US dusts off plans to use nuclear reactors in its space vehicles

Nuclear power, its growth stymied in many places on Earth, is poised to undergo a rebirth in outer space. After a 15-year hiatus, the United States has embarked on a major effort to design and develop nuclear reactors to power civilian and military space platforms.

Last year, without any fanfare, the National Aeronautics and Space Administration (NASA), the Department of Energy, and the Defense Advanced Research Project Agency signed an agreement for joint development of nuclear reactor power system technology for use in space.

In fiscal year 1983, $11 million was earmarked for the effort. This fiscal year $30 million is being spent. In 1985 a total of $70 million is budgeted. According to current estimates, an additional $350 million will probably be required to complete a small, 100 kilowatt space reactor by 1991.

Details of the civilian side of the program were presented at the Intersociety Energy Conversion Engineering Conference held here last week.

''This is the first time that the details have been presented publicly,'' says Judith Ambrus, NASA manager for the project.

Military aspects of the program are classified and were discussed at a private meeting.

Impetus for the development of space reactors comes primarily from the manned space station and the ''star wars'' intercontinental ballistic missile (ICBM) defense programs the Reagan administration favors.

According to NASA design studies, the first US space station will require about 75 kw. of power. This is low enough to be met with solar cells and batteries. But as the space station grows, it will require more energy. If the manufacture of material for computer chips and other industrial processes prove attractive, space-station power demands could easily top 1,000 kw., NASA planners say.

Once it's developed, NASA can see a number of other uses for such a space reactor. Increasingly, communications satellites are filling up the space available at geosynchronous orbit. A large number of these satellites could be replaced by nuclear-powered communications platforms. Another possibility would be a space radar system that could be used to track aircraft and ships and provide collision-avoidance warnings.

While a small space station can and probably will be built using solar power, development of space reactors are essential for a space-based missile defense system. Lasers or particle beam generators in orbit large enough to shoot down ICBMs or their warheads require tremendous amounts of power.

The exact figures are secret, but experts say the Pentagon is interested in multimegawatt-size machines. (Commercial nuclear reactors on the ground are typically 1,000 megawatts.)

In space, solar cells work superbly as a source for small amounts of electricity. But as energy requirements rise they become less attractive. To double the power from a solar cell array requires twice the area and twice the mass.

''You can use solar up to 300 kilowatts maximum. After that there is only nuclear,'' Dr. Ambrus says.

Nuclear reactors have the potential for being compact sources of large amounts of power. According to David Buden, a researcher at Los Alamos National Laboratory in New Mexico, a 100 kw. reactor can weigh around 6,600 pounds, while the best solar power system with the same output weighs in at 2.5 times more. As the power requirements go up, nuclear has an increasing advantage, because to double its output, a reactor's mass must increase only by one-third.

Of course, a host of offsetting problems must be solved. Because of the severe size and weight limitations involved in space operations, the concepts being developed are substantially different from conventional reactors.

In space, they will also be expected to work untended for 10 years with unprecedented reliability. In addition, the possibility of accidents during launch or of reentry into Earth's atmosphere means safety requirements are even more stringent than those for terrestrial reactors.

According to the researchers involved, program planners are keenly aware of the importance of safety. ''That's why we're spending so much time and money in this early design phase,'' says Ambrus.

The toughest technical safety problem, says James Stevens of the General Electric Company, is designing the reactor so it will remain inactive if it is dunked into the ocean, while the toughest policy question is how to handle the problem of reentry.

Currently, the program guidelines call for a reactor to break up into small pieces that will burn up in reentry. But this policy is the center of a heated internal debate. There is concern that this will create public anxiety. The alternative is to design the reactor so its core will come down intact.

Guaranteeing this is easier than dispersal, the engineers involved say. But these reactors contain weapons-grade nuclear material, so intact reentry raises severe security problems.