One of the systems keeping spent atomic fuel cool at the Fukushima nuclear plant temporarily failed on Friday, the second outage in a matter of weeks, underlining the precarious fix at the plant.

Tokyo Electric Power (TEPCO) said an alarm sounded at the facility at 2:27 pm (0527 GMT), and technicians soon confirmed that the cooling system for the pool attached to reactor 3 was not working.

Nuclear fuel, even after use, has to be kept cool to prevent it from overheating and beginning a self-sustaining atomic reaction that could lead to meltdown.

The problem, which was fixed in about three hours, occurred as work crew placed a metal mesh around a switchboard in a bid to prevent small animals from touching it, a TEPCO spokesman told a press conference.

The measures were taken after a rat got inside the switchboard last month, causing a short-circuit that knocked out power for sections of the crippled plant and stopped cooling systems for four storage pools.

That time, it took nearly 30 hours for TEPCO to fully fix the problem.

The TEPCO spokesman said a wire or the mesh might have touched the ground while crews put the mesh in place, unintentionally grounding the equipment and knocking it offline.

TEPCO apologised for the problem, but stressed that it had not posed any immediate danger.

However, the incident served as a reminder of the precarious state of the Fukushima plant, more than two years after it was hit by the giant tsunami of March 2011, and critics were quick to jump on the fault.

"Rather than TEPCO assuring us there is no safety threat, both the company and government officials must now follow up with robust and effective action," Greenpeace International energy campaign team leader Jan Beranek said.

"Japan needs to focus its efforts and capacities on maintaining the troubled reactors, instead of rushing to restart other risky nuclear plants," Beranek said.

TEPCO and the government said in December 2011 that the reactors were "in a state of cold shutdown" -- a phrase carefully chosen, commentators said, to imply normality in units so broken that standard descriptions did not apply.

Authorities insist they are getting on top of the problem and the reactors are not leaking significant amounts of radiation.

But the plant, which TEPCO and the government plan to dismantle over the next four decades, is kept stable only with makeshift systems to supply power, cool reactors and clean radioactive materials from water used as coolant.

As of 2 pm on Friday, the time of the outage, the temperature inside the pool affected by the latest glitch stood at 15.1 degrees Celsius (59.18 Fahrenheit), well below the safety limit of 65 degrees Celsius (149 degrees Fahrenheit), TEPCO said.

Equipment measuring radioactivity, placed in and around the plant, has shown no new abnormality related to the latest trouble, the company added.

Fukushima was the site of the worst nuclear crisis in a generation. Reactors went into meltdown and spewed radiation over a wide area, forcing tens of thousands of people from their homes and polluting farmland.

Although the natural disaster that spawned the emergency claimed around 19,000 lives, no one is officially recorded as having died as a direct result of the atomic catastrophe.

However, pressure groups like Greenpeace maintain the long-term health effects for people in the area are being vastly under-estimated by a government they say is in hock to a powerful nuclear industry.

Although many voters now distrust the technology, Prime Minister Shinzo Abe has openly said Japan must consider continued use of nuclear as a less-expensive energy source to power the world's third-largest economy.