Preventing a Nightmare Scenario: Terrorist Attacks Using Russian Nuclear Weapons and Materials / [USA] The Heritage Foundation, 20 May 2005

http://www.heritage.org/Research/NationalSecurity/bg1854.cfm
Arial Cohen
"As sources of unsecured nuclear weapons and material, Russia and the other former Soviet repub­lics remain major proliferation concerns for a num­ber of reasons. First, the Soviet Union was an empire with a strong external perimeter and weak internal safeguards."