INFLUENCE OF MODIFICATIONS AND BACKFITTINGS OF WWER-440/230 REACTORS ON OPERATIONAL SAFETY

摘要

Extensive modification and backfitting activities have been undertaken at Kozloduy NPP with the aim of restoring and improving the nuclear safety of Units 1 to 4 with WWER-440/230 type reactors. A programme for safety upgrading of Kozloduy NPP has been implemented with the assistance of the Commission of the European Communities (CEC), the IAEA, the World Association of Nuclear Operators (WANO), the consortium of western technical expert organizations, and through bilateral programmes with the United States of America, France, Japan, Germany, etc. After the implementation of the first stage measures of the safety upgrading programme significant improvements in plant safety were achieved. Upgrading measures have been performed for all safety systems and installations, process engineering systems, electrical equipment, instrumentation and control. Additional scram signals have been installed. Reliability and testability of emergency core cooling and decay heat removal systems have been improved. After the instalment of additional protection of the turbine in order to ensure reactor trip in case of low steam generator inventory, a tendency to an increasing number of reactor scrams was noted. The reason was inappropriate logic for actuation of the turbine protection system. The logical scheme of the turbine protection was modified from double protection logic to single combined protection, which is more reliable.