Complex sentence: One reason for a lack of surviving examples of early units was the use of radium on the dials, which officially made them hazardous waste, and as such were disposed of by the United States Department of Energy.
Split into simpler sentences: One reason for a lack of surviving examples of early units was the use of radium on the dials. This in essence made them hazardous waste, and therefore they were disposed of by the United States Department of Energy.