THE EMBATTLED CHECHENS have threatened to use terrorism to carry their war to Russia. Taking the threat seriously, Russian officials have ordered police sweeps through train and subway stations.

Although the authorities are reluctant to express their fears publicly, they worry about the vulnerability of their nuclear power plants and stocks of nuclear material to terrorist attack and theft.Washington worries, too. Last month, 1,300 pounds of highly enriched uranium was secretly airlifted from the republic of Kazakhstan to the United States.

One reason was the fear that the weapons-grade material - enough for more than two dozen atomic bombs - was vulnerable to theft. The uranium had been stored under inadequate safeguards at a metallurgy plant.

A nuclear bomb can be built with as little as 33 pounds of weapons-grade uranium or 11 pounds of plutonium. The Soviet Union produced hundreds of tons of these materials, which are now stored or used at dozens of different sites.

Importantly, weapons-grade material intended for civilian uses like nuclear energy and research, in Russia and in other post-Soviet states, may pose a more serious proliferation threat than material intended for military use.

Last summer, Russian officials sought to minimize the importance of six-tenths of a pound of smuggled plutonium found in Munich in a Lufthansa plane bound from Moscow. They said it was not from nuclear weapons. Some Western experts surmise that the material originated in a civilian plant.

Russian officials have sought to minimize the dangers posed by such material. They stress that it could not be used for nuclear weapons without further processing.

Actually, a national nuclear weapons program - Iran's or Pakistan's, for example - could complete such processing in months and have in hand bomb material that might otherwise require 10 years to manufacture.

By refusing to halt the extraction of plutonium in Russia, the Atomic Energy Ministry is adding substantially to the nation's stocks and increasing the danger of diversion every day.

The Atomic Energy Ministry has dragged its feet in working to develop plans for a high-security storage site for plutonium removed from dismantled nuclear weapons.

It has also shown reluctance to adopt modern nuclear-material accounting practices, even though uncertainties about the inventory in the ministry's vast nuclear enterprise must be substantial.

Considerable headway has been made in removing nuclear weapons from the former non-Russian republics. But there has been far less progress in protecting the nuclear material that remains, especially in Russia.

Unless remedial steps are taken soon, nuclear terrorism may become a reality.