A proposal by two Massachusetts congressmen would put on hold for nearly a decade attempts by the Seabrook Station nuclear power plant to win federal approval to operate until 2050.

The bill, filed last week by US Representatives Edward J. Markey and John F. Tierney, was proposed a week after the Nuclear Regulatory Commission stepped up an inves­tigation of the New Hampshire plant’s structural integrity ­after weakened concrete was discovered about three years ago.

The legislation would prevent the NRC from approving reactor ­renewals until a decade before a plant’s 40-year license expires. ­Seabrook went on line in 1990 and has a license to operate until 2030. NextEra, the plant’s owner, applied for a 20-year extension in 2010.

It is unclear, however, if Congress will be willing to intervene in a long-standing NRC rule that ­allows nuclear power plants to ­apply for relicensing after 20 years.

“Allowing the NRC to give a 60-year-long clean bill of health to ­reactors that are in their nuclear adolescence, especially one with documented safety issues such as Seabrook, is like allowing a doctor to assure a 20-year-old smoker they will never get lung cancer,’’ said Markey, a longtime critic of nuclear power. “It makes no sense.”

‘How are we going to know this is going to hold up until 2050?’

NextEra has already been told by the NRC that a license renewal would be delayed until at least 2014 so that the federal agency can better assess concerns about the plant’s structural integrity resulting from a chemical reaction known as alkali-­silica, or ASR. When ­water comes into contact with concrete, the reaction can progressively crack and weaken concrete. It is a well-known problem in sidewalks, walls, and airport runways.

Yet the issue had never been identified at a US nuclear power plant until NextEra discovered a 22 percent loss of strength in some concrete surrounding an electric tunnel ­after it was saturated with ground water for more than a decade.

NextEra said its ongoing inves­tigation shows the structural integrity of Seabrook is sound and the concrete degradation in the tunnel is not as ­serious as first reported. The energy company said it wants a relicensing decision soon so it can make long-term investment and business decisions.

“ASR is not a condition that impacts the safety of Seabrook Station in any way,’’ NextEra spokesman Alan Griffith said. “Seabrook has a comprehensive strategy to effectively manage ASR in the short and long term, and this effort is being supported by some of the foremost structural engineering ASR ­experts across the country.”

Neil Sheehan, a spokesman for the NRC, said in an e-mail that the agency believes plants should have the right to seek ­relicensing after 20 years.

“Our view is that after the plant has been in operation for at least 20 years, the facility has a well-developed operational history,’’ Sheehan wrote. He said merely because a plant gets relicensed does not mean NRC oversight wanes on aging ­issues.

Seabrook went off line last week for refueling and maintenance, a planned outage unrelated to the concrete problem. An NRC spokesman said the agency is sending a concrete and metal specialist during the plant hiatus to review NextEra’s assessment of the concrete degradation, including an evaluation of the steel containment liner, which helps prevent ­release of radioactivity in case of a severe accident.

“As always, if our inspectors identify any conditions that would call into question the safe operation of the plant, the NRC has the authority to ­require changes or even prevent the plant from restarting,” Sheehan said.

The NRC has not decided on a request from the C-10 ­Research & Education Foundation, a nuclear watchdog, and the Union of Concerned Scientists, a research and advocacy group, to more fully test for concrete problems in containment areas, which would be more accessible in the hiatus.

There is another complication for Seabrook’s bid to be re­licensed: A recent edict by the NRC has put final decisions on pending license renewals on hold until the agency conducts an environmental assessment of the storage of spent fuel at nuclear plant sites.

That is ­expected to take at least two years.

Watchdog groups said they want assurances the plant is safe.

“We are worried [the plant] will be fine until it fails,’’ said Debbie Grinnell of C-10. “How are we going to know this is ­going to hold up until 2050?”