Played a major role in both Tube Alloys, Britain's nuclear weapon program, as well as the subsequent Manhattan Project, which produced nuclear weapons for use in WW2.

Peirels came under scrutiny after Klaus Fuchs, a compatriot whom he had recruited to the Manhattan Project, was exposed as a Soviet spy in 1950. The scrutiny remained for many years, restricting Peierls' ability to operate as a scientist in America.