Improving the Breeder

In the world of nuclear energy, the fast-breeder reactor is considered the power source of the future. Not only do breeders have longer lasting fuel elements than standard, water-moderated reactors, but they also produce more nuclear fuel than they consume. When enough breeders have been built, utility executives believe, the cost of nuclear fuel can be drastically reduced. Thus there was great dismay in October 1966, when Michigan's $85 million Enrico Fermi atomic power plant, the first commercial power-producing fast-breeder reactor in the U.S., had to be shut down because of an accident. Only recently have scientists learned the cause: a small piece of metal was left behind in the cooling system during construction of the plant.

Moving freely in the cooling-system piping, the 4-in. by 8-in. sheet of metal eventually drifted into a position blocking the flow of liquid sodium coolant to two of the 105 uranium elements in the reactor core. The elements, in which uranium atoms are constantly splitting and producing heat, quickly rose to an extreme temperature and melted, releasing radioactive fission products into the coolant. About 15 minutes later, the radioactive materials leaked out of the cooling system and were spotted by sensitive detectors. The detectors triggered an automatic shutdown of the reactor before more damage could be done.

Computerized Warning. In the months since, scientists have gingerly dismantled the highly radioactive reactor enough to probe its inner workings with custom-built periscopes. It was through these periscopes that the two melted elements were first discovered and that the offending piece of sheet metal was eventually seen and photographed. Using remote-control tools on 40-ft. arms, reactor technicians plan to remove the sheet this month either by cutting it into pieces small enough to be extracted through a 1 1/2-in. hole in the reactor wall or by crushing it into tiny fragments that will not interfere with the flow of the cooling liquid sodium. If they succeed, the Fermi plant can be producing electric power again within six months.

Despite the accident, design problems and other difficulties that have put operation of the Fermi plant three years behind schedule, most scientists and utility executives still believe in fast breeders. But they have learned several lessons from their experience in Michigan. In the future, they recommend, reactors should be built with strainers in the cooling system. Warning systems should be computerized to detect trouble faster, so that the reactor can be shut down long before the elements melt. Finally, reactors should be designed so that they can be dismantled, inspected and repaired in far less time than the year it took to find the Fermi's elusive sheet of metal.

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