The answer is 2053, or possibly 2055. The league table of countries responsible is as follows:
North Korea 2 (unconfirmed)
A much more powerful way to understand these data has been created by Isao Hashimoto, which consists of a time-lapse map of the world recording the time and location of each individual explosion and tallying the totals for each country cumulatively as the months pass, accompanied by oddly sad and emotive bleeps for each one, in different tones for each country. Did you know that the last British nuclear test took place as recently as 1991? Or that the only continents not to have experienced nuclear explosions are Greenland, Antarctica, and South America?
I came across this in an article by Dan Rowinski on the ReadWriteWeb site, 10th February 2011, at http://www.readwriteweb.com/archives/video_artists_time-lapse_map_of_the_worlds_2053_nu.php?utm_source=ReadWriteWeb+Newsletters&utm_campaign=1714b79d29-RWWDailyNewsletter&utm_medium=email
Hashimoto is a curator at the Lalique Museum in Hakone, Japan. The video was created in 2003 as a series expressing Hashimoto's view of, "the fear and the folly of nuclear weapons." The video represents nuclear tests with a colored dot and a beep on a map. It starts slow in 1945, showing a world view of a couple flashes in the southwestern United States before zooming in on the two bombs dropped in Japan. The video then pans out and continues for the duration from a birds-eye view of the world. The climax comes between 1955 and 1970 as the Soviet Union joined the U.S. as a nuclear power and England, France, India and Pakistan eventually joined the arms race. The U.S. had the most nuclear tests, by a large margin, with most occurring in the southwest. The Soviet Union performed most of its tests in and around what is now Kazakhstan and the Lake Balkhash region with many also coming in northern Siberia and Nordic border with Finland. When the British entered the nuclear race, their first tests were in the desolate regions of west Australia. The French were several years behind but made up for coming late by being very active with nuclear tests in the South Pacific, the most vast and uninhabited region on Earth. India and Pakistan tested nuclear bombs mostly in the northern section of the Indian subcontinent. China tested many of its nuclear weapons at Lop Nur in the northwestern part of the country. Hashimoto's data is based on research from the Swedish Defense Research Establishment and Stockholm International Peace Institute. It does not include two supposed nuclear tests by North Korea in 1998 that may or may not have actually happened. Pakistan was the last to test nuclear bombs in 1998.
"This piece of work is a bird's eye view of the history by scaling down a month length of time into one second. No letter is used for equal messaging to all viewers without language barrier. The blinking light, sound and the numbers on the world map show when, where and how many experiments each country have conducted. I created this work for the means of an interface to the people who are yet to know of the extremely grave, but present problem of the world" Isao Hashimoto, quoted by Rowinski.
The video is hypnotic, chilling, and thought-provoking. You can see it here: http://www.youtube.com/watch?feature=player_embedded&v=LLCF7vPanrY