Complex sentence: 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 these type 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 these type were disposed of by the United States Department of Energy.