This document 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 document applies to light water reactors (pressurized water and boiling water reactors) loaded with a nuclear fuel mixture consisting of 235U and 238U. Application of the fission product contribution to decay heat developed using this document to other thermal reactor designs, including heavy water reactors, is permissible provided that the other contributions from actinides and neutron capture are determined for the specific reactor type. Its application to recycled nuclear fuel, like mixed-oxide or reprocessed uranium, is not permissible.

The calculation procedures apply to decay heat periods from 0 s to 10^9 s.