Question: What is the name given to the final series of 7 US nuclear tests that took place in 1991 to 1992?

Context paragraphs:
1989–91 French nuclear tests: The France's 1989-1991 nuclear test series was a group of 21 nuclear tests conducted in 1989-1991.  These tests followed the "1986–88 French nuclear tests" series and preceded the "1995–96 French nuclear tests" series.

1990 Soviet nuclear tests: The Soviet Union's 1990 nuclear test series was a group of 2 nuclear tests conducted in 1990-1991.  These tests followed the "1989 Soviet nuclear tests" series .

Julin Bristol: Julin Bristol is the codename given to a nuclear weapon test conducted at the Nevada Test Site on 26 November 1991.  Conducted as part of Operation Julin, the final series of US nuclear tests, it was the last test explosion of a British nuclear weapon.  Both the US and UK, as signatories of the Comprehensive Nuclear-Test-Ban Treaty, ceased nuclear testing after Operation Julin.

Castle Union: Castle Union was the code name given to one of the tests in the Operation Castle series of United States nuclear tests.  It was the first test of the TX-14 thermonuclear weapon (initially the "emergency capability" EC-14), one of the first deployed U.S. thermonuclear bombs.

Operation Julin: The United States's Julin nuclear test series was a group of 7 nuclear tests conducted in 1991–1992.  These tests followed the "Operation Sculpin" series, and were the last before negotiations began for the Comprehensive Test Ban Treaty.

Castle Romeo: Castle Romeo was the code name given to one of the tests in the Operation Castle series of American nuclear tests.  It was the first test of the TX-17 thermonuclear weapon (initially the "emergency capability" EC-17), the first deployed thermonuclear bomb.

1989 Soviet nuclear tests: The Soviet Union's 1989 nuclear test series was a group of 7 nuclear tests conducted in 1989.  These tests followed the "1988 Soviet nuclear tests" series and preceded the "1990 Soviet nuclear tests" series.

Operation Hardtack I: Operation Hardtack I was a series of 35 nuclear tests conducted by the United States from April 28 to August 18 in 1958 at the "Pacific Proving Grounds".  At the time of testing, the Operation Hardtack I test series included more nuclear detonations than all prior nuclear explosions in the Pacific Ocean put together.  These tests followed the "Project 58/58A" series, which occurred from 1957 December 6 to 1958, March 14, and preceded the "Operation Argus" series, which took place in 1958 from August 27 to September 6.

List of nuclear weapons tests of the Soviet Union: The nuclear weapons tests of the Soviet Union were performed between 1949 and 1990 as part of the nuclear arms race.  The Soviet Union conducted 715 nuclear tests using 969 total devices by official count, including 219 atmospheric, underwater, and space tests and 124 peaceful use tests.  Most of the tests took place at the Southern Test Site in Semipalatinsk, Kazakhstan and the Northern Test Site at Novaya Zemlya.  Other tests took place at various locations within the Soviet Union, including now-independent Kazakhstan, Uzbekistan, Ukraine and Turkmenistan.

1955 Soviet nuclear tests: The Soviet Union's 1955 nuclear test series was a group of 7 nuclear tests conducted in 1955.  These tests followed the "1954 Soviet nuclear tests" series and preceded the "1956 Soviet nuclear tests" series.

Supporting paragraphs: Julin Bristol Operation Julin
Answer: Operation Julin