EDIT Inventor of the Radioactive Sign: Nels Garden, Berkeley, 1946 The radioactive warning symbol was doodled out at the University of California radiation lab in Berkeley in 1946 by a small group of people. The key guy responsible was Nels Garden. They had these printed up, magenta on blue, and use of the design spread around the country. Blue as a background was a poor choice, since blue was not to be used on warning signs and besides, it faded, especially outdoors. The use of yellow as a background color was probably standardized by Oak Ridge National Lab in early 1948. That is informed speculation. Modifications to the Berkeley design were suggested and implemented locally into the early 50s e.g. the addition of straight or wavy arrows between or inside the propellor blades but an ANSI standard and federal regs finalized the current version by the mid 50s. Today the color black is an acceptable substitute for magenta.

Why did the Berkeley people chose this symbol? There is no answer, only speculation. One idea has it is that this symbol was used at a naval base drydock near Berkeley to warn of spinning propellors. Others imagine that the central circle is a radiation source and that the three blades represent radiation, perhaps one blade each for alpha, beta and gamma.

Whatever the reason, it was a good choice because it is simple, readily identifiable, i.e. not similar to other warning symbols, and discernible at a large distance.

Source:
"A Brief History of a 20th Century Danger Sign" Stephens and Barrett that appeared in Health Physics Vol. 36 (May) pp. 565-571. "Health Physics: A Backward Glance", Kathren and Ziemer, Pergamon Press.

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