[en] The FUMACS code package (an acronym of FUel MAnagement Code System) was initially developed at Rudjer Boskovic Institute in the year 1991 with the aim to enable in-core fuel management analysis of the NPP Krsko core for nominal (full power) conditions. Following NPP Krsko modernization process, the new version of the FUMACS code package was made in the year 2001 to enable modeling of the NPP Krsko core for uprated conditions and longer cycles. A graphical user interface was incorporated into the code package to make the code package more use friendly. The FUMACS code package has also been upgraded in the year 2005 to include modeling of integral fuel burnable absorbers (IFBAs) containing enriched boron. Since Westinghouse plans to use next generation fuel (NGF) and integral burnable absorbers containing gadolinium in the following cycles, the existing version of the FUMACS code package would not be able to model the NPP Krsko core properly. Therefore, the new options for modeling NGF and gadolinium depletion have been introduced into the FUMACS code package. The new, upgraded, FUMACS code package has been verified and validated for NPP Krsko operating cycles.(author).