WASHINGTON — As more nuclear reactors across the country are closed, the problem of what to do with their waste is becoming more urgent, government officials and private experts said at a conference here this week.

To address the problem, a bipartisan group of four senators introduced a bill on Thursday that would provide for temporary, centralized storage, even as House leaders remained focused on trying to revive plans for the Yucca Mountain nuclear waste repository that the Obama administration has declared dead.

Nuclear waste is accumulating in steel and concrete storage casks at reactor sites around the country. But the casks — sealed boxes of many tons — cannot be sent to any repository because they are not compatible, said Jeff Williams, director of the Energy Department’s Nuclear Fuel Storage and Transportation Planning Project.

In addition, a growing number of the sites no longer have an operating reactor or the associated fuel-handling equipment, so they have no way to move the highly radioactive fuel to another storage package.