Nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...

splits a heavy nucleus such as

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...

into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission. Yield can be broken down by: # Individual

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

Chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

spanning several isotopes of different

mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approxima ...

but same

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

. # Nuclei of a given

regardless of

. Known as "chain yield" because it represents a

decay chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay dire ...

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

. Isotope and element yields will change as the fission products undergo beta decay, while chain yields do not change after completion of

neutron emission Neutron emission is a mode of radioactive decay in which one or more neutrons are ejected from a nucleus. It occurs in the most neutron-rich/proton-deficient nuclides, and also from excited states of other nuclides as in photoneutron emission and ...

by a few neutron-rich initial fission products ( delayed neutrons), with

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

measured in seconds. A few isotopes can be produced directly by fission, but not by beta decay because the would-be precursor with atomic number one greater is stable and does not decay. Chain yields do not account for these "shadowed" isotopes; however, they have very low yields (less than a millionth as much as common fission products) because they are far less neutron-rich than the original heavy nuclei. Yield is usually stated as percentage per fission, so that the total yield percentages sum to 200%. Less often, it is stated as percentage of all fission products, so that the percentages sum to 100%. Ternary fission, about 0.2–0.4% of fissions, also produces a third light nucleus such as helium-4 (90%) or

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...

(7%).

Mass vs. yield curve

If a graph of the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...

yield of fission products against the

of the fragments is drawn then it has two peaks, one in the area

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

through to

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...

and one at

xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

through to

neodymium Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishe ...

. This is because the fission event causes the nucleus to split in an asymmetric manner, as nuclei closer to magic numbers are more stable. Yield vs. Z - This is a typical distribution for the fission of

. Note that in the calculations used to make this graph the activation of fission products was ignored and the fission was assumed to occur in a single moment rather than a length of time. In this bar chart results are shown for different cooling times (time after fission). Fission yield volatile 2

Because of the stability of nuclei with even numbers of protons and/or

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s the curve of yield against element is not a smooth curve. It tends to alternate. In general, the higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the

neutron energy The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

increases and/or the energy of the

fissile 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 t ...

atom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass for Pu-239 has a more shallow valley than that observed for

U-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exist ...

, when the neutrons are

thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium wi ...

s. The curves for the fission of the later

actinides The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...

tend to make even more shallow valleys. In extreme cases such as ²⁵⁹Fm, only one peak is seen. Yield is usually expressed relative to number of fissioning nuclei, not the number of fission product nuclei, that is, yields should sum to 200%. The table in the next section ("Ordered by yield") gives yields for notable

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

(with half-lives greater than one year, plus

iodine-131 Iodine-131 (131I, I-131) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with nu ...

) fission products, and (the few most absorptive)

neutron poison In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable eff ...

fission products, from

fission of

(typical of

nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced ...

reactors), computed fro

The yields in the table sum to only 45.5522%, including 34.8401% which have half-lives greater than one year: The remainder and the unlisted 54.4478% decay with half-lives less than one year into nonradioactive nuclei. This is before accounting for the effects of any subsequent neutron capture; e.g.: * Xenon-135, ¹³⁵Xe capturing a neutron and becoming nearly stable ¹³⁶Xe, rather than decaying to ¹³⁵Cs which is radioactive with a half-life of 2.3 million years * Nonradioactive ¹³³Cs capturing a neutron and becoming ¹³⁴Cs, which is radioactive with a half-life of 2 years * Many of the fission products with mass 147 or greater such as ¹⁴⁷Pm, ¹⁴⁹Sm, ¹⁵¹Sm, and ¹⁵⁵Eu have significant cross sections for neutron capture, so that one heavy fission product atom can undergo multiple successive neutron captures. Besides fission products, the other types of radioactive products are *

containing ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, and ²⁴²Pu, * minor actinides including ²³⁷Np, ²⁴¹Am, ²⁴³Am, curium isotopes, and perhaps

californium Californium is a radioactive chemical element with the symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding ...

reprocessed uranium Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material s ...

containing ²³⁶U and other isotopes *

activation products Activation products are materials made radioactive by neutron activation. Fission products and actinides produced by neutron absorption of nuclear fuel itself are normally referred to by those specific names, and ''activation product'' reserved fo ...

of neutron capture by the reactor or bomb structure or the environment

Fission products from U-235

Cumulative fission yields

Cumulative fission yields give the amounts of nuclides produced either directly in the fission or by decay of other nuclides.

Ordered by mass number

Decays, even if lengthy, are given down to the stable nuclide. Decays with half lives longer than a century are marked with a single asterisk (), while decays with a half life longer than a hundred million years are marked with two asterisks ().

Half lives, decay modes, and branching fractions

Ordered by thermal neutron absorption cross section

{, class="wikitable sortable" ! Barns !! Yield !! Isotope !! ''t''_½ !! Comment , - , align= , , 6.3333% , , ¹³⁵I → ¹³⁵Xe , , 6.57 h , , Most important

;

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

rapidly converts ¹³⁵Xe to ¹³⁶Xe; remainder decays (9.14 h) to ¹³⁵Cs (2.3 My). , - , align= , , 0.0065% , , ¹⁵⁷Gd , , ∞ , ,

Neutron poison In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable eff ...

, but low yield. , - , align= , , 1.0888% , , ¹⁴⁹Sm , , ∞ , , 2nd most important

. , - , align= , , 0.0003% , , ^113mCd , , 14.1 y , , Most will be destroyed by neutron capture. , - , align= , , 0.4203% , , ¹⁵¹Sm , , 90 y , , Most will be destroyed by neutron capture. , - , align=
60,900 , , 0.0330% , , ¹⁵⁵Eu → ¹⁵⁵Gd , , 4.76 y , , Both

s. , - , align= , , 2.2713% , , ¹⁴⁷Pm , , 2.62 y, , Suitable for radioisotope thermoelectric generators with annual or semi-annual refueling. , - , align= , , 2.8336% , , ¹³¹I , , 8.02 d , - , align=
140 , , 6.7896% , , ¹³³Cs → ¹³⁴Cs , , ∞
2.065 y , ,

Neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

converts a few percent of nonradioactive ¹³³Cs to ¹³⁴Cs, which has very low direct yield because

stops at ¹³⁴Xe; further capture will add to long-lived ¹³⁵Cs. , - , align= , , 6.0507% , , ⁹⁹Tc , , 211 ky , , Candidate for disposal by nuclear transmutation. , - , align= , , 0.6576% , , ¹²⁹I , , 15.7 My , , Candidate for disposal by nuclear transmutation. , - , align= , , 6.2956% , , ⁹³Zr , , 1.53 My , , Transmutation impractical. , - , align= , , 0.1629% , , ¹⁰⁷Pd , , 6.5 My , - , align= , , 0.2717% , , ⁸⁵Kr , , 10.78 y , - , align= , , 5.7518% , , ⁹⁰Sr , , 28.9 y , - , align= , , 0.3912% , , ¹⁰⁶Ru , , 373.6 d , - , align= , , 6.0899% , , ¹³⁷Cs , , 30.17 y , - , , , 0.0297% , , ¹²⁵Sb , , 2.76 y , - , , , 0.0236% , , ¹²⁶Sn , , 230 ky , - , , , 0.0508% , , ⁷⁹Se , , {{ntsh, 327000 327 ky , -

References

External links

HANDBOOK OF NUCLEAR DATA FOR SAFEGUARDS: DATABASE EXTENSIONS, AUGUST 2008
* '
The Live Chart of Nuclides - IAEA
'' Color-map of yields, and detailed data by click on a nuclide. Nuclear fission Nuclear chemistry