RAPID economic growth in China and a desire to bring back Hongkong and Taiwan into the fold may only split the country by early next century, according to a study. China is oscillating between confidence and insecurity, the authoritative International Institute for Strategic Studies in London says in the 1993 edition of its Strategic Survey yearbook just published. The confidence derives from the fact the death of the Soviet Union means China is more secure from external threat than it has been at any time for several centuries. ''Longer term confidence could be derived from the superb performance of its economy and the real prospect of overtaking Japan and the United States,'' the institute said. But in its study of China's strategic needs and interests, the highly respected institute adds: ''The logic of a very different China could emerge from the intervening years. Hence the insecurity.'' It says: ''Rapid growth might require so much regional diversity that China might not be recognisable as a centre-driven, single state by early in the next century. ''Under such circumstances, China's pursuit of irredentism in Hongkong, Taiwan or even the South China Sea, might only help split China. Rapid growth might also demand so much international interdependence that China would not be free to punish those in the outside world who have a different agenda for China. ''If interdependence leads to greater fragmentation at home, less ability to hold democratic forces at bay, and less ability to wage effective trade war or counter Western influence around the world, China's remaining hardline leaders may wonder what therush to growth was all about.'' The study says that certainly until the death of paramount leader Mr Deng Xiaoping China will settle for compromise in its international affairs.