In 1944 Richard Feynman recruited Reines to work in the Theoretical Division at the Manhattan Project's Los Alamos Laboratory, where he would remain for the next fifteen years.
He joined Feynman's T-4 (Diffusion Problems) Group, which was part of Hans Bethe's T (Theoretical) Division.
Diffusion was an important aspect of critical mass calculations.
In June 1946,  he became a group leader, heading the T-1 (Theory of Dragon) Group.
An outgrowth of the "tickling the Dragon's tail" experiment, the Dragon was a machine that could attain a critical state for short bursts of time, which could be used as a research tool or power source.

Reines participated in a number of nuclear tests, and writing reports on their results.
These included Operation Crossroads at Bikini Atoll in 1946, Operation Sandstone at Eniwetok Atoll in 1948, and Operation Ranger and Operation Buster–Jangle  at the Nevada Test Site.
In 1951 he was the director of Operation Greenhouse series of nuclear tests in the Pacific.
This saw the first American tests of boosted fission weapons, an important step towards thermonuclear weapons.
He studied the effects of nuclear blasts, and co-authored a paper with John von Neumann on Mach stem formation, an important aspect of an air blast wave.

In spite or perhaps because of his role in these nuclear tests, Reines was concerned about the dangers of radioactive pollution from atmospheric nuclear tests, and became an advocate of underground nuclear testing.
In the wake of the Sputnik crisis, he participated in John Archibald Wheeler's Project 137, which evolved into JASON.
He was also a delegate at the Atoms for Peace Conference in Geneva in 1958.
In 1944 Richard Feynman recruited Reines to work in the Theoretical Division at the Manhattan Project's Los Alamos Laboratory, where he would remain for the next fifteen years. He joined Feynman's T-4 (Diffusion Problems) Group, which was part of Hans Bethe's T (Theoretical) Division. Diffusion was an important aspect of critical mass calculations. In June 1946,  he became a group leader, heading the T-1 (Theory of Dragon) Group. An outgrowth of the "tickling the Dragon's tail" experiment, the Dragon was a machine that could attain a critical state for short bursts of time, which could be used as a research tool or power source.
Reines participated in a number of nuclear tests, and writing reports on their results. These included Operation Crossroads at Bikini Atoll in 1946, Operation Sandstone at Eniwetok Atoll in 1948, and Operation Ranger and Operation Buster–Jangle  at the Nevada Test Site. In 1951 he was the director of Operation Greenhouse series of nuclear tests in the Pacific. This saw the first American tests of boosted fission weapons, an important step towards thermonuclear weapons. He studied the effects of nuclear blasts, and co-authored a paper with John von Neumann on Mach stem formation, an important aspect of an air blast wave.
In spite or perhaps because of his role in these nuclear tests, Reines was concerned about the dangers of radioactive pollution from atmospheric nuclear tests, and became an advocate of underground nuclear testing. In the wake of the Sputnik crisis, he participated in John Archibald Wheeler's Project 137, which evolved into JASON. He was also a delegate at the Atoms for Peace Conference in Geneva in 1958.
Frederick Reines