Fission product yield

Long-lived fission products
Nuclide	t_1⁄2	Yield	Q	βγ
	(Ma)	(%)	(keV)
⁹⁹Tc	0.211	6.1385	294	β
¹²⁶Sn	0.230	0.1084	4050	βγ
⁷⁹Se	0.327	0.0447	151	β
¹³⁵Cs	1.33	6.9110	269	β
⁹³Zr	1.61	5.4575	91	βγ
¹⁰⁷Pd	6.5	1.2499	33	β
¹²⁹I	16.14	0.8410	194	βγ
↑ Decay energy is split among β, neutrino, and γ if any. ↑ Per 65 thermal neutron fissions of ²³⁵U and 35 of ²³⁹Pu. ↑ Has decay energy 380 keV, but its decay product ¹²⁶Sb has decay energy 3.67 MeV. ↑ Lower in thermal reactors because ¹³⁵Xe, its predecessor, readily absorbs neutrons.

Medium-lived fission products
	t_½ (year)	Yield (%)	Q (keV)	βγ
¹⁵⁵Eu	4.76	0.0803	252	βγ
⁸⁵Kr	10.76	0.2180	687	βγ
^113mCd	14.1	0.0008	316	β
⁹⁰Sr	28.9	4.505	2826	β
¹³⁷Cs	30.23	6.337	1176	βγ
^121mSn	43.9	0.00005	390	βγ
¹⁵¹Sm	94.6	0.5314	77	β

Nuclear fission splits a heavy nucleus such as uranium or plutonium 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
Chemical element spanning several isotopes of different mass number but same atomic number.
Nuclei of a given mass number regardless of atomic number. Known as "chain yield" because it represents a decay chain of beta decay.

Isotope and element yields will change as the fission products undergo beta decay, while chain yields do not change after completion of neutron emission by a few neutron-rich initial fission products (delayed neutrons), with half-life 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 less is stable and does not decay (atomic number grows by 1 during beta 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 product.

[1] Decay energy is split among β, neutrino, and γ if any.

[2] Per 65 thermal neutron fissions of ²³⁵U and 35 of ²³⁹Pu.

[3] Has decay energy 380 keV, but its decay product ¹²⁶Sb has decay energy 3.67 MeV.

[4] Lower in thermal reactors because ¹³⁵Xe, its predecessor, readily absorbs neutrons.