WASHINGTON - Guards at a US government plant for storing weapons-grade uranium failed to spot activists, including an 82-year-old nun, who cut through its fences until they walked up to an officer's car and surrendered, an official report said on Friday.

The report from the Energy Department's inspector general, Gregory Friedman, criticized multiple failures of sophisticated security systems and "troubling displays of ineptitude" at the plant in Oak Ridge, Tennessee in July.

Three anti-nuclear activists, including an 82-year-old nun, were not initially spotted or detained as they cut through three perimeter fences on July 28.

They painted slogans and threw what they said was human blood on the outer wall of a building where highly enriched uranium, a key component of nuclear bombs, is stored.

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