It's a notice that just couldn't be ignored. The Oct. 25 Federal Register (page 52160) says that the office of assistant secretary of energy for international affairs has approved shipment of 100 milligrams of radioactive uranium, enriched to 3 percent, and 0.5 milligrams of plutonium to the Soviet Union. A similar shipment was also proposed for Czechoslovakia. Both elements, small as they are, could be used in a nuclear weapon. Or could they?

A DOE official offers this explanation. The tiny amounts of radioactive material are part of a worldwide measuring exercise being conducted by the International Atomic Energy Agency. These two elements are being measured in each country to see the type of isotopes that can be found by the country using its own measuring equpment. The plutonium, for example, contains plutonium 239, plutonium 240 and plutonium 241. What the Soviets find when they make their measurement will be compared with other countries using other devices, and the averages will enable each country to get more precise calibrations on its measuring equipment.

What happens to the samples? "They remain there forever," according to the DOE offical. But don't worry. It takes about four kilograms to make a bomb, so the Soviets will need millions of samples to have something militarily worthwhile.