This International Standard provides the basis for calculating the decay heat power of non-recycled nuclear fuel of light water reactors. For this purpose the following components are considered: — the contribution of the fission products from nuclear fission; — the contribution of the actinides; — the contribution of isotopes resulting from neutron capture in fission products. This International Standard applies to light water reactors (pressurized water and boiling water reactors) loaded with a nuclear fuel mixture consisting of 235U and 238U. Its application to recycled nuclear fuel is not permissible. The calculation procedures apply to decay heat periods of 0 and 109 s.