Nikkei opened higher Tuesday after the stocks closed at a three-week low below the 8,300 line the previous day. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 31.17 points, or 0.38 percent, from Monday to 8,327. 29. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was up 2.20 points, or 0.31 percent, to 718. 58.