos per 238U nucleus (one per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth.[3] The decay of 238U to daughter isotopes is extensively used in radiometric dating, particularly for material older than approximately 1 million years.

Depleted uranium has an even higher concentration of the 238U 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 238U, 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.[4]

Nuclear energy applications

In a fission nuclear reactor, uranium-238 can be used to generate plutonium-239, which itself can be used in a nuclear 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 of 239Pu, which is not supplied as a fuel to the reactor, but rather, produced from 238U.[5] A certain amount of production of 239
Pu from 238
U is unavoidable wherever it is exposed to neutron radiation. Depending on burnup and neutron temperature, different shares of the 239
Pu are converted to 240
Pu, which determines the "grade" of produced plutonium, ranging from weapons grade, through reactor grade, to plutonium so high in 240
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.

Breeder reactors

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 of 238U 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[6] produced in each fission have enough speed to continue a chain reaction.

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 of transmutation to convert the fertile isotope 238U into fissile 239Pu. It has been estimated that there is anywhere from 10,000 to five billion years worth of 238U for use in these power plants.[7] Breeder technology has been used in several experimental nuclear reactors.[8]

By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk 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's Monju 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.

The breeder reactor as its name implies creates even larger quantities of 239Pu or 233U than the fission nuclear reactor.

The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to control delayed neutrons, will potentially be able to use 238U as fuel once the reactor is started with Low-enriched uranium (LEU) fuel. This design is still in the early stages of development.

CANDU reactors

Natural uranium, with 0.711% 235
U
, is usable as nuclear fuel in reactors designed specifically to make use of naturally occurring uranium, such as CANDU reactors. 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.

Radiation shielding

238U is also used as a radiation shield – its alpha radiation is easily stopped by the non-radioactive casing of the shielding and the uranium's high atomic weight and high number of electrons are highly effective in absorbing gamma rays and X-rays. It is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium and depleted uranium dioxide are used for radiation shielding. Uranium is about five times better as a gamma ray shield than lead, so a shield with the same effectiveness can be packed into a thinner layer.

DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storage systems to store radioactive waste.

Downblending

The opposite of enriching is downblending. Surplus highly 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 recycled 239Pu from nuclear weapons stockpiles for making mixed 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.

Nuclear weapons

Most modern nuclear weapons utilize 238U as a "tamper" material (see nuclear weapon design). A tamper which surrounds a fissile core works to reflect neutrons and to add inertia to the compression of the 239Pu charge. As such, it increases the efficiency of the weapon and reduces the critical mass required. In the case of a thermonuclear weapon, 238U can be used to encase the fusion fuel, the high flux of very energetic neutrons from the resulting fusion reaction causes 238U nuclei to split and adds more energy to the "yield" of the weapon. Such weapons are referred to as fission-fusion-fission weapons after the order in which each reaction takes place. An example of such a weapon is Castle Bravo.

The larger portion of the total explosive yield in this design comes from the final fission stage fueled by 238U, producing enormous amounts of radioactive fission products. For example, an estimated 77% of the 10.4-megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper. Because depleted uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The Soviet Union's test of the Tsar Bomba in 1961 produced "only" 50 megatons of explosive power, over 90% of which came from fusion because the 238U final stage had been replaced with lead. Had 238U been used instead, the yield of the Tsar Bomba could have been well above 100 megatons, and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time.

Radium series (or uranium series)

The decay chain of 238U is commonly called the "radium series" (sometimes "uranium series"). Beginning with naturally occurring uranium-238, this series includes the following elements: astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium. All of the decay products are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral. The decay proceeds as:

Parent nuclide Historic name (short)[9] Historic name (long) Atomic mass [RS 1] Decay mode [RS 2] Branch chance [RS 2] Half life [RS 2] Energy released, MeV [RS 2] Daughter nuclide [RS 2] Subtotal, MeV
238U UI Uranium I 238.051 α 100 % 4.468·109 a 4.26975 234Th 4.2698
234Th UX1 Uranium X1 234.044 β− 100 % 24.10 d 0.273088 234mPa 4.5428
234mPa UX2, Bv Uranium X2, Brevium 234.043 IT 0.16 % 1.159 min 0.07392 234Pa 4.6168
β− 99.84 % 1.159 min 2.268205 234U 6.8110
234Pa UZ Uranium Z 234.043 β− 100 % 6.70 h 2.194285 234U 6.8110
234U UII Uranium II 234.041 α 100 % 2.455·105 a 4.8598 230Th 11.6708
230Th Io Ionium 230.033 α 100 % 7.538·104 a 4.76975 226Ra 16.4406
226Ra Ra Radium 226.025 α 100 % 1600 a 4.87062 222Rn 21.3112
222Rn Rn Radon, Radium Emanation 222.018 α 100 % 3.8235 d 5.59031 218Po 26.9015
218Po RaA Radium A 218.009 β− 0.020 % 3.098 min 0.259913 218At 27.1614
α 99.980 % 3.098 min 6.11468 214Pb 33.0162
218At 218.009 β− 0.1 % 1.5 s 2.881314 218Rn 30.0428
α 99.9 % 1.5 s 6.874 214Bi 34.0354
218Rn 218.006 α 100 % 35 ms 7.26254 214Po 37.3053
214Pb RaB Radium B 214.000 β− 100 % 26.8 min 1.019237 214Bi 34.0354
214Bi RaC Radium C 213.999 β− 99.979 % 19.9 min 3.269857 214Po 37.3053
α 0.021 % 19.9 min 5.62119 210Tl 39.6566
214Po RaCI Radium CI 213.995 α 100 % 164.3 μs 7.83346 210Pb 45.1388
210Tl RaCII Radium CII 209.990 β− 100 % 1.30 min 5.48213 210Pb 45.1388
210Pb RaD Radium D 209.984 β− 100 % 22.20 a 0.063487 210Bi 45.2022
α 1.9·10−6 % 22.20 a 3.7923 206Hg 48.9311
210Bi RaE Radium E 209.984 β− 100 % 5.012 d 1.161234 210Po 46.3635
α 1.32·10−4 % 5.012 d 5.03647 206Tl 50.2387
210Po RaF Radium F 209.983 α 100 % 138.376 d 5.40745 206Pb 51.7709
206Hg 205.978 β− 100 % 8.32 min 1.307649 206Tl 50.2387
206Tl RaEII Radium EII 205.976 β− 100 % 4.202 min 1.532221 206Pb 51.7709
206Pb RaG Radium G 205.974 stable – – – – 51.7709
  1. "The Risk Assessment Information System: Radionuclide Decay Chain". The University of Tennessee.
  2. "Evaluated Nuclear Structure Data File". National Nuclear Data Center.

The mean lifetime of 238U is 1.41×1017 seconds divided by ln(2) ≈ 0.693 (or multiplied by 1/ln(2) ≈  1.443), i.e. ca. 2×1017 seconds, so 1 mole of 238U emits 3×106 alpha particles per second, producing the same number of thorium-234 atoms. In a closed system an equilibrium would be reached, with all amounts except for lead-206 and 238U in fixed ratios, in slowly decreasing amounts. The amount of 206Pb will increase accordingly while that of 238U decreases; all steps in the decay chain have this same rate of 3×106 decayed particles per second per mole 238U.

Thorium-234 has a mean lifetime of 3×106 seconds, so there is equilibrium if one mole of 238U contains 9×1012 atoms of thorium-234, which is 1.5×10−11 mole (the ratio of the two half-lives). Similarly, in an equilibrium in a closed system the amount of each decay product, except the end product lead, is proportional to its half-life.

While 238U is minimally radioactive, its decay products, thorium-234 and protactinium-234, are beta particle emitters with half-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 this isotope 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 pure 238U will emit three times the radiation due to 238U itself, and most of this radiation is beta particles.

As already touched upon above, when starting with pure 238U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of 238U, 3×106 times per second one alpha and two beta particles and a gamma ray are produced, together 6.7 MeV, a rate of 3 μW.[10][11]

238U 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.[12][13]

Radioactive dating

238U abundance and its decay to daughter isotopes comprises multiple uranium dating techniques and is one of the most common radioactive isotopes used in radiometric dating. The most common dating method is uranium-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.[14]

The relation between 238U and 234U gives an indication of the age of sediments and seawater that are between 100,000 years and 1,200,000 years in age.[15]

The 238U daughter product, 206Pb, is an integral part of lead–lead dating, which is most famous for the determination of the age of the Earth.[16]

The Voyager program spacecraft carry small amounts of initially pure 238U on the covers of their golden records to facilitate dating in the same manner.[17]

Health concerns

Uranium emits alpha particles through the process of alpha decay. External exposure has limited effect. Significant internal exposure to tiny particles of uranium or its decay products, such as thorium-230, radium-226 and radon-222, can cause severe health effects, such as cancer of the bone or liver.

Uranium is also a toxic chemical, 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.[18][19]

See also

References
  1. Mcclain, D. E.; Miller, A. C.; Kalinich, J. F. (December 20, 2007). "Status of Health Concerns about Military Use of Depleted Uranium and Surrogate Metals in Armor-Penetrating Munitions" (PDF). NATO. Archived from the original (PDF) on April 19, 2011. Retrieved November 14, 2010.
  2. Arevalo, Ricardo; McDonough, William F.; Luong, Mario (2009). "The K-U ratio of the silicate Earth: Insights into mantle composition, structure and thermal evolution". Earth and Planetary Science Letters. 278 (3–4): 361–369. Bibcode:2009E&PSL.278..361A. doi:10.1016/j.epsl.2008.12.023.
  3. Araki, T.; Enomoto, S.; Furuno, K.; Gando, Y.; Ichimura, K.; Ikeda, H.; Inoue, K.; Kishimoto, Y.; Koga, M. (2005). "Experimental investigation of geologically produced antineutrinos with KamLAND". Nature. 436 (7050): 499–503. Bibcode:2005Natur.436..499A. doi:10.1038/nature03980. PMID 16049478. S2CID 4367737.
  4. Nuclear France: Materials and sites. "Uranium from reprocessing". Archived from the original on October 19, 2007. Retrieved March 27, 2013.
  5. "Plutonium - World Nuclear Association".
  6. "Physics of Uranium and Nuclear Energy". World Nuclear Association. Retrieved November 17, 2017.
  7. Facts from Cohen Archived 2007-04-10 at the Wayback Machine. Formal.stanford.edu (2007-01-26). Retrieved on 2010-10-24.
  8. Advanced Nuclear Power Reactors | Generation III+ Nuclear Reactors Archived June 15, 2010, at the Wayback Machine. World-nuclear.org. Retrieved on 2010-10-24.
  9. Thoennessen, M. (2016). The Discovery of Isotopes: A Complete Compilation. Springer. p. 19. doi:10.1007/978-3-319-31763-2. ISBN 978-3-319-31761-8. LCCN 2016935977.
  10. Enghauser, Michael (April 1, 2018). Uranium Gamma Spectroscopy Training Revision 00 (Report). OSTI 1525592.
  11. "5.3: Types of Radiation". Chemistry LibreTexts. July 26, 2017. Retrieved May 16, 2023.
  12. Huy, N. Q.; Luyen, T. V. (December 1, 2004). "A method to determine 238U activity in environmental soil samples by using 63.3-keV-photopeak-gamma HPGe spectrometer". Applied Radiation and Isotopes. 61 (6): 1419–1424. doi:10.1016/j.apradiso.2004.04.016. ISSN 0969-8043.
  13. Clark, DeLynn (December 1996). "U235: A Gamma Ray Analysis Code for Uranium Isotopic Determination" (PDF). Retrieved May 21, 2023.
  14. Valley, John W.; Reinhard, David A.; Cavosie, Aaron J.; Ushikubo, Takayuki; Lawrence, Daniel F.; Larson, David J.; Kelly, Thomas F.; Snoeyenbos, David R.; Strickland, Ariel (July 1, 2015). "Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals" (PDF). American Mineralogist. 100 (7): 1355–1377. Bibcode:2015AmMin.100.1355V. doi:10.2138/am-2015-5134. ISSN 0003-004X.
  15. Henderson, Gideon M (2002). "Seawater (234U/238U) during the last 800 thousand years". Earth and Planetary Science Letters. 199 (1–2): 97–110. Bibcode:2002E&PSL.199...97H. doi:10.1016/S0012-821X(02)00556-3.
  16. Patterson, Claire (October 1, 1956). "Age of meteorites and the earth". Geochimica et Cosmochimica Acta. 10 (4): 230–237. Bibcode:1956GeCoA..10..230P. doi:10.1016/0016-7037(56)90036-9.
  17. "Voyager - Making of the Golden Record". voyager.jpl.nasa.gov. Retrieved March 28, 2020.
  18. Radioisotope Brief CDC (accessed November 8, 2021)
  19. Uranium Mining in Virginia: Scientific, Technical, Environmental, Human Health and Safety, and Regulatory Aspects of Uranium Mining and Processing in Virginia, Ch. 5. Potential Human Health Effects of Uranium Mining, Processing, and Reclamation. National Academies Press (US); December 19, 2011.
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