The known ORNL ORIGEN code is widely spread over the world for inventory, activity and decay heat tasks and is used stand-alone or implemented in activation, shielding or burn-up systems. More than 1000 isotopes with more than six coupled neutron capture and radioactive decay channels are handled simultaneously by the code. The characteristics of the calculated inventories, e.g., masses, activities, neutron and photon source terms or the decay heat during short or long decay time steps are achieved by summing over all isotopes, characterized in the ORIGEN libraries. An extended nuclear GRS-ORIGENX data library is now developed for practical appliance. The library was checked for activation tasks of structure material isotopes and for actinide and fission product burn-up calculations compared with experiments and standard methods. The paper is directed to the LWR decay heat calculation features of the new library and shows the differences of dynamical and time integrated results of ENDF/B-VI based and elder ENDF/B-V based libraries for decay heat tasks compared to fission burst experiments, ANS curves and some other published data. A multi-group time exponential evaluation is given for the fission burst power of 4 important fission materials, to be used in quick LWR reactor accident decay heat calculation tools. 1 Development of a GRS improved, ENDF/B-VI based ORIGEN data library A new nuclear data library GRS-ORIGENX [11] is now developed for practical appliance. In a first step of development, called LIBMAST04, some problems in the former ORIGEN calculation method [2] and/or in the data libraries for structural material activation calculations (LIB1), for the actinide buildup (LIB2) and the fission product generation (LIB3) could be solved, e.g., the tritium, 14C, 22Na, 26Al, 60Fe or 93mNb production problem. This was achieved by extending the number of neutron reaction channels, the energy groups and the energy range. All cross sections and build-up channels are completely recalculated by point data files JEF-2.2, ENDF/BVI, JENDL3.2 and EAF97. But the decay data and fission yields of LIBMAST04 were based on ENDF/B-V as in the burn-up program system OREST-96 [10]. In a second step of development LIBMAST06 the decay data – decay energies, probabilities and channels – and 25 fission yield sets are now taken from ENDF/B-VI data bases. The decay energies were analyzed and improved for reactor accident calculation to avoid the slight under-predictions of the reactor decay heat in the first 1000 seconds in ENDF/BVI. Especially the beta and gamma energies of 70 important fission products were enlarged by 5%. The library was checked for structure material isotope activation, and for actinide and fission product burn-up inventories compared with experiments and standard calculations. The overall data a Presenting author, e-mail: Ulrich.Hesse@grs.de of the new library, compared to elder evaluations, are listed in table 1, last column: Table 1. Nuclide numbers and data range LIBMAST06 and former ORIGEN versions. Library ORIGEN ref. [10] LIBMAST-04 LIBMAST-06 ref. [2] *) *) **) Number of nuclides LIB1 253 700 98