The man President Carter nominated to be the fifth member of the Nuclear Regulatory Commission said yesterday that radioactive waste disposal is the most critical issue nuclear power faces today.

"The future of nuclear power depends entirely on the next three to five years," John F. Ahearne told the Senate subcommittee on nuclear regulation, which will recommend for or against his confirmation. "It depends on the resolution of the waste disposal issue and on whether the public accepts that resolution."

An aide to Energy Secretary James R. Schlesinger Jr., Ahearne is the second Carter administration nominee for the fifth and last seat on the NRC.The first was Massachusetts Institute of Technology nuclear engineer Kent Hansen, who was rejected by the Senate subcommittee earlier this year.

Ahearne said that by 1983 the storage pools where spent nuclear fuel is being left today will be "filled to capacity." When that happens, he said, "something else will clearly have to be done" to handle the rising tide of spent nuclear fuel in the United States.

The NRC nominee said that it was his hope that the Energy Department would have a plan approved by then to store spent fuel "away from the reactors," where it is now stored. He said he regarded that as an interim solution that would be supplanted by a long-term method of permanent nuclear waste disposal.

"It's important that whatever this proposal is that the public accept it," Ahearne said. "I think the public will accept it if some credible government agency, and I hope that is the Nuclear Regulatory Commission, has reviewed it and says it has found it credible."