**3. Overview of reactor types for NTP**

A plethora of different NTP reactors were proposed and some of them were tested. Before considering particular examples, distinguishing features of reactor types are discussed. This allows for the development of a taxonomy of NTP reactors where one can more easily appreciate the differences in reactor physics characteristics and performance. We discuss different neutron spectra, fuel element geometry concepts, the use of low enriched and highly enriched uranium, fuel compact type, and the interplay of these factors when considering example designs.
