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Fissile material

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"Fissility" redirects here. For the topic in geology, seeFissility (geology).

Innuclear engineering,fissile materialis material that can undergonuclear fissionwhen struck by aneutronof low energy.[1]A self-sustaining thermalchain reactioncan only be achieved with fissile material. The predominantneutron energyin a system may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuelthermal-neutron reactors,fast-neutron reactorsandnuclear explosives.

Fissile vs fissionable[edit]

According to the RonenFissile rule,[2]for a heavy element with90≤Z≤100,itsisotopeswith2 ×Z−N= 43 ± 2,with few exceptions, are fissile (whereN= number ofneutronsandZ= number ofprotons).[3][4][note 1]

Region of relative stability: radium-226 to einsteinium-252
88 89 90 91 92 93 94 95 96 97 98 99
154
Half-lifeKey
1 10 100
1k 10k 100k
1M 10M 100M
1G 10G (a)
250Cm 252Cf 154
153 251Cf 252Es 153
152 248Cm 250Cf 152
151 247Cm 248Bk 249Cf 151
150 244Pu 246Cm 247Bk 150
149 245Cm 149
148 242Pu 243Am 244Cm 148
147 241Pu
242m⁂
 243Cm 147
146 238U 240Pu 241Am 146
145 239Pu 145
144 236U 237Np 238Pu 144
143 235U 236Np 143
142 232Th 234U 235Np 236Pu 142
141 233U 141
140 228Ra 230Th 231Pa 232U
Table Axes
Neutrons (N)
Protons (Z)
140
139 229Th 139
138 226Ra 227Ac 228Th 138
88 89 90 91 92 93 94 95 96 97 98 99
Only nuclides with a half-life of at least one year are shown on this table.

The termfissileis distinct fromfissionable.Anuclidecapable of undergoingnuclear fission(even with a low probability) after capturing a neutron of high or low energy[5]is referred to asfissionable.A fissionable nuclide that can be induced to fission with low-energythermal neutronswith a high probability is referred to asfissile.[6]Fissionable materials include those (such asuranium-238) for which fission can be induced only by high-energy neutrons. As a result, fissile materials (such asuranium-235) are asubsetof fissionable materials.

Uranium-235 fissions with low-energy thermal neutrons because thebinding energyresulting from the absorption of a neutron is greater than thecritical energyrequired for fission; therefore uranium-235 is fissile. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is fissionable but not fissile.[7][8]

An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable) and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting. Under this definition, the only nuclides that are fissionable but not fissile are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain anuclear chain reaction.As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In thearms controlcontext, particularly in proposals for aFissile Material Cutoff Treaty,the termfissileis often used to describe materials that can be used in the fission primary of a nuclear weapon.[9]These are materials that sustain an explosivefast neutronnuclear fissionchain reaction.

Under all definitions above, uranium-238 (238
U
) is fissionable, but not fissile. Neutrons produced by fission of238
U
have lowerenergiesthan the original neutron (they behave as in aninelastic scattering), usually below 1MeV(i.e., a speed of about 14,000km/s), the fission threshold to cause subsequent fission of238
U
,so fission of238
U
does not sustain anuclear chain reaction.

Fast fission of238
U
in the secondary stage of a thermonuclear weapon, due to the production of high-energy neutrons fromnuclear fusion,contributes greatly to theyieldand tofalloutof such weapons. Fast fission of238
U
tampers has also been evident in pure fission weapons.[10]The fast fission of238
U
also makes a significant contribution to the power output of somefast-neutron reactors.

Fissile nuclides[edit]

Actinides and fission products by half-life
Actinides[11]bydecay chain Half-life
range (a) 		Fission productsof235Ubyyield[12]
4n 4n+ 1 4n+ 2 4n+ 3 4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 a 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[13] 249Cfƒ 242mAmƒ 141–351 a

No fission products have ahalf-life
in the range of 100 a–210 ka...

241Amƒ 251Cfƒ[14] 430–900 a
226Ra№ 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th№ 231Pa№ 32–76 ka
236Npƒ 233Uƒ 234U№ 150–250 ka 99Tc₡ 126Sn
248Cm 242Pu 327–375 ka 79Se₡
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs₡ 107Pd
236U 247Cmƒ 15–24 Ma 129I₡
244Pu 80 Ma

... nor beyond 15.7 Ma[15]

232Th№ 238U№ 235Uƒ№ 0.7–14.1 Ga

In general, mostactinideisotopes with an oddneutron numberare fissile. Most nuclear fuels have an oddatomic mass number(A=Z+N= the total number ofnucleons), and an evenatomic numberZ.This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from thepairing effectwhich favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile.

More generally, nuclides with an even number of protons and an even number of neutrons, and located near awell-known curvein nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else toabsorb the neutronbut without gaining enough energy from the process to deform the nucleus enough for it to fission. These"even-even"isotopes are also less likely to undergospontaneous fission,and they also have relatively much longerpartial half-livesforAlphaorbetadecay. Examples of these isotopes are uranium-238 andthorium-232.On the other hand, other than the lightest nuclides, nuclides with an odd number of protons and an odd number of neutrons (oddZ,oddN) are usually short-lived (a notable exception isneptunium-236with a half-life of 154,000 years) because they readilydecay by beta-particle emissionto theirisobarswith an even number of protons and an even number of neutrons (evenZ,evenN) becoming much more stable. The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing. The relatively short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.

Nuclear fuel[edit]

Main article:Nuclear fuel

To be a useful fuel for nuclear fission chain reactions, the material must:

  • Be in the region of thebinding energycurve where a fission chain reaction is possible (i.e., aboveradium)
  • Have a high probability of fission onneutron capture
  • Release more than one neutron on average per neutron capture. (Enough of them on each fission, to compensate for non-fissions and absorptions in non-fuel material)
  • Have a reasonably longhalf-life
  • Be available in suitable quantities
Capture-fission ratios of fissile nuclides
Thermal neutrons[16] Epithermal neutrons
σF(b) σγ(b) % σF(b) σγ(b) %
531 46 8.0% 233U 760 140 16%
585 99 14.5% 235U 275 140 34%
750 271 26.5% 239Pu 300 200 40%
1010 361 26.3% 241Pu 570 160 22%

Fissilenuclidesin nuclear fuels include:

Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capturecross sectionsfor fission (σF), the cross section for neutron capture with emission of agamma ray(σγ), and the percentage of non-fissions are in the table at right.

Fertile nuclides in nuclear fuels include:

  • Thorium-232,which breeds uranium-233 by neutron capture with intermediate decays steps omitted.
  • Uranium-238,which breeds plutonium-239 by neutron capture with intermediate decays steps omitted.
  • Plutonium-240,bred from plutonium-239 directly by neutron capture.

See also[edit]

Notes[edit]

  1. ^The fissile rule thus formulated indicates 33 isotopes as likely fissile: Th-225, 227, 229; Pa-228, 230, 232; U-231, 233, 235; Np-234, 236, 238; Pu-237, 239, 241; Am-240, 242, 244; Cm-243, 245, 247; Bk-246, 248, 250; Cf-249, 251, 253; Es-252, 254, 256; Fm-255, 257, 259. Only fourteen (including a long-livedmetastablenuclear isomer) have half-lives of at least a year: Th-229, U-233, U-235, Np-236, Pu-239, Pu-241, Am-242m, Cm-243, Cm-245, Cm-247, Bk-248, Cf-249, Cf-251 and Es-252. Of these, only U-235 isnaturally occurring.It is possible tobreedU-233 and Pu-239 from more common naturally occurring isotopes (Th-232 and U-238 respectively) by singleneutron capture.The others are typically produced in smaller quantities throughfurther neutron absorption.

References[edit]

  1. ^"NRC: Glossary -- Fissile material".nrc.gov.
  2. ^"Nuclear Science and Engineering -- ANS / Publications / Journals / Nuclear Science and Engineering".
  3. ^Ronen Y., 2006. A rule for determining fissile isotopes.Nucl. Sci. Eng.,152:3, pages 334-335.[1]
  4. ^Ronen, Y. (2010). "Some remarks on the fissile isotopes".Annals of Nuclear Energy.37(12): 1783–1784.Bibcode:2010AnNuE..37.1783R.doi:10.1016/j.anucene.2010.07.006.
  5. ^"NRC: Glossary -- Fissionable material".nrc.gov.
  6. ^"Slides-Part one: Kinetics".UNENE University Network of Excellence in Nuclear Engineering.Retrieved3 January2013.
  7. ^James J. Duderstadt and Louis J. Hamilton (1976).Nuclear Reactor Analysis.John Wiley & Sons, Inc.ISBN0-471-22363-8.
  8. ^John R. Lamarsh and Anthony John Baratta (Third Edition) (2001).Introduction to Nuclear Engineering.Prentice Hall.ISBN0-201-82498-1.
  9. ^Fissile Materials and Nuclear WeaponsArchived2012-02-06 at theWayback Machine,International Panel on Fissile Materials
  10. ^Semkow, Thomas; Parekh, Pravin; Haines, Douglas (2006). "Modeling the Effects of the Trinity Test".Applied Modeling and Computations in Nuclear Science.ACS Symposium Series. Vol. ACS Symposium Series. pp. 142–159.doi:10.1021/bk-2007-0945.ch011.ISBN9780841239821.
  11. ^Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability afterpolonium(84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap isradon-222with a half life of less than fourdays). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  12. ^Specifically fromthermal neutronfission of uranium-235, e.g. in a typicalnuclear reactor.
  13. ^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.Bibcode:1965NucPh..71..299M.doi:10.1016/0029-5582(65)90719-4.
    "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 Bk248with a half-life greater than 9 [years]. No growth of Cf248was detected, and a lower limit for the β−half-life can be set at about 104[years]. No Alpha activity attributable to the new isomer has been detected; the Alpha half-life is probably greater than 300 [years]. "
  14. ^This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
  15. ^Excluding those "classically stable"nuclides with half-lives significantly in excess of232Th; e.g., while113mCd has a half-life of only fourteen years, that of113Cd is eightquadrillionyears.
  16. ^"Interactive Chart of Nuclides".Brookhaven National Laboratory. Archived fromthe originalon 2017-01-24.Retrieved2013-08-12.
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