The political and economic changes in the states of Central and Eastern Europe have also altered the boundary conditions for the nuclear power plants in operation and under construction. The approximately twenty plants of the Russian VVER type operated in the Czech Republic, an Slovakia, Hungary, and Bulgaria produce a considerable share of the national electricity generation in each of these countries and thus represent a major potential. As a consequence, backfitting concepts are elaborated and put into effect for these plants on the basis of international safety standards in order to allow these plants to be operated up to the end of their projected operating lives. On the site of Bohunice in the Slovak Republic, with two nuclear generating units each of the Russian VVER 440/230 and VVER 440/213 lines, backfitting measures elaborated since the mid-nineties and approved by the Slovak regulatory authority, UJD, have been carried out by the international REKON consortium. Within this framework, Siemens carried out the basic engineering for most of the measures, for instance, for reconstruction of the emergency core cooling system and the confinement spray system, modernization of the instrumentation and control systems, increasing the reliability of the emergency power supply systems, the systems technology provisions for feed-and-bleed measures and fire protection measures, and a comprehensive improvement in measures against seismic impacts. The measures were carried out step by step, i.e., for one unit at a time, so as to allow the experience accumulated to be fed back into work on the next unit. These measures have been successful and represent the most extensive backfitting exercise yet in a VVER plant in operation. The safety level achieved in this way is internationally considered to be exemplary.
