VIENNA (Reuters) - A senior Finnish nuclear official is to become new chief United Nations nuclear inspector in charge of monitoring Iran's disputed atomic activities and other sensitive issues, diplomats said on Friday.

Tero Varjoranta, now head of Finland's Radiation and Nuclear Safety Authority (STUK), is to succeed Herman Nackaerts as deputy director general for safeguards of the International Atomic Energy Agency (IAEA), they said.

One Western envoy said he was an "excellent" choice.

Varjoranta in 2010-11 worked at the Vienna-based IAEA as director of a unit dealing with nuclear fuel and waste technology after a three-decade career at STUK.

Nackaerts, a Belgian national, is due to retire later this year after three years in charge of the IAEA's department handling nuclear proliferation inspections globally, including high-profile cases such as Iran and Syria.

Under Nackaerts, the safeguards department prepared a landmark report to IAEA member states in late 2011 that gave a wealth of intelligence information pointing to research and other work in Iran relevant for nuclear weapons development.

Nackaerts and a team of senior officials have since then traveled to Tehran several times to press Iran to address questions raised by the IAEA report, so far without success.

Western diplomats blame Iranian stonewalling.

The IAEA, whose mission is to prevent the spread of atomic arms in the world, regularly inspects Iranian nuclear sites, which produce material that Western powers and Israel suspect could be used to build bombs. Iran denies any such aim.

But the U.N. agency says its inspectors need wider access to sites, officials and documents as part of a long-stalled investigation into suspected atom bomb research by Iran. Iran says its nuclear program is entirely peaceful.

Varjoranta's appointment is expected to be announced by the IAEA at a meeting next week of the U.N. agency's 35-nation governing board, the diplomats said. Another Finn, Olli Heinonen, held the post before Nackaerts.

It was not immediately clear when Varjoranta would start his new job. One diplomat said he believed it would be after the summer.

The March 4-8 board meeting is also expected to approve a second four-year term for IAEA Director General Yukiya Amano, who has no rivals for the post, unlike for his close election victory in 2009.

(Reporting by Fredrik Dahl; Editing by Janet Lawrence)