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Fissile material

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In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.

Fissile vs fissionable

According to the Ronen Fissile rule, for a heavy element with 90 ≤ Z ≤ 100, its isotopes with 2 × ZN = 43 ± 2, with few exceptions, are fissile (where N = number of neutrons and Z = number of protons).

Region of relative stability: radium-226 to einsteinium-252
       88 89 90 91 92 93 94 95 96 97 98 99       
   
 154 
Half-life Key
  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  238 240Pu 241Am  146 
 145  239Pu  145 
 144  236 237Np 238Pu  144 
 143  235 236Np  143 
 142  232Th 234 235Np 236Pu  142 
 141  233  141 
 140  228Ra 230Th 231Pa 232
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 term fissile is distinct from fissionable. A nuclide capable of undergoing fission (even with a low probability) after capturing a neutron of high or low energy is referred to as fissionable. A fissionable nuclide that can be induced to fission with low-energy thermal neutrons with a high probability is referred to as fissile. Fissionable materials include also those (such as uranium-238) for which fission can be induced only by high-energy neutrons. As a result, fissile materials (such as uranium-235) are a subset of fissionable materials.

Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required 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.

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 a nuclear chain reaction. As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control context, particularly in proposals for a Fissile Material Cutoff Treaty, the term fissile is often used to describe materials that can be used in the fission primary of a nuclear weapon. These are materials that sustain an explosive fast neutron nuclear fission chain reaction.

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

Fast fission of 238
U
in the secondary stage of a thermonuclear weapon, due to the production of high-energy neutrons from nuclear fusion, contributes greatly to the yield and to fallout of such weapons. Fast fission of 238
U
tampers has also been evident in pure fission weapons. The fast fission of 238
U
also makes a significant contribution to the power output of some fast-neutron reactors.

Fissile nuclides

Actinides by decay chain Half-life
range (a)
Fission products of 235U by yield
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 249Cfƒ 242mAmƒ 141–351 a

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

241Amƒ 251Cfƒ 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

232Th 238U 235Uƒ№ 0.7–14.1 Ga

In general, most actinide isotopes with an odd neutron number are fissile. Most nuclear fuels have an odd atomic mass number (A = Z + N = the total number of nucleons), and an even atomic number Z. 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 the pairing effect which 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 a well-known curve in 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 to absorb the neutron but 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 undergo spontaneous fission, and they also have relatively much longer partial half-lives for alpha or beta decay. Examples of these isotopes are uranium-238 and thorium-232. On the other hand, other than the lightest nuclides, nuclides with an odd number of protons and an odd number of neutrons (odd Z, odd N) are usually short-lived (a notable exception is neptunium-236 with a half-life of 154,000 years) because they readily decay by beta-particle emission to their isobars with an even number of protons and an even number of neutrons (even Z, even N) 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

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

  • Be in the region of the binding energy curve where a fission chain reaction is possible (i.e., above radium)
  • Have a high probability of fission on neutron 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 long half-life
  • Be available in suitable quantities
Capture-fission ratios of fissile nuclides
Thermal neutrons 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%

Fissile nuclides in 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 capture cross sections for fission (σF), the cross section for neutron capture with emission of a gamma rayγ), and the percentage of non-fissions are in the table at right.

Fertile nuclides in nuclear fuels include:

See also


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