Frances H. Arnold is the Dickinson Professor of Chemical Engineering, Bioengineering, and Biochemistry at the California Institute of Technology. She is a pioneer of “directed evolution,” which mimics Darwinian evolution in the laboratory to create new biological molecules. Her research is particula...

The Molten Salt Reactor Experiment achieved its first self-sustaining nuclear reaction on June 1, 1965. Three years later, on Oct. 8, 1968, it became the first reactor ever to run on uranium-233.

MSRE was noteworthy in at least three respects. Beside running on U-233 and acting as an economic...