As the Trump administration races to start talks with North Korea on what it calls “rapid denuclearization,” a top federal government adviser who has repeatedly visited the North’s sprawling atomic complex is warning that the disarmament process could take far longer, up to 15 years.

The adviser, Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico, and now a Stanford professor, argues that the best the United States can hope for is a phased denuclearization that goes after the most dangerous parts of the North’s program first.

The disarmament steps and timetable are laid out in a new report, circulated recently in Washington, that Dr. Hecker compiled with two colleagues at Stanford’s Center for International Security and Cooperation. Dr. Hecker has toured that nation’s secretive labyrinth of nuclear plants four times and remains the only American scientist to see its facility for enriching uranium, a bomb fuel. American intelligence agencies had missed the plant’s construction.