President Dalia Grybauskaitė signed a decree on Demchenko's appointment on Thursday, following a recommendation from Prime Minister Andrius Kubilius.
Demchenko, who is currently aborad, could not be reached by BNS for comment on Friday.
Demchenko replaced Gytis Maksimovas in 2009. The latter resigned as the VATESI head to take up a job with the International Atomic Energy Agency (IAEA), which involves taking part in inspections of nuclear energy facilities worldwide.
The Parliament dropped the idea of merging the Radiation Protection Center with VATESI last summer.
The head of VATESI draws a monthly salary of around 12,000 litas (EUR 3,500), which amounts to four average salaries in the energy sector.
