Maybe they did steal it.
Reagan restarted neutron bomb production in '81 (first paragraph under 'History'"): http://en.wikipedia.org/wiki/Neutron_bomb
Hersh reports (without any stated source) that Israel has developed an extensive array of tactical nuclear weapons: efficient compact boosted fission bombs, neutron bombs (allegedly numbering in the hundreds by the mid-eighties), nuclear artillery shells, and nuclear mines. With an arsenal that is quite possibly in excess of 100 weapons it is likely that some of the nuclear materials would be applied tactical weapons. Boosted bombs are doubtful, as are neutron bombs, due to problems with development in the absence of a significant testing program. Neutron bombs also require very large amounts of tritium (20-30 g per weapon) which would impact the production of plutonium quite seriously (each gram of tritium displaces 80 grams of plutonium production). Artillery shells are also doubtful due to their wastefulness in plutonium. Tactical weapons are probably aircraft or missile delivered, or are pre-emplaced mines. Burrows and Windrem claim (without indicating a source) that Israel has produced 300 warheads, including those that have since been dismantled. They place the current arsenal at about 200 weapons.
http://nuclearweaponarchive.org/Israel/index.html
Interview with neutron bomb inventor Sam Cohen
Cohen (Hebrew: כֹּהֵן kōhēn', means: 'A Priest') is a Jewish surname of ...