Markets also fell in Asia and Europe after Japan said the crisis at its crippled nuclear plant was as serious as the 1986 Chernobyl accident, though much less radiation has leaked.
The plant in northern Japan was damaged in the March 11 earthquake and tsunami disaster.
"It means slower growing coming out of Japan in the short term, and that's going to weigh on global growth," said Peter Cardillo, chief market economist at Avalon Partners Inc.
The Dow Jones industrial average fell 117.53 points, or 0.9 percent, to close at 12,263.58. That's the largest drop since March 16, when the Dow lost 242 points on fears of a nuclear meltdown in Japan.
The Standard & Poor's 500 index fell 10.30, or 0.8 percent, to 1,314.16. The Nasdaq composite index fell 26.72, or 1 percent, to 2,744.79.
The situation in Japan and an unexpected drop in U.S. exports sent bond prices sharply higher and their yields lower. The yield on the 10-year note fell to 3.49 percent from 3.58 percent late Monday.
The Commerce Department said in its monthly trade report that exports fell 1.4 percent in February, more than economists had expected and a worrisome sign for U.S. economic growth. Exports had climbed 2.6 percent in January to an all-time high.
Alcoa dropped 6 percent to $16.70 after the aluminum maker's first-quarter revenues fell short of expectations. Alcoa, one of the 30 companies that make up the Dow average, is the first large company to report quarterly earnings.
Other big companies reporting results this week include JPMorgan Chase & Co., Google Inc. and Bank of America Corp.
The energy industry lost the most of the 10 that make up the S&P 500 index. Oil prices slid 3 percent to settle at $106.25 per barrel, the lowest price this month.
Goldman Sachs, which had expected higher oil prices, surprised oil traders with a report early Tuesday saying it now expects a "substantial pullback." Chevron Corp. lost 3 percent and Exxon Mobil Corp. 2 percent.
Slot machine makers dropped after WMS Industries Inc. warned that its latest earnings will fall short of expectations. WMS sank 17 percent. International Game Technology dropped 3 percent.
Three shares fell for every one that rose on the New York Stock Exchange. Trading volume was 4.4 billion shares.