SEABROOK — NextEra Energy this week scored two significant wins in its eight-year battle to gain final approval of a 20-year license extension for its Seabrook nuclear power plant from the Nuclear Regulatory Commission.

The Advisory Committee on Reactor Safeguards on Wednesday sent a letter to NRC Chairwoman Kristine Svinicki. The ACRS, an independent body of experts advising the commissioners on nuclear safety matters, recommended: “NextEra’s application for renewal of the operating license for Seabrook (Station) should be approved.”

According to the letter, its recommendation came after ACRS members reviewed NextEra’s license amendment application, as well as the NRC staff’s final safety evaluations and documentation related to Seabrook Station and its problems with a concrete-degradation phenomenon known as alkali-silica reaction.

According to its letter, the ACRS concluded the methods NextEra established and has committed to for monitoring and managing ASR in the plant’s structures, “provide reasonable assurance that Seabrook can be operated in accordance with its licensing basis for the extended operation without undue risk of the health and safety of the public.”

The Dec. 19 letter comes after the ACRS issued its initial letter Dec. 14 to the NRC. That letter offered the committee’s opinion on NextEra’s license amendment request, which outlines how it will address ASR at Seabrook Station.

In the Dec. 14 letter, ACRS concluded that while the plant’s structures are degraded due to ASR, “they are fully capable of performing their credited function through the requested (period of extended operation) under the committed enhanced monitoring and evaluations.”

ASR occurs in concrete when in the presence of moisture. It is not uncommon in dams and bridges.

Discovered by Seabrook Station employees in 2009 and immediately reported to the NRC, the ASR at Seabrook Station is attributed to the result of moisture combined with the chemically reactive aggregate used when constructing the plant, which came from a Maine quarry. A slow developing phenomenon, ASR can take up to 15 years to show its characteristics, which are micro-cracking and staining of the concrete, as exhibited on walls at Seabrook Station.

Other nuclear power plants in the world deal with ASR, but so far Seabrook Station is the only plant in the United States to show signs of the phenomenon.

Once reported, the ASR problem at Seabrook Station became an obstacle in NextEra’s 2010 application to extend its operating license from 2030 to 2050. The issue raised a furor among anti-nuclear organizations, some demanding immediate closure of the plant.

NRC staff did extensive and ongoing reviews and inspections at the site over the past eight years. According to Sheehan, NRC staff repeatedly concluded there are no immediate safety concerns regarding ASR at the Seabrook Station, due to, among other things, existing safety margins such as 2-foot thick, steel-reinforced concrete walls, and the slow progression of ASR and ongoing monitoring.

However, the NRC let NextEra know the final disposition of its license extension request would only be decided after the NRC was sure NextEra could successfully manage the problem.

According to Sheehan, the ASR assessment and monitoring programs put forward by NextEra in the plant’s license amendment request come in part after a multi-year large-scale test program conducted for NextEra at the Ferguson Structural Engineering Laboratory at the University of Texas, as well as two new aging management programs that monitor for ASR and building deformation.

The ACRS’s two letters present major positive findings and will be considered by NRC commissioners when they make their final decisions on whether to grant NextEra’s license amendment request and license extension, Sheehan said.

Other issues still remain, however. One is an upcoming hearing on a contention to NextEra’s license amendment request from Newburyport, Mass.-based nuclear-watchdog C-10.

The organization contends the testing program undertaken by NextEra at the Ferguson Laboratory “yielded data that are not ‘representative’ of the progressing of ASR at Seabrook (Station).” As a result, C-10 believes NextEra’s license amendment monitoring and inspection schedule isn’t adequate.

Sheehan expects the hearing will come next summer before the Atomic Safety Licensing Board, the quasi-judicial arm of the NRC.