I was involved with some JIMO work. There were two major design variations, i.e. Brayton cycle, thermoelectric. Thermoelectric was very attractive because it involved almost no moving parts and had extreme reliability. However Brayton cycle blew the doors off of thermoelectric in terms of energy conversion efficiency. Less efficiency means more power is required for the reactor, which means a bigger reactor, which means more mass required by the reactor, which means more ion propulsion required, which means more energy required, which means.....

Gary