The Nikkei 225 index slipped 94.65 points, or 0.99 percent, to 9,441.48 in afternoon trade on Monday. The Topix index was off 4.05 points, or 0.47 percent, to 853.33.
"There was disappointment that no progress was seen over the weekend and there are growing concerns that the nuclear issue may drag on for a protracted period," said Masayoshi Yano, a senior market analyst at Meiwa Securities.
Dangerous levels of radiation detected in water thought to be leaking from a stricken reactor have dealt a setback to efforts to avert a nuclear disaster at the stricken Fukushima Daiichi plant.
Radiation in puddles near reactor two at the plant was more than 1,000 millisieverts per hour, operator Tokyo Electric Power (TEPCO) said.
The plant's cooling systems were crippled by the March 11 magnitude 9.0 quake and the devastating tsunami it unleashed, which tore into the northeast coast and erased entire towns.
After last week saw buy-backs following a post-earthquake sell-off, investors were now focusing on the impact of production setbacks and wider impact of the twin disaster on companies, dealers said.
"While last week was mostly about buying back of shares following the post-earthquake sell-off, investors will be paying a closer look at the specific fallout (on corporations) from the quake," Yutaka Miura, a technical senior analyst at Mizuho Securities, told Dow Jones Newswires.
Shares in Fukushima Daiichi plant operator TEPCO tumbled 14.77 percent at 721.
Japan Tobacco was off 2.38 percent at 306,500 yen after it said it will halt cigarette shipments from March 30 to April 10.
The yen was trading at 81.75 to the dollar on Monday from 81.34 yen on Friday, with the dollar buoyed by news the US economy grew 3.1 percent in fourth quarter 2010, up from a previous estimate of 2.8 percent.
The yen was at 114.90 to the euro, from 114.60 yen Friday.