TOKYO — Workers at the stricken Fukushima nuclear power plant who were installing wire nets Friday to keep rats away from a vital cooling system instead tripped that system, causing it to fail for the second time in weeks.

The spent-fuel pool at the site’s No. 3 reactor went without fresh cooling water for almost three hours on Friday afternoon, said the plant’s operator, the Tokyo Electric Power Company, or Tepco.

Cooling was restored by late evening on Friday, and there was no imminent danger to the 566 nuclear fuel rods stored in the pool, according to the company. It would have taken at least two weeks for the pool to have risen above the safe level of 149 degrees Fahrenheit, Tepco said.

Still, the recent power failures have raised concerns over continued vulnerabilities at the plant two years after a large earthquake and tsunami knocked out its vital cooling systems, resulting in multiple fuel meltdowns and forcing 160,000 people to evacuate.