Tokyo stocks opened flat Tuesday, as moves to lock in gains after a recent rally were offset by buying amid continued optimism for Japanese equities. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 23.91 points, or 0.06 percent, from Monday to 40,438.03. The broader Topix index was down 1.76 points, or 0.06 percent, at 2,775.88. On the top-tier Prime Market, gainers were led by mining, insurance and machinery shares. The main decliners were land transportation, farm and fishery, and air transportation issues. At 9 a.m., the U.S. dollar fetched 151.36-39 yen compared…