Dimona Nuclear Complex - The largest (not the only, thanks for the correction ariels - there is another, smaller, at Nahal Soreq) nuclear reactor in Israel. It is widely assumed that the reactor is used for the production of nuclear weapons.
The desire of Israel to create a nuclear weapons program grew largely from its desire to ensure that the Holocaust could never happen again, and so the Israeli army had been planning the program almost from the birth of the country in 1948. The Negev desert, in the southern portion of the country, was chosen as the site of a reactor to produce weapons grade plutonium in order to take advantage of small amounts of uranium recoverable from phosphate deposits in the region. By 1953, the Israelis had created a new method for making heavy water and learned how to extract uranium from the Negev. They coorperated with the French on the construction of the 24 MWt reactor (with cooling facilities large enough to accomadate one 3 times as powerful) at Dimona, beginning with an agreement in 1957. The reactor was constructed in secret, without IAEA inspections and under the leadership of the IDF.
The Israeli government insisted that the reactor be constructed in the utmost secrecy, which was difficult for such a large project. Some 1500 workers were employed on the project at one point, and an entire agency was created by the Israeli government to provide intelligence and security for the project. Customs officials were told that the reactor parts were slated to become part of a desalination plant in Latin America, and heavy water was secretly flown into the country from Norway by the French. The US noticed the construction site in aerial photos taken by the U2 spy plane in 1958, and Israel offered many explanations to mask the true nature of the site. They said at various times that it was used for textile manufacturing, metallurgical research, and agricultural research. In the early 1960's the French began to urge Israel to reveal the reactor to the IAEA and submit to inspections, but the Israeli government would not hear of it. They reached a compromise where France continued to supply parts, but the construction was completed without them, and the Israeli government promised not to produce nuclear weapons and to reveal the existence of the reactor, although not allowing inspections.
The reactor was revealed as a research facility for 'peaceful purposes' in 1960. This was, of course, not the truth, and it is estimated that Israel may have had nuclear weapons from the reactor as early as 1966, only two years after the reactor became operational in 1964. The United States was at least partially aware of the Israeli nuclear program, and while it did not encourage it, did nothing to stop it. Israeli officials intentionally deceived US weapons inspectors who visited the reactor at Dimona, creating fake control rooms and hiding doors and elevators. Adding to the problem, the American ambassador to Israel, Walworth Barbour did everything he could to downplay the Israeli nuclear program to the president, allowing it to grow and flourish. In 1968, the CIA issued a report concluding that Israel had nuclear weapons, and so the true purpose of the Dimona facility was recognized.
It is now estimated that the Dimona facility has produced enough material for 100-200 nuclear weapons, although this number, or even the existence of such weapons, is not aknowledged by the Israeli government. The number of weapons that have been produced depends heavily on the power level of the Dimona reactor, which is also unknown. It is estimated to be between 75 and 200 MWt. There is some evidence to suggest the power level has been increased, based on satellite images, but this is not certain. We also have the testimony of a dismissed Israeli nuclear technician, Mordecai Vanunu, who leaked information to the general public about the Israeli nuclear program in 1986, but this too is inconclusive. Therefore, the 100-200 range is our best estimate as to Israel's nuclear stockpile, with all of the nuclear material for these weapons being produced at the Dimona reactor complex.
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