Trends for further perfection and improvement in safety of the reactor plants of VVER-type

Three stages of VVER plant development projects have been completed in the USSR. The first stage is for projects of NPS with reactors of the first generation, followed by the second for those of NPS with VVER-440 reactors and the third for those with VVER-1000 reactor plants and reactor plant VVER-440 of the third generation. A series of safety increase oriented measures have been developed for operating NPS with VVER. At present works are under way for perfection of VVER plants according to the following directions: (1) to increase the use of passive methods of system operation, (2) to install diagnostic systems, (3) to increase the efficiency of mechanical control members, (4) to decrease the core power peaking factors, (5) to use refuelling conditions with low neutron leak, (6) to develop packed cores to increase the fuel breeding factor, (7) to use 'ventilated' spacing grids in steam generators wherein corrosion products and sludge do not deposit, (8) to previde NPS manoeuvrability, (9) to provide selfdiagnostics in system securing safety, (10) to improve metal quality used for equipment manufacturing, (11) to provide obligation of probabilistic safety and risk assessment, (12) to provide in control systems availability of expert system and (13) to increase the service life. Measures planned for improvement in safety of VVER-1000 and VVER-440 and those required for development of VVER-1800 reactor plants are listed. (Nogami, K.)