The VVER-1000 model exists in several versions. The ‘small series’ plants could be considered as pioneers of this model. The VVER-1000/320 is the large series version of the design. Developed after 1975, VVER-1000/320 type plants are operated in Bulgaria, the Czech Republic, Russia, Ukraine and China. Modernized versions of VVER-1000 plants are under construc­tion in five countries (Bulgaria, China, India, Iran and Russia).

In regard to lifetime management, the VVER-1000/320 plants have great­est practical importance. The ‘small series’ plants show some specific design features, but the lifetime management practice of these plants does not dif­fer essentially from the VVER-1000/320 version.

The VVER-1000 is a four loop PWR with horizontal steam generators. Each loop consists of a hot leg, a horizontal steam generator, a main circu­lating pump and a cold leg. Main isolating valves on the hot and cold legs of each loop equip the non-standard VVER-1000 primary loops. The standard V-320 design and the new clones of the VVER-1000 do not have isolating valves on the primary loop. A pressurizer is connected to the hot leg of one of the loops and the spray line to the cold leg. Operating conditions are Thot=322°C, Tcold=290°C, p=15.7 MPa. The reactor, the primary and the safety systems are all placed within a full pressure, dry, pre-stressed concrete containment.

The design bases, and also the technical solutions applied, are very similar to the PWRs operated in Western countries. The safety concerns about the VVER-1000 plants are discussed in detail in IAEA reports (1996b; 2000). The main safety concern regarding the VVER-1000 plants lies in the qual­ity and reliability of the individual equipment, especially the I&C equip­ment. The plant layout has weaknesses that make the redundant system parts vulnerable to hazardous systems interactions and common cause fail­ures by fires, internal floods or external hazards. At all plants, many of these deficiencies have been addressed by plant modifications and an acceptable safety level has thus been achieved.

There are several advanced VVER-1000 plants presently under con­struction, more than 20 new projects of advanced VVER design are under preparation or consideration and several are in the bidding phase. The most advanced versions of VVER design, showing features of Generation III reactors, are being considered for future bids for large generating capacity reactors.