RBMK-1500 reactors are the most advanced version of the RBMK reactor design series. Compared to the Chernobyl NPP, these reactors are more powerful (1500 MW versus 1000 MW), and is provided with an improved Accident Confinement System (ACS). In most other respects, the plants are quite similar to their predecessors. They have two cooling loops, a direct cycle, fuel clusters are loaded into individual channels rather than a single pressure vessel, the neutron spectrum is thermalized by a massive graphite moderator block. The plant can be refueled on line and uses slightly enriched nuclear fuel.Because of their online refueling capability RBMK-type reactors were not exported, and were built exclusively in the territory of the former Soviet Union. There are presently plants at Saint Petersburg (Sosnovy Bor), Kursk, Chernobyl and Smolensk. A total of 17 such reactors have been built and 15 are currently in operation.