Fears that a nuclear reactor in Japan may be in the midst of a partial meltdown is shaking financial markets. Stock indexes lost 2% and gave up nearly all of their gains for the year today. All 10 company groups that make up the Standard & Poor's 500 index fell. The Dow Jones industrial average fell 242 points to 11,613. It was the biggest drop for the index since August 11. The Standard & Poor's 500 index lost 25 to 1,257, and Nasdaq fell 51 to 2,617. (More Dow Jones stories.)