The Clamshell Alliance's campaign against the Seabrook nuclear plant on the New Hampshire coast created the modern mass-action model: affinity groups, consensus decision-making, and nonviolence trainings borrowed from the civil-rights movement. On April 30–May 1, 1977, over two thousand occupied the site; 1,414 were arrested — and when they refused bail solidarity, Governor Meldrim Thomson held them two weeks in National Guard armories, an improvised mass imprisonment that became a school of the movement.

The armory fortnight — meetings, trainings, and songs under guard — welded a generation of organizers and spread the Clamshell model to the Abalone, Shad, and Catfish alliances nationwide. Seabrook's second reactor was never built; the first opened over a decade late, into an industry the occupations had helped halt at 100-odd cancelled plants.