Table 1.

Properties of key reactor isotopes

Property	U-235	U-238	Pu-239	Pu-241
Thermal (0.0253 eV) neutron induced fission cross-section20	582.6 ± 1.1 barns	0.000003 barns	748.1 ± 2.0 barns	1011.1 ± 6.2 barns
Thermal energy per fission21	201.7 ± 0.6 MeV	205.0 ± 0.9 MeV	210.0 ± 0.9 MeV	212.4 ± 1.0 MeV

Equation (2) relies upon the energy released per fission, as well as the cross-section for fission, for each isotope. These quantities are presented above for isotopes which typically account for 99.9% of the power in a reactor core: U-235, U-238, Pu-239, and Pu-241¹⁰