Reading further after receiving this answer, I saw the section on nuclear power in the Wikipedia article on the Northrop Grumman RQ-4 Global Hawk:

Nuclear power

Sandia National Laboratories and Northrop Grumman have studied the possibility of equipping the Global Hawk with a nuclear power plant with US Air Force Research Laboratory funding. A nuclear powered Global Hawk would have a very large range and endurance, and more power for on-board systems. Apart from the technical feasibility, the major drawback however is the fact that drones sometimes crash. So whether this type of propulsion will ever be used on Global Hawk (or any aircraft) is questionable.64, 65

Those references do not give may details on the plan.

Radioisotope thermoelectric generators or RTGs can certainly be compact enough, and generate hundreds of watts or more, but I don't know if they could produce enough power to drive an electric engine.

Was the idea to use electric propulsion, or to somehow generate thrust through direct heating of the air? Or something else?

Nuclear powered aircraft are usually envisioned as quite large, so the proposal to add nuclear power to a Global Hawk may have been substantially different.

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    $\begingroup$ @qqjkztd that's better than using a rotating shaft connecting the two and a pair of constant velocity joints; one at the propeller and the other at the motor co-located with the nuclear power plant. $\endgroup$ – uhoh Mar 28 '18 at 8:24
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    $\begingroup$ This article explains the technology available for nuclear powered flight Putins Nuclear Power Missile $\endgroup$ – jCisco Mar 30 '18 at 1:59
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    $\begingroup$ @jCisco that is an excellent article, thanks! I think the plan for the Global Hawk did not include a conventional reactor based on fissionable material, but instead called for something more unusual. $\endgroup$ – uhoh Apr 4 '18 at 7:54
  • $\begingroup$ @uhoh - see the bottom of jCisco's article for a discussion of compact nuclear reactors. Small reactors exist cna.org/CNA_files/PDF/D0023932.A5.pdf, en.wikipedia.org/wiki/High_Flux_Isotope_Reactor. Even in the Pluto/SLAM days, a "conventional" reactor could already power small vehicles like cruise missiles and potentially recon drones. Even Sandia's 2012 paper for a nuclear UAV for Northrop suggests a mature/conventional reactor: fas.org/irp/eprint/sand-uav.pdf. I don't think the explanation requires exotic schemes, eg Hafnium nuclear isomers. $\endgroup$ – Hephaestus Aetnaean Apr 4 '18 at 21:33
  • $\begingroup$ You know, if a nuclear UAV didn't want to RTB there's not much the operators could do about it -- it could remain an airborne menace for years. $\endgroup$ – A. I. Breveleri Apr 5 '18 at 1:08

I didn't find any direct evidence that Northrop Grumman actually proposed a nuclear-powered Global Hawk.

Citation 64 (2 April 2012) in the question doesn't reference a nuclear-powered Global Hawk, only "drones." The pictured Global Hawk was apparently just an illustration.

Citation 65 (August 2003?) in the question does reference a nuclear-powered Global Hawk:

The US Air Force Research Laboratory has funded at least two feasibility studies into nuclear powered versions of Global Hawk., [sic] which could extend the vehicles flight time from hours to months.

But this August 2003 page (above) appears to be copied from an earlier February 2003 article in New Scientist (below):

The US Air Force Research Laboratory (AFRL) has funded at least two feasibility studies on nuclear-powered versions of the Northrop-Grumman Global Hawk UAV (pictured). The latest study, revealed earlier in February at an aerospace technology conference in Albuquerque, New Mexico, concluded that a nuclear engine could extend the UAV’s flight time from hours to months.

This is the only reference to a nuclear-powered Global Hawk I've found. I'm not sure this is a reliable source, however, for two reasons. First, this article was published in 2003, just five years after the Global Hawk started flying. That's not a lot of time for AFRL to conduct two feasibility studies for nuclear-powered Global Hawks, though not impossible. I've been unable to find these studies. It's possible these studies were for nuclear-powered ISR drones in general, not the Global Hawk specifically. Second, the New Scientist article references a problematic study by Carl Collins on using a metastable nuclear isomer of Hafnium (Hafnium-178m2) as a gamma/heat source by using xrays to induce the transition/gamma release. The claim has since been discredited (though this may not have been apparent at the time of publication). I'm not saying the author is unreliable (he seems like a decent STEM writer for the MIT Tech Review and Scientific American), but he doesn't seem to have a background in defense or aviation. So there's a possibility he conflated "Global Hawk" with "drone," which wouldn't be unusual of even good journalists then or now.

Northrop's nuclear-powered UAV work seems to predate Global Hawk:

Northrop Grumman is known to have patented a drone equipped with a helium-cooled nuclear reactor as long ago as 1986, and has previously worked on nuclear projects with the US air force research laboratory. Designs for nuclear-powered aircraft are known to go back as far as the 1950s. The Guardian

Feasibility of nuclear-powered UAVs

jCisco's arstechnica article gives a surprisingly good overview of nuclear-powered aviation (there's a couple photos I've never seen before). They touch on ANP, the nuclear Bounder hoax, Pluto/SLAM, and Putin's nuclear-powered cruise missiles and torpedoes.

Some discussion of nuclear propulsion: Why don't aircraft use nuclear propulsion?. I'll repeat just one comment I made:

@vsz the water boiler, turbines etc, have to be there and are heavy You can actually dispense with the steam turbines entirely. The HTRE series were basically modded J47s that sent air through a reactor instead of a combustion chamber. HTRE-3 pic. The whole engine+reactor assembly is actually lighter than a conventional engine+fuel, especially at long range. Rather, it's the shielding that's quite heavy. See SLAM/Project Pluto, the Mach 4 nuclear powered ramjet – Hephaestus Aetnaean Nov 28 '17 at 17:20

Northrop Grumman (maker of the Global Hawk) did sponsor Sandia to study the feasibility of using 'non-hydrocarbon' fuels for an "ultrapersistent" (read: nuclear) UAV. (Note that even this report doesn't mention the Global Hawk.) 2012 Report summary. Quotes:

Upon completion of the CRADA, Sandia provided a final out-brief to NGIS UMS [Northrop Grumman Integrated Systems Unmanned Systems]. Accomplishments: The effort concentrated on propulsion and power technologies that went well beyond existing hydrocarbon technologies. It contrasted and compared eight heat sources technologies, three power conversion, two dual cycle propulsion system configurations [indirect cycle?], and a single electrical power generation scheme [to power the ever-hungrier avionics mentioned earlier]. Overall performance, specific power parameters, technical complexities, security, safety, and other operational features were successfully investigated. Large and medium sized UAV systems [probably Global Hawk or X-47B and larger] were envisioned and operational flight profiles were developed for each concept. Heat source creation and support challenges for domestic and expeditionary operations were considered. Fundamental cost driver analysis was also performed. System development plans were drafted in order to determine where the technological and programmatic critical paths lay. NGIS UMS and SNL felt that the technical goals for the project were accomplished. NGIS UMS was quite pleased with the results of analysis and design although it was disappointing to all that the political realities would not allow use of the results.

Taking this assessment at face value, nuclear-powered UAVs sound technically feasible.

The powerplant is well understood, suggesting a mature/pre-existing concept (eg a "conventional" nuclear fission reactor) rather than an exotic new concept:

Basic application of the advanced propulsion and power approach is well understood

New power/propulsion concepts would probably fall outside the scope of this study, which sounds fairly small and limited to what the sponsor could potentially accomplish in the short/medium term.

Note: "Conventional" is used in the same sense as the OP (I hope): "conventional reactor based on fissionable material." I do not use it here to refer to a specific reactor generation, fuel type, fuel phase, or coolant type.

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    $\begingroup$ Thank you for looking into this so thoroughly and taking the time to writing this up. It seems to be fairly clear what happened now. Great answer! $\endgroup$ – uhoh Apr 5 '18 at 3:22

A nuclear powered aircraft was explored in the US, with the Convair NB36H. The idea was to use the heat from the nuclear reactor instead of burning kerosene to run a gas turbine engine. GE actually built a prototype nuclear powered turbine engine, but it was never used in an aircraft.

The NB36H carried a functioning reactor aloft. A lot of shielding had to be added to protect the crew, which added considerably to the weight of the aircraft. Plus, in the event of a crash, the reactor could release a lot of radiation.

The Soviets also experimented with a nuclear powered aircraft, the Tupolev Tu95LAL. It would have used a similar design... a gas turbine that used the reactor heat instead of burning fuel. Like the NB36H, the TU95LAL made many flights with an active reactor on board.

Both projects were abandoned, partially from fear that the thing might crash over their own country with massive radioactive contamination, and partially because the ICBM proved to be a more effective (and less costly) delivery system.

Both the US and USSR also looked into a nuclear powered cruise missile, using a ramjet with nuclear created heat. The radiation trail left behind by this setup in flight was considerable, and both such investigations were terminated.

In the 1950's and 1960's, both nations did things with nuclear technology that, considering what we now know, seem a bit kooky.

I strongly suspect that any attempt to nuclear power a drone will be stopped for environmental reasons (in the event of a crash). A more likely solution to a long distance/long range drone would be aerial refueling via a drone tanker. The US Navy is looking into a stealthy drone tanker, to refuel it's F35's.

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    $\begingroup$ Thanks for your answer, but double check my last sentence in the question, and consider how much smaller the Northrop Grumman RQ-4 Global Hawk is compared to a Convair or a Tupolev. I am not sure the technology you are writing about here would be applicable to a small UAV. That's what's motivated this question. $\endgroup$ – uhoh Mar 30 '18 at 11:28

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