Problems and future development of validation and application of computer codes for WWER thermal-hydraulic analyses in INRNE-BAS

Thermal-hydraulic analyses of nuclear reactors are important for their safety operation. Design features of WWER require additional validation of used computer codes. Bulgaria is may be the only operator of WWER that never have performed own experimental investigations for the codes' validation. The need to use new up to date codes for non-analysed before accidents put this question for discussion again. European Union program for safety upgrading of NPP with WWER includes the establishment of verification matrix for thermalhydraulic system codes applied for WWER analysis. For our country is very important to participate in such projects and investigations. This way experimental data obtained by other countries and validated codes will be available to us. Next step have to be the establishment of own data bases for codes' verification and then real licensing of the codes will be possible in the country. The main activities of the Reactor Safety and Thermal-Hydraulics Section of INRNE are the development of computer codes for WWER thermal-hydraulic analyses related to their safety. This way we are going to meet the requirements to use second system code in the country, 3D codes for non-analysed before accidents, performing of independent and parallel analyses. Validation of the codes by documented operational transients, creation of input data decks for Kozloduy NPP units are the required part of our activity. Establishment of open data bases on these two subjects will improve the quality of performed thermal-hydraulic analyses. The support of NPP and Regulatory Committee as main users of the results is very important in these activities