Washington DC - More than two decades after the collapse of the Soviet Union, the planet's largest stockpile of nuclear weapons materials remains insecure, according to a series of US intelligence reports obtained by Al Jazeera's Investigative Unit.
In the wake of the Soviet collapse into 15 independent states, hundreds and perhaps thousands of grammes of nuclear material - including highly enriched uranium used in atomic bombs - were spirited away from Russia's nuclear heartland.
"We assess that undetected smuggling of weapons-usable nuclear material has occurred, but we do not know the total amount of material that has been diverted or stolen since the dissolution of the Soviet Union,'' the US Director of National Intelligence (DNI) said in a 2011 report, the latest unclassified document released by the intelligence community.
"We judge it highly unlikely that Russian authorities have been able to recover all of the stolen material."
The conventional wisdom is that a dirty bomb is far more likely than a conventional nuclear bomb.
The release of the report comes as questions arise about embarrassing lapses in the security of the United States' own nuclear stockpile. US Defense Secretary Chuck Hagel ordered a review of the nation's nuclear forces last month after a series of problems, including rampant cheating by officers on proficiency tests at a nuclear missile launch site in Montana, and an investigation of 10 Air Force officers accused of possessing recreational drugs.
Separately, in 2012, three peace activists breached security at the Y-12 National Security Complex in Oak Ridge, Tennessee, where enriched uranium for nuclear bombs is stored. Nuclear operations were temporarily shut down.
The DNI report on Russia was recently obtained by Al Jazeera's Investigative Unit under the federal Freedom of Information Act. At its peak, the Soviet Union controlled 45,000 nuclear weapons, according to a study published in the Bulletin of Atomic Scientists, an online magazine that focuses on global security issues. The 1991 collapse of the nation and its central economy resulted in lax security, desertions and thefts from secret "atomic cities" located in southern Siberia, where nuclear weapons were manufactured and stored.
Experts agree an attack using a fully functioning nuclear device built with at least six kilogrammes of plutonium - the same amount used in the US bomb dropped on Nagasaki during World War II - is most likely beyond the capability of separatist organisations operating in southern Russia.
A "dirty bomb" that features conventional explosives laced with radioactive materials such as cobalt or strontium is considered a more feasible threat.
"The conventional wisdom is that a dirty bomb is far more likely than a conventional nuclear bomb,'' says Page Stoutland, vice president for nuclear materials security at the Nuclear Threat Initiative, a Washington-based nonprofit created to slow the spread of nuclear, chemical and biological weapons.
Almost four dozen United Nations members, including the United States and Russia, agreed in April 2010 to secure and account for all nuclear materials within four years. While the Russian government has rejected fears that its nuclear inventory is not secure, the DNI report questions whether the billions in aid, including money for radiation sensors at key border crossings, will prevent nuclear materials theft and smuggling.
"Russia's vast stockpile of nuclear material, scattered across multiple facilities, continues to present an attractive theft target," US officials wrote in their report. "Security of this material has improved since the fall of the Soviet Union, but we lack information on the extent of recent thefts, and vulnerabilities remain. Probable Russian-origin weapons-usable nuclear material has continued to circulate on the black market."
Russian officials have periodically confirmed and denied reports of attempted thefts of nuclear materials from the nation's arsenals. Valentin Ivanov, the former Russian deputy atomic minister, said at a 2000 press conference there had been 23 attempts to steal fissile material from nuclear sites. All but two occurred between 1991 and 1995, according to Russia's Interfax news agency.
A 2009 report for the Fletcher Forum of World Affairs, an academic journal of international relations, estimated the US Departments of Defense, State and Energy spend about $1.4 billion annually to help Russia dismantle and secure its nuclear materials. Even so, Matthew Bunn, a former White House science adviser who co-authored the report, said the money hasn't ensured complete safety. The International Atomic Energy Agency has documented 18 cases of theft or loss of plutonium or highly enriched uranium, he said.
"A key question is: How many other cases may have occurred without being detected?" he wrote. "It is sobering to note that nearly all of the stolen [highly enriched uranium] and plutonium that has been seized over the years had never been missed when it was originally stolen."
In January, a Nuclear Threat Initiative report found Russia's control of materials was in the bottom third of nuclear states, and its overall score remained unchanged from 2012. The report said Russia has the second-highest risk factors of any nuclear state, ahead of only Pakistan. Those risk factors include political instability, ineffective governance, pervasive corruption, and the presence of groups determined to obtain nuclear materials.
So far, what we're seeing is that the Russians have done a fairly decent job of preventing leakage.
The US, with the world's second-largest stockpile of nuclear weapons, slipped slightly from 2012, falling to the 11th-safest nuclear state. Its political risk factors ranked 10th in the world, tied with Poland and trailing countries that include Japan, Germany and France.
Despite repeated requests from Al Jazeera's Investigative Unit, the Russian Foreign Ministry and the Russian Embassy in Washington did not respond to questions about the DNI report.
The study was released as authorities in Moscow are hosting the $51 billion Winter Olympic Games. The authorities contend that Sochi will be a safe destination, despite its proximity to separatist movements and a history of politically motivated attacks on the Olympics.
Steven Pifer, a former US ambassador to Ukraine who leads the Brookings Arms Control and Non-Proliferation Initiative in Washington, told Al Jazeera's Investigative Unit the Russians have made strides in securing their stockpile since the early 1990s, when "there were real concerns that weapons would slip into the wrong hands".
"Part of the current problem is that the Russians aren't as transparent as we'd like them to be," Pifer says. "But they seem to be making the right efforts. And so far, what we're seeing is that the Russians have done a fairly decent job of preventing leakage."
View the reports below: