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.