After a complicated, technical discussion of "Recent Developments in Nuclear Fusion," an expert on atomic physics devoted several minutes of a question-and-answer period in Harkness Commons last night to clearing up common misconceptions about the Soviet 30-megaton bomb and nuclear bombs in general.

Student questions drew the following observations from David Rose, professor of Nuclear Engineering at M.I.T.:

* Can the United States build a bomb this big at this time? "I see no reason why not. There are no technical limitations I know of."

* What about fall-out? "Not much of the fall-out comes from the bomb itself. When the fireball hits the ground, it activates a vast quantity of dust which is then redistributed in the upper atmosphere. This fall-out settles fairly rapidly--a week to 10 days. The material from the bomb--strontium 90 and that sort of thing, is up there much longer."

* The Russians are testing in the Arctic. Would it be possible for a series of these explosions to melt enough ice to cause an appreciable rise in the sea level? "No. Nuclear explosions are hot--during a blast this size the earth is receiving more energy from them than it is from the sun. But they are much too short to melt a dangerous amount of ice. There is more total energy, from start to finish, in a good-sized hurricane than in a nuclear explosion."

Rose devoted most of the evening to considering possible ways of extracting energy from a fusion process involving deuterium, which has the advantage that it is as easily obtainable as "sagebrush in the West." Deuterium, popularly known as "heavy hydrogen," is hydrogen with an extra neutron and can be removed from sea water.

The difficulty in using the deuterium-fueled fusion process is that it requires temperatures as hot as those on the surface of the sun. Physicists can only sustain these by suspending the materials in a network of magnetic waves, since any other container would draw off heat too rapidly.

Setting up a field powerful enough to do this requires a large amount of heavy machinery. Thus, Rose explained there is little chance that the radioactively "clean" processes of nuclear fusion could be used in airplanes or rockets.