@DDE: but what astonished me was this bit I found here:
Now, DDE, you are probably aware of all this, but this was a wake-up call for me, and I am also speaking to others on this forum.
Solid fuel rods, such as are using in the old NERVA solid-core NTR, become clogged with nuclear reaction products (neutron poisons) when 15% of the fuel has been burnt (undergoing nuclear fission). A clogged rod will not support nuclear fission. The rod has to be removed and taken to a reprocessing plant. For this reason NASA's reusable nuclear shuttle has an engine life of only 10 Terra-Luna round trips before disposal, even though the rods still contain 85% of the expensive uranium-235 unburnt. Extracting the rods for reprocessing is too dangerous in NASA's eyes, they just send the nuclear shuttle into a graveyard orbit. This wastes a lot of expensive U235.
The same limit applies to a nuclear power plant. Ground based plants periodically halt operations when the solid fuel rods become clogged. They then spend a few months carefully opening the reactor, removing the clogged rods, replacing them with fresh rods, then carefully reassembling the reactor. The clogged rods are sent to a reprocessing plant to extract the unburnt U235 and using it to fabricate fresh rods. The rest of the rod goes to a long term nuclear waste disposal site.
If I am reading the above sentence correctly; they are implying that the fuel gas, after one pass through the MHD generator, can be passed through an on-board refinery. The product is already gaseous, as opposed to solid fuel rods, which simplifies refining. In the refinery the nuclear reaction products clogging the gas can be filtered out, and the unburnt U235 can be sent on another pass through the MHD generator. The alternative is removing the gas "from the circuit to the external environment", i.e., wastefully jettisoning the gas into deep space along with all its expensive unburnt U235.
Now that my nose has been rubbed in the fact, I realize that a nuclear lightbulb gas core engine should also require an on-board reprocessing plant. But in all the nuclear lightbulb documents I've read, a re-processor is conspicuous by its absence. I'm going to have to review the documents.