The previous Necsa board included prominent backers of plans to expand South Africa's nuclear power capacity which were shelved by President Cyril Ramaphosa after he succeeded scandal-plagued Jacob Zuma as head of state in February.
Necsa exports medical radioisotopes and promotes research and development in nuclear energy.
"The Necsa board failed to execute its statutory mandate in a satisfactory and prudent manner," Radebe told a news conference in Pretoria.
He added that several board members had shown "ineptitude and deliberate acts of defiance" which had resulted in setbacks including a halt in production of radioisotopes at Necsa's NTP subsidiary.
The new board will be headed by Rob Adam, a former Necsa executive who has also served as director of the Square Kilometre Array telescope project. (Reporting by Alexander Winning Editing by James Macharia)
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