A few years ago, Israeli and American intelligence developed a computer virus with a specific military objective: damaging Iranian nuclear facilities. Stuxnet was spread via USB sticks and settled silently on Windows PCs. From there it looked into networks for specific industrial centrifuges using Siemens SCADA control devices spinning at highspeed to seperate Uranium-235 (the bomb stuff) from Uranium-238 (the non-bomb stuff).
Iran, like many other countries, has a nuclear program for power generation and the production of isotopes for medical applications. Most countries buy the latter from specialists like the Netherlands that produces medical isotopes in a special reactor at ECN. The western boycott of Iran makes it impossible to purchase isotopes on the open market. Making them yourself is far from ideal, but the only option that remains as import blocked.
Why the boycott? Officially, according to the U.S. because Iran does not want to give sufficient openness about its weapons programs. In particular, military applications of nuclear program is an official source of concern. This concern is a fairly recent and for some reason has only been reactivated after the US attack on Iraq (a lot of the original nuclear equipment in Iran was supplied by American and German companies with funding from the World Bank before the 1979 revolution). The most curious of all allegations of Western governments about Iran is that they are never more than vague insinuations. When all 16 U.S. intelligence agencies in 2007 produced a joint study there was a clear conclusion: Iran is not developing a nuclear weapon (recent speech by the leader of this study here).