The International Atomic Energy Agency (IAEA) obtained satellite pictures of the container at Parchin, near Tehran, and other evidence that lent credence to allegations by IAEA member states that the installation was intended for nuclear-related explosives testing, the sources said.
The IAEA will also reveal evidence that Iran carried out computer modelling of a nuclear weapon, one source said.
Western diplomats say the keenly awaited report will strengthen suspicions that Tehran is seeking to develop a capability to make atomic bombs but stop short of explicitly saying it is doing so.
It was unclear when the container was built or whether it was actually used for nuclear-related work.
Iranian officials were not immediately available for comment.
Suspicions of nuclear arms work at the Parchin military complex southeast of Tehran date back at least to 2004 when a prominent nuclear expert said satellite images showed it may be a site for research, testing and production of nuclear weapons.
Iran, which says its nuclear programme is peaceful, has previously denied the allegations. In 2005, Iran allowed U.N. nuclear inspectors to visit Parchin.
The IAEA in May listed seven areas of concern regarding possible military dimensions to Iran’s nuclear programme, including high explosives manufacture and testing and the development, manufacture and testing of explosive components.
One of the main hurdles in making an atomic bomb is designing a ring of conventional explosives used to compress atomic material in the warhead core, igniting a nuclear chain reaction. Experiments have to be carried out to test the impact of explosions on bomb components.
The IAEA report is expected to include other evidence of research and other activities that make little sense if not weapons-related, Western diplomats said.
Western powers believe Iran is secretly trying to develop nuclear weapons, but Tehran denies this, saying it is enriching uranium only to power reactors for electricity generation.
Any evidence of nuclear weapons activities would strengthen calls for further sanctions against Iran.
(Editing by Louise Ireland)
