TOKYO — The Tokyo Electric Power Company said Saturday that the filtration system it had struggled to put into operation at the damaged Fukushima Daiichi nuclear power plant had broken down after just five hours, a disappointing setback in its efforts to cool the reactors.

The company said that the sprawling system, which is designed to siphon oil, radioactive materials and salt from the water used to cool the reactors, was shut down because of readings that indicated one of the filters had filled up with radioactive cesium. The rapid depletion of a filter that was supposed to have lasted several weeks suggested the presence of far greater radioactive material than anticipated.

The filtration system was built ad hoc and rushed into service because Tokyo Electric, or Tepco, is quickly running out of space to store the tens of thousands of tons of water that have been contaminated after being poured into the reactors and spent-fuel pools.

Some of the tanks, basements and other storage facilities at the power plant have inches to spare and could overflow within days. Tepco hoped to reduce the amount of contaminated water by reusing the newly filtered water. The company is also bringing in hundreds of extra tanks.