1951 

The second British plutonium reactor starts operation in Windscale, Cumberland, to manufacture plutonium for nuclear weapons. In 1957 it caught fire and caused radioactive contamination of a wide area. To help the public forget, the town was renamed Sellafield.

January
U.S. mathematician Stanislaw Ulam proposes radically new design for hydrogen bomb. Edward Teller embraces and refines the concept.

January 11
President Harry Truman approves the establishment of the Nevada Proving Grounds, later called the Nevada Test Site (NTS).

January 27
The first atmospheric test at the Nevada Test Site, Able , takes place 1060 feet above Frenchman Flat.

March 29
Julius and Ethel Rosenberg are convicted and sentenced to death at the Federal Courthouse in Foley Square, New York City, for passing information on atomic weapons to the USSR.

April 4
Edward Teller submits report on new design for hydrogen bomb.

April 5
U.S. Joint Chiefs of Staff order atomic retaliation against air bases in case of "a major attack" against UN forces in Korea. 

April 6
President Harry Truman approves military request to use atomic weapons in Manchuria if large numbers of Chinese troops join the Korean War or if bombers are launched against United Nations forces from Manchurian bases. 

April 11
President Harry Truman discharges General General Douglas MacArthur for insubordination after MacArthur repeatedly criticizes the limited objectives of the war in Korea.

May 9
U.S. nuclear test shot "George" ( Operation Greenhouse ) confirmes for the first time that a fission device can produce the conditions needed to ignite a thermonuclear reaction. 

September 17
U.S. physicist Marshall Holloway is named leader of hydrogen bomb project. Edward Teller leaves Los Alamos, New Mexico shortly afterwards. 

September 24
Soviet Union conducts its second nuclear test, an improved plutonium bomb. 

December
A four-man team at RAND begins to study the likely effects of the hydrogen bomb.