The United States is completing a deal to buy an advanced type of nuclear reactor built by the Soviet Union to power systems in space, federal officials say.
The transaction would be the first major sale between the former antagonists of a sensitive space technology with military potential.
Such a development, which would have been unthinkable a year or two ago, points up the extraordinary changes that have accompanied the end of the Cold War.
The purchase is to be announced today in New Mexico at a scientific meeting.
The reactor is an advanced version of devices that have powered Soviet spy satellites for decades.
The United States has no working nuclear reactors in orbit, although it has programs to develop them. The United States has launched spacecraft with nuclear reactors, but they have been sent beyond Earth orbit.
A federal official, speaking on the condition of anonymity, said the aim of the purchase was to learn about Soviet technology rather than to use the device in space.
He said that, on its own, the United States had studied this type of advanced reactor but never built one.
"This purchase is a way to leap-frog the process, to start from operational hardware and make a variation," he said in an interview.
Space reactors are compact energy sources that can generate more electricity than the large arrays of solar cells usually found on satellites.
In theory, they can energize electrical systems on any type of spacecraft, but they are seen as vital for missions requiring great amounts of electrical power, including certain kinds of spy satellites, space weapons and civilian spacecraft.
The existence of the advanced device sought by the U.S. government came to light in January 1989, when Soviet scientists said they had developed a new class of reactor that was very efficient, long-lived and powerful and disclosed that they had launched two experimental devices into space.
In a surprising move, they offered at the same time to sell the reactor to the West, although its mere existence had previously been a state secret.
Although there have been reports that U.S. scientists wanted to acquire the reactor from the Soviets, the actual negotiations had remained a secret until now.
The device is about 6 feet wide and 12 feet long, weighing a little more than a ton and generating between 6,000 and 10,000 watts of electricity.
The purchase price, including plans, manuals, a stand for ground testing and a Soviet team to start operations, is said to be around $10 million.
Although the Soviet Union has lagged in some aspects of high technology, its scientists are seen more and more by their U.S. counterparts as metallurgical wizards who have mastered high-strength, high-temperature alloys that are virtually unknown in the West.
These metals can be important in the design of advanced reactors, which generate great amounts of heat.