Some nuclear experts are warning that spent fuel rods at a damaged plant in Japan could trigger a major catastrophe despite the government’s declaration in December that the emergency phase of the nation’s worst nuclear disaster was over.

Fifteen months after a magnitude-9 earthquake and tsunami struck the Fukushima Daiichi plant and led to meltdowns, fears about reactor 4 have grown as its building holds a storage pool filled with 1,535 nuclear fuel rod assemblies.

The pool, which is 30 metres above ground, has been left uncovered since a hydrogen blast last year blew off the upper part of the outer wall of the containment building.

Most of the assemblies are spent fuel rods with a total amount of radioactive caesium equal to 5,000 atomic bombs of the kind that destroyed Hiroshima, said Hiroaki Koide, assistant professor at the Kyoto University Research Reactor Institute.

The government estimated the amount of caesium—137 already released by the Fukushima disaster were equal to that of 168 Hiroshima bombs.

If a large quake or other event were to cause the pool to crack and drain, it could lead to a new catastrophe, Koide said.

“We just all have to pray that an earthquake does not happen before that fuel is removed,” Arnie Gundersen, nuclear engineer of US-based Fairewinds Energy Education, said on his website.

The plant suffered meltdowns at three of its six reactors. But each of the three units holds fewer assemblies than reactor 4.

Mitsuhei Murata, Japan’s former ambassador to Switzerland, also sent a letter to UN Secretary General Ban Ki—moon in late March, saying, “It is no exaggeration to say that the fate of Japan and the whole world depends on the No 4 reactor.” US Senator Ron Wyden, who visited the complex in April, expressed concern in a letter to the Japanese ambassador to the United States.

“The precarious status of the Fukushima Daiichi nuclear units and the risk presented by the enormous inventory of radioactive materials and spent fuel in the event of further earthquake threats should be of concern to all and a focus of greater international support and assistance.” “The true earthquake risk for the site was seriously underestimated and remains unresolved,” Wyden added.

Gundersen argued that the top priority of the operator Tokyo Electric Power Co (TEPCO) and the government “should be to move the fuel out of that pool just as quickly as possible.” The operator and the government also “need to strengthen that pool to make sure that it can withstand an earthquake” in the meantime, Gundersen said. “It is a serious concern.” He said he does not believe TEPCO and the government are taking it seriously enough.

Goshi Hosono, the minister in charge of the nuclear disaster who inspected the complex in May, said the floor of the reactor building and supporting structure under the pool appeared to be intact.

But cranes that were supposed to carry spent fuel rods to a safer place collapsed during the disaster last year. TEPCO would need to erect a new building to support cranes.

The operator said it would start to remove the fuel in December 2013. The cleanup of the complex is expected to take decades.

Koide said he thinks TEPCO has been working on it with a sense of crisis, but that the government is eager to restart idled reactors and appears to lack that sense.

Critics say the government and major media are focusing too much on issues of the restart and an increase in the sales tax.

When he took office in September, Prime Mnister Yoshihiko Noda stressed that the rebirth of Fukushima was his government’s top priority.

“There is no rebirth of Japan without the rebirth of Fukushima,” he said.

In December, Noda declared a cold shutdown had been achieved at the Fukushima plant.

A “major fear factor” had been eliminated with the cold shutdown, the premier said then.