Israel has amassed approximately 115 nuclear warheads since it developed its first nuclear weapon shortly before the Six Day War in 1967, according to a report published this week by a US think tank.

The Institute for Science and International Security (ISIS) published a report on November 19 alleging that Israel possesses about 660 kilograms of plutonium, produced at the Dimona reactor since operations began in 1963, which would allow it to have an estimated 115 nuclear weapons today.

The author does acknowledge that the actual number is a closely guarded secret. Other studies have put the number of Israel’s nuclear weapons at between 80 and 200.

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A single nuclear weapon has between three to five kilograms of plutonium, according to the ISIS study

“Based on the total production of plutonium, the median for the number of nuclear weapons is about 165 with a standard deviation of 33 and a full range of about 90-290 weapons. Likely, Israel did not build this many nuclear weapons. A reasonable assumption is that the number of deployed weapons is 30 percent lower, or 115 nuclear weapons as of the end of 2014,” according to the report, written by former UN nuclear inspector David Albright.

The study also alleges that “Israel has a wide range of delivery vehicles for its nuclear weapons.”

Since the 1960s when Israel developed the nuclear-capable Jericho ballistic missile with France’s help, it has developed “several improved missiles since then on its own, as well as nuclear-capable cruise missiles,” according to Albright, who added that Israel “also has aircraft that can deliver nuclear weapons and may have the capability to launch nuclear-tipped cruise missiles from its submarines.”

Israel maintains a policy of nuclear ambiguity, neither openly admitting nor denying that it possesses a nuclear program. It is also not a signatory to the Nuclear Non-Proliferation Treaty.

Israel’s nuclear activities came into sharp focus in 1986 when Mordechai Vanunu — a technician from 1976-1985 at Israel’s nuclear facility at Dimona — revealed overwhelming evidence of Israel’s nuclear program to Britain’s The Sunday Times, including dozens of photographs, enabling nuclear experts to conclude that Israel had produced at least 100 nuclear warheads.

Charged with treason, he spent 18 years in jail and was released under very strict conditions in 2004.