example, most solar neutrinos have energies on the order of 100 keV – 1 MeV , so the fraction of neutrinos with "wrong" helicity among them cannot exceed 10 −10 . [ 52 ] [ 53 ] Nuclear reactors are the major source of human-generated neutrinos. Antineutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the antineutrino flux are 235 U , 238 U , 239 Pu and 241 Pu (i.e. via the antineutrinos emitted during beta-minus decay of their respective fission fragments). The average nuclear fission releases about 200 MeV of energy, of which roughly 4.5% (or about 9 MeV ) [ 54 ] is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of 4 000 MW , meaning that the core produces this much heat, and an electrical power generation of 1 300 MW , the total power production from fissioning atoms is actually 4 185 MW , of which 185 MW is radiated away as antineutrino radiation and never appears in the engineering. This is to say, 185 MW of fission energy is lost from this reactor and does not appear as heat available to run turbines, since the antineutrinos penetrate all building materials essentially without any trace, and disappear. [ nb 7 ] The antineutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission antineutrinos on average have slightly more energy than those from