TOKYO — The operator of the stricken Fukushima nuclear plant said Wednesday that it had found a leak in one of the hundreds of steel tanks used to store radioactive water at the plant, raising renewed questions about the company’s ability to handle the plant’s cleanup.

The discovery comes a day after the operator, the Tokyo Electric Power Company, or Tepco, admitted that it had found cesium particles in groundwater flowing into the Fukushima Daiichi plant, reversing its earlier assertion that the water was uncontaminated.

The company stressed that the size of the tank leak was small — the equivalent of about a quart had dripped out so far, it said — and that the level of radioactivity in groundwater was within safe levels. However, the problems are the latest in a string of mistakes and mishaps that have added to mounting criticism of the government’s decision to leave the tricky cleanup in the hands of Tepco, the company that many say allowed the triple meltdown two years ago to happen in the first place.