Since 1991, the EC has been supporting the transition in Eastern European and former Soviet Union countries to market economy through the Technical Assistance to the Commonwealth of Independent States (TACIS) and Poland Hungary Aid for Reconstruction of the Economy (PHARE) Programmes. One of the priority areas for TACIS and PHARE funding was improvement of nuclear safety. In these programmes, the Joint Research Centre (JRC) of the European Commission (EC) was providing Technical and Scientific Expertise in different areas of nuclear safety to the EC Directorates responsible for TACIS and PHARE programming and implementation. Several TACIS and PHARE projects were focussed on thermalhydraulic issues associated with VVER and RBMK reactor safety: accident analyses, severe accident analyses, accident management, experimental facilities and results databases, etc. These projects have included the application and evaluation of Eastern codes in addition to the transfer, adaptation and use of Western codes together with Western technologies, analysis experience, know-how and training of Eastern experts on the use of the codes, methodology and phenomena. This paper gives an overview of some of the projects carried out so far during more than two decades of TACIS and PHARE assistance. It illustrates in particular the objectives and main achievements, classifying the areas covered, and summarising the open issues. In four projects the Russian EDO Gidropress participated as a sub-contractor of the Western consultant, and the contractor for the experimental activities performed at EDO Gidropress mixing facility (project R2.02/02). For those projects the results obtained have been briefly summarized. The evaluation of the technological impact of the project results was beyond the scope of this paper, however some important conclusions were highlighted. In general projects have demonstrated that Western codes initially developed for PWR analysis appeared to be broadly valid for RBMK and VVER reactor types. In addition to transfer of Western codes and training on those codes, projects addressed also the transfer of Western methodologies, experience and approach in several areas (code validation, severe accident phenomena, acceptance criteria, accident and severe accident management procedures, etc). A key value of TACIS and PHARE projects was the development of a bank of experimental data addressed to reactor safety, aimed at improving phenomena understanding and further validation of codes (i.e. experimental data produced in the PSB-VVER and PSB-RBMK facilities and in the EDO Gidropress mixing facility). The application of CFD codes to nuclear safety is also increasing, in particular to model local phenomena for which a traditional approach with one dimensional system codes could be insufficient. This was demonstrated in the R2.02/02 project presented in the paper. ; JRC.F.5-Nuclear Reactor Safety Assessment