Toshiba Corp could post a 700 billion yen ($6.1 billion) loss at its U.S. nuclear reactor business, much bigger than the 500 billion yen it had earlier flagged to investors, Kyodo news agency reported. Shares in Toshiba plunged 15 percent on reports of bigger than expected losses, which come one day after it said it was considering a spin-off of its chip business to cover financial shortfalls. Toshiba declined to comment on the reports.