Civilian defense experts find in the latest sinking of a radioactivity "hot" ship a sobering hint of the problems an atomic war would bring.

When the cruiser Salt Lake City was sent down off the California coast by gun fire, bombs and torpedoes 22 months had elapsed since it got its coating of the radioactive spray and solid green water thrown up by the submarine burst of an A-bomb in the Bikini tests of 1946.

Yet the ship remained so contaminated that it was deemed un-safe for workmen to scrap her for salvage metal. This bore out the predictions of scientists that some of the Bikini ships might remain dangerous for years before the slow decay of the residual radioactive material was complete. Attempts to cleanse the target ships of the invisible but deadly poison brought little success.

Experts of the national military establishment's civil defense planning unit are giving close attention to residual radioactivity, the result of "fission products" or unexpended particles of the explosive material flung outward by the bomb when detonated in water. Translating the target ships into terms of cities, defense workers can foresee the possibility of communities made inhabitable for prolonged period if a bomb should be exploded in nearby water.

An air burst bomb, such as those used in the attacks on Hiroshima and Nagasaki, presents no problem of residual activity because the fission products ascend into the air and drift away.

It has been suggested, however, that in the absence of a large body of water near a city a bomb could be exploded in the middle of rain clouds. The dispersion of fission products over the ground might then be even more complete than in an underwater burst.

Curiously, improvements in the explosive efficiency of atomic bombs conceivably can reduce their power to produce lingering radioactive contamination. In a rough way, fission products from a bomb are something like the unburned portion of a coal fire....the better the fire, the fewer the ashes that remain.

Official statements strongly indicate that the three atomic explosions set off this spring at the Eniwetok proving grounds in the mid-Pacific were more efficient than of the first five A-bombs exploded.