[en] The International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO) was initiated in the year 2000, based on resolution of the IAEA General Conference (GC(44)/RES/21). This followed an initiative of the Russian Federation supported by a group of IAEA Member States to join forces in a broad international effort to develop innovative nuclear reactor and fuel cycle technology, recognizing that: A sustainable energy supply for humanity in the 21st century will require the large-scale deployment of nuclear power as well as other energy sources; Nuclear power is an energy technology that offers practically unlimited energy resources whose deployment can reduce environmental pollution and the volumes of waste needing management, including greenhouse gas emissions. As of December 2004, INPRO has 22 members: Argentina, Armenia, Brazil, Bulgaria, Canada, Chile, China, Czech Republic, France, Germany, India, Indonesia, Morocco, Netherlands, Republic of Korea, Pakistan, Russian Federation, South Africa, Spain, Switzerland, Turkey and the European Commission. The main objectives of INPRO are to: Help to ensure that nuclear energy is available to contribute in fulfilling energy needs in the 21st century in a sustainable manner; Bring together both technology holders and technology users to consider jointly the international and national actions required to achieve desired innovations in nuclear reactors and fuel cycles; and to Create a forum to involve all relevant stakeholders that will have an impact on, draw from, and complement the activities of existing institutions, as well as ongoing initiatives at the national and international level. To realize its objectives, INPRO has adopted a stepwise approach. In the first step, called Phase 1A, task groups established a hierarchy of Basic Principles, User Requirements and Criteria, in the areas of economics, safety, environment, waste management, proliferation resistance, and infrastructure, that must be fulfilled by an innovative nuclear energy system (INS) to meet the overall target of sustainable energy supply. As well, the initial development of the INPRO method for the assessment of nuclear energy systems was carried out. The Basic Principles, User Requirements, and Criteria and the INPRO method of assessment, taken together, comprise the INPRO methodology. The INPRO methodology provides the possibility to take into account local, regional and global boundary conditions of IAEA Member States, including those of both developing and developed countries. Phase 1A was completed in June of 2003 with the publication of IAEA-TECDOC-1362, Guidance for the Evaluation of Innovative Nuclear Reactors and Fuel Cycles, which documented the results of the Phase 1A work. The next step of INPRO was immediately launched. In this step, referred to as Phase 1B (first part), INPRO arranged for some 14 case studies to be performed, by national teams or by individual experts from seven countries, to test and provide feedback on the applicability, consistency and completeness of the INPRO methodology. This report documents changes to the basic principles, user requirements, criteria and the method of assessment that resulted from the second step of INPRO (referred to as Phase 1B (first part)), which started in June 2003 and ended in December 2004. During this step, Member States and individual experts performed 14 case studies with the objective of testing and validating the INPRO methodology. Based on the feedback from these case studies and numerous consultancies mostly held at the IAEA, the INPRO methodology has been significantly updated and revised, as documented in this report. The ongoing and future activities of INPRO will lead to further modifications to the INPRO methodology, based on the feedback received from Member States in light of their experience in applying the methodology. Thus, additional reports will be issued, as appropriate, to update the INPRO methodology