Relations between the United States and Russia may be their worst since the end of the Cold War, but that hasn’t stopped the sides from forging ahead with efforts to prevent nuclear weapons-grade materials from falling into the hands of terrorists.

In the latest measure of progress, U.S. and Russian experts have completed a project to strengthen security at one of Russia’s largest facilities for storing nuclear weapons-grade materials, U.S. officials said Tuesday.

The facility is located at the Mayak Nuclear Association, a sprawling decades-old nuclear weapons plant that suffered accidents in the 1950s and 1960s that exposed workers and residents in the surrounding area of Chelyabinsk to radiation.

The security upgrades at the storage facility, which contains the largest amount of nuclear weapons-grade material of any such site in Russia, were funded by the United States at a cost of $15 million under an agreement that President Bush signed in February 2005 with Russian Prime Minister Vladimir Putin while Putin was president.

Russia refuses to disclose how much material is being kept at Mayak. Rosatom, Russia’s state-run nuclear energy agency, and the U.S. National Nuclear Security Administration, the Department of Energy agency that oversees the U.S. nuclear arsenal, completed the work at Mayak three months ahead of deadline.

The Bush-Putin accord, known as the Bratislava Initiative, bolstered U.S.-Russian cooperation in upgrading the security of nuclear weapons-grade materials, improving emergency responses to nuclear mishaps and converting reactors around the world from using highly enriched uranium, the fuel used in nuclear bombs, to low-enriched uranium.

The work has continued unhindered despite grave tensions between Moscow and Washington over the installation of U.S. missile defense sites in Eastern Europe and this summer’s Russian invasion of the former Soviet republic of Georgia.

“It’s absolutely clear that the United States and Russia are committed to taking steps to preventing nuclear proliferation and nuclear terrorism,” NNSA Deputy Administrator Will Tobey told Nukes and Spooks in a telephone interview.

Tobey said the United States has spent close to $2 billion since the mid-1990s on helping Russia secure its nuclear weapons-grade materials, destroy excess Cold War era nuclear warheads, missiles and chemical and biological weapons and nuclear bombers, and employ Russian weapons scientists.

The United States also has a “second line of defense” program of installing radiation detectors at Russian ports, airports and border crossings to detect attempts to smuggle nuclear weapons-grade materials out of the country, he said.

U.S. and Russian experts have converted 62 research reactors in 31 countries from using highly enriched uranium to low-enriched uranium, and returned almost 2 metric tons of fresh and spent fuel to Russia, Tobey recounted.