The RBMK-1000 was the first Soviet standard type reactor, and it was one of these that failed at Chernobyl in 1986.
One of the great problems with the RBMK reactors was the provision of cooling water. It was too expensive to use closed cooling tower circuits so the designers opted for a radical simplification, a direct-flow water supply from purpose built reservoirs near the reactor site and isolated from the nearest water system.
These reservoirs acted as biological de-activators for radioactive water, the radio- nuclides precipitated to the bottom and were absorbed by the silt.
Very large reservoirs are required for this option. To cool one RBMK- 1000 the pool may be shallow ( typically 4m) but must be at least 5km- 6km in area. The Chernobyl pool covered 22km, and represented a large loss of highly productive, high grade, chernozem soil (one of the most valuable resources of Ukraine and southern Russia).
The VVER reactors were pressurised, water-cooled and water moderated reactors, developed initially to power nuclear submarines. They could function without the huge area of cooling ponds required for the RBMKs, but they did require large, fairly complex steel pressure vessels to contain the high pressure part of the system. Medvedev (1988) has described the early stages of VVER construction.