What are nations like North Korea and Iran really doing at nuclear reactors that are out of sight?

Someday, wispy subatomic particles known as antineutrinos could provide a clear view of what countries with illicit nuclear weapons programs are trying to hide.

Antineutrinos are devilishly difficult to detect, but this quality is precisely what makes them potentially ideal for monitoring international nonproliferation agreements aimed at preventing the spread of atomic weapons.

A collaboration of American and British scientists announced on Tuesday that they would build a test antineutrino detector called Watchman in a mine on the northeast coast of England. The project is sponsored by the National Nuclear Security Administration, part of the United States Department of Energy.

When completed in 2023, the apparatus is to consist of a cylinder about 50 feet in diameter and 50 feet in height, filled with 7.7 million pounds of water and located about 3,600 feet underground in the Boulby Mine, which produces salt and potash, a fertilizer. Sensors lining the inside of the cylinder will observe the occasional flashes generated when an antineutrino resulting from reactions in the Hartlepool nuclear power plant, about 15 miles away, slams into a particle in the detector liquid. The experiment would run for two years.