From: David Roberson
You mention thermonic emission as being fairly inefficient. That made me wonder how effeicient it would be if the emitting surface were well insulated from the outside world. What if the radiation loss, the convection loss and conduction losses could be essentially eliminated by good design practices? Would that ensure that mainly electrons would carry away the heat energy? The problem with that suggestion is that if you superimpose the electron emission curve (for the Edison effect) over the blackbody curve, most of the thermal heat spectrum of combustion is not very useful (steep enough), even if all heat was completely retained. The threshold for thermionic emission is high, and thermal distribution curve was never adequate to begin with (for chemical reactions). To be useful, the rejected heat must not only be retained, but upshifted. Another problem is that if the heat is from combustion, then removal of exhaust will necessarily carry away heat. However, if the heat source is nuclear, then things are different since the origin of the thermal spectrum is extremely high, MeV level - and one needs only to prevent rapid downshifting before electron emission. A case in point (and a huge missed opportunity for the USA) was the Topaz reactor fiasco. We may have missed a great opportunity for civilian use, had we joined forces with the Russians on this – since we had part of the answer (computer controls) and they had the main part (high temp hardware). Instead, petty jealousies kept everyone from benefiting (assuming that it would have evolved into a civilian reactor). http://en.wikipedia.org/wiki/Topaz_Nuclear_Reactor This design can be made to work with low enriched fuel - and also in a subcritical regime where a “desktop accelerator” provides both makeup neutrons, positive flux control, and a voltage gradient to accelerate electron “boil-off”. That kind of synergy is what would push it into civilian use. The thermal spectrum for thermionics is essentially the same with low enrichment as high. Now that we have seen the possibility of driving small electron accelerators with solid state lasers, the possibility of un-enriched thermionic reactors makes the Topaz fiasco seem even more short-sighted in the big picture perspective. (for everyone except the coal and natural gas purveyors).
