Working in top secret over a period of 17 years, Russian and American scientists collaborated to remove hundreds of pounds of plutonium and highly enriched uranium — enough to construct at least a dozen nuclear weapons — from a remote Soviet-era nuclear test site in Kazakhstan that had been overrun by impoverished metal scavengers, according to a report released last week by the Belfer Center for Science and International Affairs at Harvard.

The report sheds light on a mysterious $150 million cleanup operation paid for in large part by the United States, whose nuclear scientists feared that terrorists would discover the fissile material and use it to build a dirty bomb.

Over the years, hints emerged that something extraordinarily dangerous had been left behind in a warren of underground tunnels — like the American aerial drones that circled over the site, looking for intruders, or the steel-reinforced concrete that was poured into tunnels and over stretches of earth.

Among the report’s new revelations is that the Soviet testers left behind components, including high-purity plutonium, that could have been used to build not just a dirty bomb but a “relatively sophisticated nuclear device,” an American official told the report’s authors.