TOKYO — What passes for normal at the Fukushima Daiichi plant today would have caused shudders among even the most sanguine of experts before an earthquake and tsunami set off the world’s second most serious nuclear crisis after Chernobyl.

Fourteen months after the accident, a pool brimming with used fuel rods and filled with vast quantities of radioactive cesium still sits on the top floor of a heavily damaged reactor building, covered only with plastic.

The public’s fears about the pool have grown in recent months as some scientists have warned that it has the most potential for setting off a new catastrophe, now that the three nuclear reactors that suffered meltdowns are in a more stable state, and as frequent quakes continue to rattle the region.

The worries picked up new traction in recent days after the operator of the plant, Tokyo Electric Power Company, or Tepco, said it had found a slight bulge in one of the walls of the reactor building, stoking fears over the building’s safety.

Tomohiro Ohsumi/Bloomberg News

Reporters and Tepco workers at Reactor No. 4 at Fukushima Daiichi, which the environment and nuclear minister visited Saturday.

To try to quell such worries, the government sent the environment and nuclear minister to the plant on Saturday, where he climbed a makeshift staircase in protective garb to look at the structure supporting the pool, which he said appeared sound. The minister, Goshi Hosono, added that although the government accepted Tepco’s assurances that reinforcement work had shored up the building, it ordered the company to conduct further studies because of the bulge.

Some outside experts have also worked to allay fears, saying that the fuel in the pool is now so old that it cannot generate enough heat to start the kind of accident that would allow radioactive material to escape.

But many Japanese scoff at those assurances and point out that even if the building is strong enough, which they question, the jury-rigged cooling system for the pool has already malfunctioned several times, including a 24-hour failure in April. Had the outages continued, they would have left the rods at risk of dangerous overheating. Government critics are especially concerned, since Tepco has said the soonest it could begin emptying the pool is late 2013, dashing hopes for earlier action.

“The No. 4 reactor is visibly damaged and in a fragile state, down to the floor that holds the spent fuel pool,” said Hiroaki Koide, an assistant professor at Kyoto University’s Research Reactor Institute and one of the experts raising concerns. “Any radioactive release could be huge and go directly into the environment.”

Senator Ron Wyden, Democrat of Oregon, expressed similar concerns during a trip to Japan last month.

The fears over the pool at Reactor No. 4 are helping to undermine assurances by Tepco and the Japanese government that the Fukushima plant has been stabilized, and are highlighting how complicated the cleanup of the site, expected to take decades, will be. The concerns are also raising questions about whether Japan’s all-out effort to convince its citizens that nuclear power is safe kept the authorities from exploring other — and some say safer — options for storing used fuel rods.

“It was taboo to raise questions about the spent fuel that was piling up,” said Hideo Kimura, who worked as a nuclear fuel engineer at the Fukushima Daiichi plant in the 1990s. “But it was clear that there was nowhere for the spent fuel to go.”

The worst-case situations for Reactor No. 4 would be for the pool to run dry if there is another problem with the cooling system and the rods catch fire, releasing enormous amounts of radioactive material, or for fission to restart if the metal panels that separate the rods are knocked over in a quake. That would be especially bad because the pool, unlike reactors, lacks containment vessels to hold in radioactive materials. (Even the roof that used to exist would be no match if the rods caught fire, for instance.)

There is considerable disagreement among scientists over whether such catastrophes are possible. But some argue that whether the chances are small or large, changes should be made quickly because of the magnitude of the potential calamity.

Senator Wyden, whose state could lie in the path of any new radioactive plumes and who has studied nuclear waste issues, is among those pushing for faster action. After his recent visit to the ravaged plant, he said the pool at No. 4 poses “an extraordinary and continuing risk” and the retrieval of spent fuel “should be a priority, given the possibility of further earthquakes.”

Attention has focused on No. 4’s spent fuel pool because of the large number of assemblies filled with rods that are stored at that reactor building. Three other reactor buildings at the site are also badly damaged, but their pools hold fewer used assemblies.

According to Tepco, the pool at the No. 4 reactor, which was not operating at the time of the accident, holds 1,331 spent fuel assemblies, which each contain dozens of rods. Several thousand rods were removed from the core just three months before so the vessel could be inspected. Those rods, which were not fully used up, could more easily support chain reactions than the fully spent fuel.

While Mr. Koide and others warn that Tepco must move more quickly to transfer the fuel rods to a safer location, such transfers have been greatly complicated by the nuclear accident. Ordinarily the rods are lifted by giant cranes, but at Fukushima those cranes collapsed during the series of disasters that started with the earthquake and included explosions that destroyed portions of several reactor buildings.

Tepco has said it will need to build a separate structure next to Reactor No. 4 to support a new crane.

The presence of so many spent fuel rods at Fukushima Daiichi highlights a quandary facing the global nuclear industry: how to safely store — and eventually recycle or dispose of — spent nuclear fuel, which stays radioactive for tens of thousands of years.

In the 1960s and 1970s, recycling for reuse in plants seemed the most promising option to countries with civilian nuclear power programs. And as Japan expanded its collection of nuclear reactors, local communities were told not to worry about the spent fuel, which would be recycled.

The idea of recycling fell out of favor in some countries, including the United States, which dropped the idea because it is a potential path to nuclear weapons. Japan stuck to its nuclear fuel cycle goal, however, despite leaks and delays at a vast reprocessing plant in the north, leading utilities to store a growing stockpile of spent fuel.

As early as the 1980s, researchers, including those at the United States Nuclear Regulatory Commission, started warning of the risks of storing growing amounts of nuclear fuel in pools. The United States has since concluded that densely packed pools are safe enough, but Tepco says that it never even specifically studied the risks posed by the pools.

“Japan did not want to admit that the nuclear fuel cycle might be a failed policy, and did not think seriously about a safer, more permanent way to store spent fuel,” said Tadahiro Katsuta, an associate professor of nuclear science at Tokyo’s Meiji University.

The capacity problem was particularly pronounced at Fukushima Daiichi, which is among Japan’s oldest plants and where the oldest fuel assemblies have been stored in pools since 1973.

Eventually, the plant built an extra fuel rod pool, despite suspicions among residents that increasing capacity at the plant would mean the rods would be stored at the site far longer than promised. (They were right.)

Tepco also wanted to transfer some of the rods to sealed casks, but the community was convinced that it was a stalling tactic, and the company loaded only a limited number of casks there.

The casks, as it turns out, were the better choice. They survived the disaster unscathed.