US Nuclear Regulatory Commission inspectors arrived in Plymouth Monday to begin a three-week review of the Pilgrim Nuclear Power Station, which is under heightened scrutiny after a series of unplanned shutdowns.

About 20 people will take part in the process, according to the NRC. Early next year, the federal agency will issue a report on what it expects the plant to do to continue producing power safely before its planned shutdown in 2019.

Pilgrim ranks among the three lowest-rated plants in the country after the NRC downgraded its safety rating last year. That decision sparked the review that begins this week, along with two other special inspections earlier this year.

On Monday, the inspection team arrived at Pilgrim, met with staff from plant owner Entergy Corp., and sat down to review documents, according to Donald E. Jackson, who is leading the team of NRC inspectors.

Some involved in the effort also began looking at components of the plant, receiving authorization to visit sensitive areas, Jackson said in a conference call with reporters.

The inspection began to unfold as a group of protesters gathered near the plant, calling for its immediate shutdown.

Media reports from the scene showed a handful of demonstrators, one holding a sign that said “NRC do your job,’’ and called for Pilgrim’s immediate closure. Critics have said the plant has neglected maintenance to save money.

Monday, Jackson sought to reassure residents that the team is dedicated to ensuring safety.

“I think public involvement is fantastic. When I’m not wearing my NRC hat, I’m a member of the public,’’ he said. “Frankly, we’re doing the public’s work here.’’

Regulators and company officials have emphasized that none of the issues previously identified at Pilgrim placed the public at serious risk.

A further downgrade in the plant’s safety rating, however, could lead the NRC to order the plant to shut down sooner than expected.

Entergy said it is looking forward to the inspection as “the next step in Pilgrim’s process toward a return to industry excellence.’’

Inspectors will be at Pilgrim for two weeks, leave for the holiday season, then return in January. A report will follow within 45 days, and a public meeting will be held to discuss the findings.

Andy Rosen can be reached at andrew.rosen@globe.com. Follow him on Twitter at @andyrosen.