Eloquent post. Thank you. I may steal it for another discussion on nuclear power.
Well, thank you! But if I am to believe asnys, and he seems eminently trustworthy, my core argument (that fissionable fuels are inherently very expensive to acquire) seems to be wildly off base. 1) the fuel is just 10 percent of an OTL power plant's lifetime cycle operation cost (including initial construction of the plant I gather--or if not, it is even less). So griping about the expense of processing uranium is like arguing different rocket designs on the basis of propellant cost--per e of pi and others this is marginal since propellant costs are a very minor part of the price of a rocket launch. I suppose I have been misled by some 60's-80's antinuclear polemics that argued that nuke plants put out less energy than the energy inputs to them. This admittedly seemed screwy to claim to me, and evidently now was, but wondering how it could be that we nevertheless have the power plants led me to the weapons program funding recapture theory which per asnys was pure hogwash. Probably nations with weapons programs do indeed in effect subsidize nuke power plants, in ways asnys has already himself outlined--by subsidizing the development and even construction of nuclear processing plants for instance.
I was wrong. Evidently the high cost of nuclear power OTL mainly boils down to the high cost of building, maintaining, and retiring the power plants themselves. The future history/ATL question is, how much cheaper can these plants be?
Note that it would be possible to enter the "Atomic Age" of the OP without fully accounting for all costs. The cost of waste disposal, including safely mothballing the end-of-life power plants, is quite likely to be grossly underestimated, and it is entirely possible then that there might be a binge of nuke plant building and even cheap power, then when the bill comes due for waste disposal bad things happen. Either the true price of a kilowatt has to be adjusted upward to pay for disposal, causing major economic shock, social dislocation and likely severe political repercussions, or the governmental/industrial complex starts juggling to cover their mistake, shortchanging the safe disposal process and obfuscating generally, with the outcome of major contaminant leaks that might themselves be covered up to a maximum extent. This also would have severe consequences in all the categories above. Watergate and Vietnam rolled into one on steroids, who knows, we might get a successful Hippie Revolution in the '90s! The hellish thing is even if an angry public under a new regime with a radical environmental agenda (much more populist and deep grass roots than OTL, due to mass suffering from major releases of toxic wastes) resolves (perhaps not entirely wisely) to mothball the entire nuclear industry forthwith they are still stuck with the wastes, for thousands of years to come. Hence the likelihood of a rival "stay the course" movement that punishes particular scapegoats for mismanagement but remains committed to the nuclear juggernaut. Call it rationality or call it sunk cost fallacy--God knows the sunk liability would be tremendous enough.
So, let's suppose that an alternate reactor design could beat coal in the cost department. ...
Now, although the LWR was the one that won, there were a number of other designs floating around in the 50s, including many that had small-scale prototypes built, and even a few commercial demonstration plants. These included aqueous homogeneous reactors, several different kinds of gas-cooled reactors, oil-cooled reactors, and others. Of these, the one that I think had the most potential over the same time frame as the LWR is the high-temperature gas-cooled reactor. Others may have more long-term potential - such as the Molten Salt Reactor of fame and story - but will require considerably more research to turn into commercial hardware.
Wasn't the core mission statement of General Atomics, the operation that hired Freeman Dyson and Ted Taylor in the late '50s, to develop civil uses of nuclear power? This is why I am so head-scratching as to why apparently none of these alternative designs, or yet others, were investigated as alternatives to LWR, and why the prospect (if it seemed to exist for any of them) of a much cheaper plant structure didn't attract any serious investment.
I could speculate thus:
1) despite the ideology of capitalism that assures us that progress happens via private capitalists taking risks in order to get an edge on competition, and this is the basis of profit, the fact is big establishments tend to be risk averse and more interested in trying to control known variables. They tend to leave the risky business of blue-sky research to government agencies or at any rate undertake it only with government funding. In the 50's USA (and USSR) this meant the military mainly.
2) the government establishments concerned with nuclear power were an interlocking directorate of Defense and the Atomic Energy Commission, which both regulated and promoted nuclear power. The latter should have been funding speculative research, and developing promising alternative modes of extracting fission power, but...
3) the contractors the US Navy hired to develop and build their pressurized LWR systems had a deep vested interest in that particular mode of power generation and equated it with "fission" as such. Were the AEC to present an alternative model unsuited to submarine installation but cheaper to build and operate than LWR, the prospect of leveraging the Naval contracts into a strong commercial presence as well would be thwarted; these firms would have no advantage over competitors in making the civil power plants and would have to divide capital investment between supporting the Navy and attempting to compete in the civil market, instead of synergistically using the same capital for both. Therefore it was not in their interest to have the AEC or any other state funded agency come up with a promising alternative. I don't have to suppose they were so cynical as to openly seek to suppress alternatives, merely that their strong influence in the interlocking directorate (via contacts at the Pentagon, long-established relationships there, political influence via Congress and Presidential electoral politics) would suggest to the AEC and Congressional committees that the established LWR program had everything well in hand and there was little need to "squander" large taxpayer funds on considering alternatives.
The question still arises why, in what was in the '50s and early 60's admittedly just a handful of parallel establishments, in Britain, France and the USSR, there wasn't some chance of alternatives being explored. But obviously they would all have parallel considerations, they too would hit on LWR early, and develop their own vested interests in it. And all of these rival national establishments had far less funds available than the USA potentially did. The Soviet system, driven by close to desperation (the high Kremlin officials knew, though didn't want to put out too loudly, that the West, mainly the USA, was ahead of them in every respect and feared it was all too likely we'd preemptively attack if we knew just how weak they were) and with a command society that could allocate admittedly scarce and inefficiently used resources lavishly on regime priorities, would be the strongest alternative. Poor per capita though it was, the Soviet bloc did command a very high population with a higher level of technical knowledge than mere comparison of standard of living would suggest, and had a rather futurist ideology too. But they had to prioritize vital defense, and if they had something in hand that seemed workable they'd tend to stick with it.
...And, unlike the other designs, I think there is a potential route to giving gas-cooled reactors the same kind of investment that LWRs got: the Aircraft Nuclear Propulsion Program.
Interservice rivalry! this addresses point 3 pretty directly! A bunch of scientists who think some other way than LWR is more promising are voices crying in the wilderness--unless the Air Force needs one of them, and now that alternative versus the Navy's favored system has a voice that will be heard in the interlocking directorate!
...ANP was an enormous project at the time, spending the equivalent of about $20 billion in today's money over ten years. Not quite as big as the submarine project, but still big. And it produced working hardware, including three nuclear turbojets that were static-tested in Idaho. Although two designs were considered, I want to focus on the direct-cycle option by GE. In its final incarnation, this consisted of an air-cooled, beryllium oxide-moderated reactor with uranium oxide fuel elements. Air would enter the turbojet, be ducted to the reactor, be heated by direct contact with the fuel elements, and then be ducted back to the turbojet. Now, it's not quite as simple as replacing the air with a closed helium loop and the beryllium oxide with graphite, but this is not too different from something that could be a very good power reactor.
I want to point out that if you disdain the Soviet RBMK, "upgrading" the gas cooled loop with graphite moderator is adopting the single feature that turned Chernobyl into a radioactive abattoir. Sure, everything is fine as long as it is helium circulating, but what happens if air gets into the loop?
I suppose a design derived from something developed for an airplane engine would not be nearly as massive as the RBMK cores to be sure. There might be relatively little graphite to burn, leaving the rest of the core to "simply" melt down or emit vapors thermally, not via combustion. Still, let's not condemn Ivan too harshly if graphite moderator seems like a fine idea to you!
Now, for powering airplanes, this has some serious problems, even leaving aside the whole "crashing" thing. First, it's not going to be fast. It's just not.
I figure this is for 3 reasons. One, the weakest in itself, is sheer mass. A jet engine is pretty light for the tremendous power it develops. Not so a nuclear core obviously, especially if it is shielded to give crew a chance of survival. But this is offset by the lack of fuel, to an extent.
Two, air intake temperatures. An air based heat engine needs to pressurize the air to be efficient to be sure. Up to a certain speed, pretty high up in the supersonic regime, ram air temperatures need to be actually increased to meet the adequate pressure levels for efficient power generation. So this point seems kind of shaky, but I suppose a specific design of reactor might have rather low intake temperatures, and flying too fast would exceed those temperatures.
Three, at a given thrust level, power rises with speed. If we have a plant adequate for cruising at Mach 0.75, and wish to go at Mach 2 with the same thrust instead, we might need to nearly triple the power output. For a fission reactor that means tripling the core mass, and tripling the radiation output. The former is actually dependent on design; we could have designs that "burn hot" and consume fissionables at a higher rate and get more power out of the same mass--bringing it to depletion and end of life sooner in inverse proportion of course. But radiation is a hard correlation, so much gamma ray an and neutron flux per kilowatt (thermal kilowatts, not necessarily useful output) in a fixed proportion. Also I believe supersonic aircraft require more thrust.
What it can do is stay aloft for a couple of weeks - its endurance is limited by maintenance and the crew's sanity, not by fuel. That could still be really useful, for things like missile carriers and command planes.
Unfortunately, that's not what the Pentagon wanted - they wanted a fast, high-altitude bomber, basically the XB-70 Valkyrie (which at one point was going to be nuclear-powered). This led to regular oscillations in the program's support, as it was alternately scaled up and cut back, which wasted a huge amount of money and time. Despite that, they still managed to produce a few turbojets, and by the time the program was cancelled in 1961, they basically knew how to build a nuclear airplane. It would be big, expensive, and slow, but it would fly, and it would not be completely useless.
Um, the thing is, desiring the air-cooled version probably does relate to desiring supersonic speeds. If we settle for subsonic cruise, in principle we could develop a version of LWR to drive propellers. The Soviet/Russian Tupolev "Bear" family of bombers was the USSR's answer to the US B-47 and B-52. The Americans used jets, later upgraded to turbofans, but the Russians used contra props driven by a turboprop engine. Their outcome was higher observabilty (including sheer noise, which I'm told could be heard by submerged submarines) and a noisier radar signature, but in terms of speed the Bears are competitive with the Buffs, that are after all pushing sonic limits anyway, and I believe they got significant endurance and range benefits with the more efficient turboprop setup.
Given subsonic cruise, I've seen a number of speculative options for nuclear powered planes. I was able to access a RAND corporation study done IIRC in the mid-80s comparing concepts for a big transport, much larger than a C-5, that considered many options for propulsion systems, including alternative fuels such as methane and ammonia as well as synthetic kerosene (making that version a mere size extrapolation of the C-5), hydrogen and nuclear power. The study meant to outline options for a future in which fossil derived kerosene would no longer be available.
The nuclear option was not at all a direct air cycle; rather it was molten metal (sodium and something else mix IIRC) with a double loop; the interior reactor core loop would, through a heat exchanger, transferred in a second loop (to isolate radio-activation of the primary loop substance) outside the shielded reactor core to the heat input part of a more or less standard turbojet, where the liquid metal would be cooled in lieu of fuel combustion there, then the heated air would first drive the turbine to drive the turbo compressor and possibly, likely actually, a fan, with the exhaust then completing the thrust, as in a normal turbofan. In fact, as a safety measure this study proposed that this engine be designed bimodal, using ordinary jet fuel (presumably synthesized per the study's premise) for takeoff and landing, only cruising on nuclear power with the kerosene flow turned off. The reactor core would supposedly be robust enough to survive a crash without cracking! The jet fuel reserve for a landing would also serve as part of the reactor shielding. The layout was unusual, with the jet engines, four of them powered by a single reactor, mounted on top of the fuselage above the reactor, placed in the center of mass of the airplane pretty much at the wing roots. Otherwise its layout was conventional for a big subsonic transport of the post-707 era.
Yet other schemes involve some sort of closed cycle turbine engines, hot gas or possibly steam, driven by a reactor to drive fans serving as the jets.
Whereas of course one rather infamous and well-known alternative here was the unmanned "Pluto" cruise missile, which would IIRC carry a number of short-range stand-off ballistic missiles to be fired at various targets on its trajectory (the thing being quite huge), propelled by a nuclear reactor in the form of an air breathing ramjet. IIRC Pluto would be quite supersonic, and it was anticipated the reactor core would leak quite a lot, spewing radioactive dust in its exhaust!
So really I'm not quite so sure why we are agreeing that a nuke powered jet could not be supersonic. Perhaps because Pluto or any other version running hot enough to produce necessary supersonic thrust on any scale would necessarily be dirty, and also much too "hot" in its core radiations for flight crew to survive on it or operate it on anything but a kamikaze basis? Clearly it would be possible for the intake air to be handled even at supersonic speeds. Clearly, barring some ATL breakthrough in making very small reactor cores, it would have to be scaled up to a massive airplane and making a plane both of unprecedented size and capable of supersonic cruise might indeed be too daunting a task, especially if one had any intention of ever safely landing it!
But accepting the stipulation it must be subsonic, certainly goals such as making it stay clean with its core not decaying and leaking, keeping radiation hazards within bounds so a flight crew could contemplate serving on it and hope to live normal lives, enabling safe landing, keeping the size within the bounds of leaps forward that would be deemed possible in the late 50s, are all more sanely attainable I guess.
But under those conditions as I say, would the rational designer aim for direct heating of air by running it right through the reactor core? The idea of air, with its random contaminants and occasional ingested bird, and its steady 20 percent oxygen composition, with variable amounts of water in various states, being the medium gives me the willies. Even granting that a gas cooled reactor points to lighter structure and probably more efficient power output, I think I'd suggest from the get-go that it be a controlled gas, using ambient air flow to cool it back down perhaps via a liquid intermediary heat exchanger or even active heat pumping "air conditioning" using Freon, ammonia or some such, in a closed cycle, and that basically the turbine would be a turboshaft, driving ducted fans or even open propellers, contraprops al la B-36 or the Bear. After all using props instead of heating the air for a jet effect would lower the net power demand, which would offset efficiency losses due to extra intermediary steps and the probably serious power drain of actively pumping heat exchangers to cool the working gas for another cycle. I probably would aim for helium to be that working gas immediately, but might be discouraged by the way helium has of leaking away while costing quite a bit to replace. Nitrogen might be good enough, and very similar in general thermodynamics to air, air being 80 percent nitrogen after all--but without oxidation hazard or the unpredictability of air's tertiary components. Carbon dioxide was used in early generation gas cooled reactors such as those developed in Britain--indeed IIRC such British gas cooled reactors were in fact essentially similar to the Soviet RBMK, which also was gas cooled unless I am much confused in my recall. (Advanced gas cooled reactors, such as you are suggesting here being already an Air Force project in the 50s, differ from both the British and Soviet design in that these used the hot CO2 to boil water and run ordinary steam turbines, whereas in advanced form the gas flows directly into gas turbines. Steam turbines are limited in their efficiency by the upper temperature limits at which our materials can handle pressurized steam, which sets a Carnot efficiency upper limit of efficiency well under 50 percent, whereas gas turbines can handle considerably higher peak temperatures and thus become overall much more efficient, in the 60-80 percent range versus 40 or less.
As a working fluid helium has the advantages of being strongly unlikely to be transmuted by neutron bombardment, almost perfectly chemically inert, and the second-lightest "molecule" known to science (or given current theory, possible) and also immune to phenomena like dissociation that complicates the thermodynamic calculation. It is as close to an ideal gas as is physically possible! So I'd think it smart to go straight for it for such an application. As I said the trick, in the context of an airborne power plant, is to cool it again for another cycle. But perhaps at some sacrifice of efficiency and hence requiring a heavier core and plant, the turbine exhaust temperature can be such that heat is readily drained from it through gas/gas heat exchangers that don't create too much drag and don't weigh far too much, and cool enough to get decent efficiency from the cycle.
Now, I am personally unable to be objective about ANP. I just love it too much. But we don't need the plane to actually fly to do what we need it to do. It just needs to get further than it did historically, to produce technology that can eventually go into a power reactor. Give it more consistent support, maybe even fly a few demo flights - maybe, if you buy the arguments about radiation I'm eventually going to get around to making, have it become a major part of the Air Force fleet. That gets us tech that can eventually become a gas-cooled civilian power reactor.
Someone in the Air Force has to recognize, around the late 50s, that a giant "turboprop" is useful to the Air Force mission. Too bad the glory is aimed at supersonic bombers--though I suspect you do dismiss the supersonic option a bit too definitively. A subsonic plane like the B-52 would clearly seem to have its days numbered and of course a mega-plane is going to be made in smaller numbers, hence become a high priority target for Soviet defenses.
OtL, the Air Force had a bit of a split personality between demanding control of strategic ICBMs--
should these be deployed--versus wanting to keep priority on manned bombers. Eventually a branch of SAC emerged that was committed to Thor, Atlas, Titan, Minuteman, etc and ICBMs were there to stay.
Now suppose that some political and perhaps contractor interaction happens to give the Army the mission of CONUS based land ICBMs and perhaps allied-soil based front line IRBMs like Redstone (which actually had ranges verging on tactical) and Thor and so forth. (Thor would not exist, being essentially a replica of Jupiter capability). Von Braun's team at ABMA (or is that AMBA?) in Huntsville aka Redstone would be riding high, possibly Chrysler corporation joins the ranks of Boeing, Convair or Martin as an aerospace contractor (well, they were OTL, thanks to Von Braun's relationship with them).
With the Air Force backed into a corner, not having the fall back of owning the ICBMs (say a Congressional clique is duly impressed with the Army's work and authorizes von Braun to develop the prime ICBM contract, justified by the argument that missiles are basically artillery and not aircraft) some officers come up with a compromise, and suggest that the Air Force, like the Navy, be entrusted with an alternate system that is protected not by silos but by mobility and stealth--air launched long range missiles. Much longer range than the stand-off short range missiles that B-52s were eventually equipped with. The carrier aircraft does not penetrate enemy air space but stands off in international air space, maybe as far back as the Canadian Arctic, and the virtue of air launching a rocket versus ground launch from sea level is emphasized. Now at subsonic launch speeds, those virtues are not tremendous, but they are large enough to justify air launch satellite systems such as Minotaur OTL. If the plane could reach supersonic speed, it could contribute significantly to launch velocity and thus given the exponential nature of the rocket equation, save considerable propellant mass for a given throw weight. Recognizing that as an advanced goal, a first generation system is proposed instead that mainly benefits the rocket via launching from stratospheric altitudes, which cuts down the difference between launch and vacuum conditions and enables more efficient rocket engines thereby. Also, given good navigational techniques, a computer on board the plane, far more massive than anything that could be put on the rocket itself, inputs current coordinates at the time the go code for launch is given, computes a launch trajectory, downloads instructions into a simplified internal guidance system in the missile, and instructs the flight crew to an azimuth for launch. If location is accurately known and the crew can meet a tight specified launch condition, the missile can have high precision on target, a selling point versus early Polaris sub launched missiles since they were solid fueled and harder to steer accurately.
I'm thinking the plan is load one each Atlas missile in the fuselage of the plane, horizontally. Upon getting a go code the crew flies toward the launch point at the computed azimuth. Using extra thrust from conventional jet engines or perhaps rockets installed in the plane, they do a pull up maneuver to say 45 degrees climb. They shut down the auxiliary engines and throttle back the nukes, and coast on a ballistic "Vomit Comet" sort of roller coaster trajectory. As they approach the target release altitude on the right launch azimuth, they open a hatch in the rear of the plane, and the missile rolls out on rollers in the fuselage to emerge, pulled out by drogue parachute. Once the missile is clear the plane does a hard dive--or actually a mushy one as the air will be quite thin, but not so thin considering at say 300 m/sec they only have to climb one mile or so to launch the missile. After a suitable delay, when it has gotten hundreds of meters clear and is coasting on the right angle for a suborbital trajectory to target, the missile fires. The plane crew track it to make sure it is flying true and then head for a landing strip, some 120 tonnes lighter.
Why an Atlas? Because the Air Force was procuring it OTL, and it is so light that it can deliver a warhead to a target while massing all up not much over 100 tonnes. The fragility of the missile is less of an issue since it will be cradled within the fuselage of the plane until launched.
Some other features might be to have LOX in the missile but not kerosene fuel, this being kept in tanks on the plane (serving as extra radiation shielding) until the launch order is given, then rapidly pumped into it. The LOX will boil off, but since we have nuclear engines it is possible to power coolers that reliquefy it and return it to the tank; gradually the fuselage storage bay is chilled down a whole lot. Another feature might be to have the warhead separated, and only mount it upon getting the go code; that way if the plane has an emergency the missile can be dumped without risking releasing the warhead. Since there is only LOX in the missile the explosion risk is lowered considerably.
Similarly, when the plane is about to come in for a landing after a two or three week tour airborne, it is possible to lighten the load considerably by dumping the LOX. Making more LOX and loading it in to the missile probably costs relatively little.
Hey, for that matter might it be possible for such a nuclear plane to include an on board LOX generation facility? Then it could take off with the missile unloaded, and spend a day or two gradually condensing LOX out of the air and loading it in. This would make a certain portion of the Air Force's missile deterrent off line at any given time, as does having to land and change crews anyway, and necessary down time for aircraft maintenance and checking out the missile. In a way that's good though; it guarantees there is always a reserve of weapons that would not be fired in one spasm. Just so long as the operational portion is adequate!
A second generation launcher plane is also envisioned, something between a B-58 Hustler bomber with supersonic dash capability and the B-70 Valkyrie meant to cruise at Mach 3, and scaled up enough for over 100 tonnes missile load. This version would also cruise around at a high subsonic speed the nuclear engines are designed to maintain, but its roller coaster climb will be assisted with much more powerful thrust that first shoves it through the sound barrier, while also accomplishing a very fast rate of climb putting it into a roller coaster trajectory that reaches a much higher launch speed, one that does significantly cut down on the burden the missile must bear to reach a given target, therefore either the missiles can be lighter or can carry bigger bombs. Using more aggressive auxiliary thrust systems than the subsonic cruiser, such as ramjets, rockets, ejector ramjet, or what have you, when this one goes into a climb to launch it also surges up to Mach 3 or so, around 1000 m/sec. Unlike the Valkyrie there is no need to make it cruise for hours at this speed; it is a quick surge to release a missile or two, and then dive to escape their exhaust plumes, then very temporarily endure the high speed in lower atmosphere while braking down to more sustainable speeds. The nuke drive can sustain flight above Mach 1 though it might be poor at pushing through the sound barrier; normally it sustains flight at below the speed of sound, thus avoiding sonic booms and putting minimal strain on the loaded airplane and the plane only goes supersonic for an attack run. But, having been thus boosted to very high Mach speeds and past the "sound barrier" (which is a thing, you know--obviously not an absolute barrier, but right at sonic speed and within 5 percent or so of it on the low side and 15-20 on the high side, drag is high, lift is low and stability is dubious--a plane cruising just below sonic speed would need a surge of thrust to push to and past sonic speed, and would wish to get going well beyond sonic speed because it is hell trying just maintain it!) the plane would keep going at Mach 1.3 or so, damn the sonic booms--there is evidently a war on now after all! At high speed it would recover from its surge, turn around and head for a base to land at presumably to be reloaded with another set of missiles--even if it is hoped no more salvos will be launched, it is important to restore the deterrent again--presumably the USA has more than one potential enemy.
The extraordinary costs of such an airborne ICBM system are evident enough, so the Air Force would need to stress the advantages:
1) impossible for enemies to preemptively destroy. This might prove unnervingly less obvious after something like STARFISH, revealing the vulnerability to EMP. But in the conceptual stage that might not be anticipated, or it might be and means of shielding against it included in the design. Short of a hemispheric area bomb, how are airplanes flying at semi random locations on racetrack paths hundreds or thousands of miles long in the stratosphere going to be located and targeted independently? Even if the enemy has spy sats that can identify each plane by visual signatures and relay their exact flight paths in real time, what sort of missile can be launched that could locate, track and home in on each plane to get it preemptively? And would not any launch to try that give the US so much warning that the missiles could be launched even if the planes are doomed?
2) Flexible deployment--with their weeks of endurance, the missile bombers can be placed on standby patrol anywhere in the world where the USAF or allies enjoy air supremacy. They cannot be placed where enemy snipers have a good chance of shooting one down of course, although a certain risk of losing one or two to essentially commando/guerrilla operations can be accepted, with the proviso that the Air Force and allied forces have the political freedom to pound hell out of the ground bases attackers are observed to come from. So in peacetime one obviously could not ever fly one over enemy borders--no "Indian Country" missions. If you flew one of these over some Soviet client state in Africa, and the ostensible state air force were to attack, we'd have only ourselves to blame for losing the plane. But if it were flown over some territory whose state government is supposedly allied to the USA, like say Zaire under Mobutu or a Latin American state with a military junta put in with CIA help, and it happens the jungles or mountains are lousy with insurgents with ample SAMs, and they take a shot or three at them, some other USAF planes or say the Brazilian Air Force can come blast them with counterinsurgency gear. Naturally the sane thing to do is fly the planes over allied territory where conditions are pretty settled. The open sea is less favored than say the Canadian Arctic or the Australian outback since hostile submarines may lurk in the oceans--and even if one shoots at one of this missile planes, it may not be politically expedient for the USA to shoot back!
But anyway within constraints of political prudence, these missile planes can be sent to orbit around anywhere in the world, for even at subsonic speeds the endurance is so great it can deploy from any base in the world, reach a loiter area somewhere else in the world, then return to the base. In principle one could build just one airfield to handle these monsters, and deploy them anywhere. In practice one would want diverse fields they could operate from so enemies can't cripple the whole system by taking out one target. But all of these can be deep in the heart of CONUS, none need to be "forward" as a matter of giving the system reach. (Distant bases in Australia and so forth are useful in giving a damaged plane options besides ditching!)
So, if the USSR is the threat today, most of the planes can be sent to loiter over Canada. But say the Soviet Union has a political meltdown and fragments into seven pieces, none of which maintain the technical prowess of the unified former state and many of which declare friendship for the USA and seek aid from it...But meanwhile the PRC has built up a substantial threat, it is possible for the same air craft within days to be patrolling over Siberia (say that part of it was one of the new friend states to come out of the Soviet breakdown), Japan (well, maybe not because of the peace treaty, but say in waters just east of Japan, the Philippines and Australia or maybe Indonesia.
Also with over 60 degrees of range, it isn't really necessary to get really close to the targets, this just reduces warning time and introduces the possibility of heavier throw weights.
Now do I think the USAF could have the subsonic version of these babies deployed before 1965 and the supersonic ones come on line by 1970? I don't know if the actual deployment would happen but anyway perhaps serious R&D money might be spent on them before they finally do get cancelled. And they'll be cancelled only if the Air Force is granted, or settles for, an alternate strategic nuclear role to supplement conventional bombers.
Also, even if the missile carrier version is ultimately judged not cost effective, it could be that nuclear powered airframes that derive from the missile fleet study but are repurposed to other roles may be built and flown--as AWACS, as super-transports, as airborne refueling tankers, conceivably as attack plane carriers (the latter allowing USAF assets to be deployed to distant unprepared theaters and fight for air supremacy there so ground bases can be established).
Let's also add in the idea of Small Modular Reactors sixty years early. Similar ideas were already in the air, mostly related to powering remote military bases and villages, but they never quite gelled into the idea of mass-producing small reactors on assembly lines. But they easily could have.
Combine high-temperature gas-cooled reactors with the idea of the SMR, and what do you have? You might just have a power reactor that can beat coal. We can't know without actually spending a lot of money in the real world. But, for the purposes of fiction, I think it works.
Turbine gas reactors seem likely to have a limit to how low they scale in terms of practical power reactors. We would not be talking about something on a scale that would power say a single tank, would we? (Unless said "tank" is a Maus or some other hyperbolic dream from a Japanese cartoon anyway!) The reactor core might be compact and the turbine rather light (though if it is not air breathing, which poses the horrible prospect of a malfunctioning reactor injecting its shattered guts directly into the air as dust and vapor, it also needs some kind of heat exchanger that cools down the working gas again). Say power generator designs always require a steady supply of cooling water as from a fair sized river. In mass the plant may be petite compared to one of OTL, but if it puts out hundreds of megawatts, it can't be called mini, can it?
Of course in principle turbines can be scaled down, but they tend to become more challenging and less efficient doing so, and I think the same is true of reactor cores.
I gather that smaller cores are achieved by "burning hotter," by fissioning a greater percentage of the remaining available nuclei per second. Therefore these cores would reach EOL sooner. I daresay they might burn more efficiently, pushing the envelope of depletion farther before being too depleted to sustain operations. And with smaller cores it is easier to replace them.
Safety is something you'd have to explain very carefully. I notice in later posts you say RBMK had no containment for instance--but it did, it just wasn't a dome like Western PLWRs tend to have. It got blasted apart of course! But would a Western style dome over the core have contained the blast any better? I grant that RBMK appears to have had design features posing risks we would not allow--but really, isn't claiming a nuke plant is safe like claiming a rocket design is sure to launch without incident? Isn't there always a risk of some sort of failure, inherent in making the design capable of the useful performance you ask from it, and isn't it always asking too much to design against every conceivable failure mode?
The whole point of your thesis is, nuclear power is rare in the modern OTL world because the reactor designs are too expensive! Now clearly if this is the case, safety is in conflict with utility. It may be that PLWR were the wrong way to go and had we done something else we'd have the same degree of safety, at least per KW/hr produced, at a much lower cost and the nuclear business would be profitable, with lots of reactors of the alternate design existing and putting out much more fission generated power than OTL. That said if safety is defined on a per-plant year of operation basis, and we are running far more plants, the same degree of safety translates to a lot more accidents and accidental releases happening over the decades. (Not to mention an order of magnitude or more accumulation of atomic waste). To lower the degree of public contamination to OTL levels then we need not equivalent but superior standards of safety. And if we have lowered the cost of other aspects of operation, we need to spend a greater and greater proportion of the capital and operating budget on safety! Whereas, human irrationalities being what they are, if the cost is 90 percent safety features, either people will be deterred from using the tech even if you can show them how they are objectively worse off with alternatives. Or they will disregard some of the safety measures since doing so greatly lowers costs and thus creates a huge competitive edge for the scofflaws, whose profits might seem to outweigh any potential risk. Then when something bad happens they might be held up as scapegoats, if they didn't die, but the people most responsible probably will not be identified as such, because they would be very rich success stories. It will be hapless underlings of theirs who are blamed. And serious measures to enforce stringent safety standards would be opposed on the grounds they impose severe costs; someone will argue the general progress that comes from cheaper power is worth a few hundred thousand cancer cases more, or when put to it, a few ten million of them! Most won't want to put themselves in such a confrontational, Herman Kahn-esque position, but will be quietly pushing to downplay the importance of safety all the same.
Outcome I think is inevitably, a lot more accidents and a lot more radioactive materials release than OTL. Which is more or less accepted as normal and inevitable. The net outcome might be a severe polarization between the Technophiles and the Luddites, with the stakes much starker and stronger.
Naturally much depends on the details of the design of the reactors!
Reading Fred Pohl's novelization of the Chernobyl disaster, which I believe attempted within the limits of hard science fiction standards of technical accuracy to properly represent the nature of the reactor explosion and subsequent fire, I get the impression there are inherent resemblances between a fission reaction and a chemical fire such as in a fireplace or bonfire. There are inherent positive feedback effects--a fluctuation in favor of fission releases more neutrons, which inherently tend to expand the scale and speed of the reaction, just as allowing a wood fire to grow hotter accelerates the circulation of air which brings in more oxygen to extend combustion--vice versa, attempts to damp out reactions can also involve positive feedback. In the context of fission producing certain isotopes that have a strong tendency to absorb neutrons, with many designs (including RBMK, and also PLWR) the ambient levels of these isotopes, predominantly IIRC a xenon isotope, combined with a commanded reduction in neutron flux, can cause the reactor to plummet below sustainable fission and for the pile to become impossible to restart fission in until the isotopes involved have been removed or decay into others; since there is in many designs no practical way to accomplish purging them one must wait for the latter process to allow restart. This relates to the scenario in Pohl's novel (titled
Chernobyl after the plant) where supposedly the power ministry was interested in conducting an experiment whereby the possibility of extracting useful power from a shut-down but still hot core was being examined--this meant bringing the core to a state of near shut-down, but avoiding actually stopping the reaction completely since this would terminate the experiment and take that core (one of four at the plant) offline for weeks. The near-shutdown zone was presented as one of very dangerous instability and therefore safety features which
had been designed in and installed to avoid this hazardous zone
were ordered turned off in order to conduct the test. (Pohl, in a possible deviation from OTL reality, had a heroic and competent Assistant Plant Manager who did the real work of operating the plant, his nominal superior being a creature of Kremlin politics and dealing with all that, adamantly opposed to the experiment despite clear Ministry demands for it, and therefore had his authority circumvented by conducting the experiment when he was home asleep--under KGB supervision). I believe the parts about the nature of the hazard, the nature of the breakdown (the fission level dropped to near the red line of natural and irrevocable shutdown and in the course of trying to bring the levels up a bit one section surged to such a degree an actual if quite small fission explosion occurred, this cracked the inner containment separating the graphite moderators from air, and then of course the big kaboom as these burnt and blasted the outer containment roof to smithereens, leaving the exposed core to burn uncontrollably) were accurate, as was the part about shutting off the safety controls and attempting something quite ill advised considering the basic design.
Now I have also read of approaches to controlled reaction that avoid some of these pitfalls. For instance it is apparently possible to engineer in negative feedback to countervail the inherent positive feedback, whereby if some or all of a core gets too hot the neutron absorption cross section of non-fissionable materials rises sharply enough to naturally check the surge; perhaps this also means that vice versa if levels go low and the reactor core cools, the reaction is easier to elevate at will. Perhaps some designs can purge the xenon and other gases naturally so their "poisoning" effect becomes a minor factor--though that is only indirectly a safety hazard; inherently they make it easier to shut down the reaction with assurance which is directly more safe, though obviously in view of the Chernobyl scenario a perverse incentive to keep the reactor running at some risk, as is the case not only for economic reasons but say in a warship power plant where a loss of power for weeks can equate to becoming useless in combat at best and in actual combat situations, a death sentence for the crew and an expensive loss to the Navy.
Various other designs you hinted at, such as aqueous reactions (I gather this means putting fuel and moderator into water solution in some way so that it is all dissolved together and control is a matter of controlling the solutions and use of solid or otherwise concentrated moderator controls) might offer a combination of inherent safety with profitable and compact reactor design, maybe. Such a design would certainly make the job of purging fission daughter product and neutron activated contaminants more straightforward, at least in principle--let gases bubble out, use chemistry to filter out the worst contaminants. Another design that caught my interest some years ago was a German proposal for forming fuel and moderator into prefabricated spherical small pebbles, and dropping them into a hopper; when enough fresh pellets are on the pile it reaches critical density and the pebbles get hot. The thing was a version of gas cooled, with helium flowing through the heap of pebbles (being spherical, if they do not physically crumble or melt, there is a fair amount of volume for the helium to filter through) and thus heated, drive a gas turbine. The neat thing here is that once running, you get a natural gradation of the pebbles by height, the newest and fresher ones being on top, where the main reaction occurs, the expended ones on the bottom. There is a valve to allow pebbles to be removed, one by one, from the bottom and taken away to reprocessing for reuses or disposal, while adding new ones on top sustains the reaction at a desired level. Thus there is no need to periodically take the reactor off line, remove the core and replace it with a fresh one. Another feature was a metal damper substance that would be liquid at normal reactor temperature, held above the hopper by a diaphragm of hotter-melting metal; should the reaction run out of control such that merely emptying it from the bottom is not adequate to check it, temperatures would rise but at a temperature below danger to the hopper walls, the safety diaphragm would melt and dump the damper mixture right on top of the pile, presumably the liquid damper would sink in through the interstices and shut down the reaction definitively. Creating of course a nasty blob of fuel pellets in a frozen matrix of damper metal which would have to be removed, broken down carefully, and the damper scraped or otherwise remove from the cold hopper walls; perhaps removing the blob would crack or otherwise undermine the hopper walls and it would need to be replaced. But this is an emergency measure after all.
Given the nature of radioactive decay, I am not sure any gas cycle can guarantee containment of decay or neutron activated undesirable wastes. When a nucleus undergoes fission, about 1/1000 of its total mass-energy is released mainly in the form of kinetic energy of neutrons and the two (are there ever more) daughter nuclei, and some gamma rays. The daughter nuclei are highly charged and though quite energetic are moving at much lower speeds than the neutrons. Charged particles interact very strongly with all other charged substances around them, including the components of atoms generally, and so lose their energy fast in a dense environment, and will be completely stopped by a finite amount of material. Thus I suppose most daughter nuclei can be guaranteed to be initially stopped within a solid element designed to catch them, and not get out immediately. However these core elements, even when refractory enough to remain nominally solid, are going to be quite hot and some elements, such as I presume the xenon, will subsequently migrate at random and some will reach the outer walls and escape into whatever fluid might be flowing there. Thus in a water cooled core, this stuff is gradually contaminating the water--which is why PLWR reactors always use a heat exchanger to confine the core water to the core and recycle it, and use the secondary heated loop which will have very little radioactive exposure (only from whatever does get into the water, which is mostly a matter of neutron activation of this water) and can be run through conventional steam turbines for power. Gas/gas heat exchangers don't work so well so their design would be heavy and bulky, which is why the gas-cooled reactors actually built OTL have used the hot gas, CO2 rather than helium, to boil water in a heat exchanger instead.
Clearly the Air Force OTL plan for a direct air cooled reactor would take ambient air, presumably pressurized by turbo compressors as in ordinary jet engines, blast that through the reactor core directly, and exhaust it either through a turbine or directly into jet exhaust, and not have to worry at all about recooling the working fluid for another cycle--this is accomplished for free by the atmosphere, and the hot exhaust is left behind in any case. With this design, neutron activation of air molecules seems inevitable though the exposure is brief, and any seepage of daughter fission nuclei or neutron-transmuted nuclei would wind up in the atmosphere. If that seepage is low I suppose a case can be made it is overall safer than conventional alternatives--Jerry Pournelle used to rant quite a lot that natural radioactive isotopes contained in coal would be released into the air at rates far exceeding what any conventional US nuclear power plant would be allowed to do in normal operation. Presumably the same is true of kerosene so it then becomes a question of which isotopes are being vaporized and released, whether the particular ones released by fission are more dangerous gram for gram, and if so if their levels of release can be reduced in proportion so the real hazard is equivalent. All this assumes of course that no physical erosion of the reactor core surfaces exposed to air flow happens, or if it does it happens at a very low rate, otherwise the engine is dumping radioactive grit at rates that I presume would rapidly exceed any sane safety standard.
A thing to remember when dealing with proposed military nuclear jets or rocket engine is that their deployment is often by design contingent on fighting an all-out war, presumably a nuclear one with a peer power like the USSR or PRC. The enemy plans to dump gigatons of bombs on our territory and those of allies we are defending, killing hundreds of millions at a stroke. In that context, hazards posed by release of radioactive materials and long-term man-years of human life lost to poisoning are set against deterring or foiling such wholesale immediate slaughter. Thus when I read about nuclear rockets like say Timberwind, even if I can accept their alleged thrust/weight and other features at face value, I am not at all confident they are designed to contain radiation and radionuclide contamination dangers effectively, since the premise of Timberwind launched weapons was to enable SDI, presumably shooting down incoming enemy H-bomb warheads before they can reach their targets, thus saving lives by the scores of millions--in that context, radiation releases that could shorten those same lives by decades might be justified as acceptable overall. And if the system deters enemy attack completely, then none of the hazard actually happens, except of course for necessary test launches.
These Nuke bombers, both the OTL envisaged nuclear Valkyries and my proposed missile carriers, as well as derivative transports, AWACs, fighter/attack plane carriers, tankers and so on fall in between. On one hand, even in peacetime they'd have to be operated quite a lot, for training and proficiency practice as well as for operational missions, the missile carriers in particular being useless if they are all kept idle on the runways. They have to be kept flying, otherwise they are vulnerable to preemptive strikes or even sabotage! So their emissions had better be kept at levels acceptable for sustained routine use, which implies they can and should be used for civil purposes too if such markets for such big planes emerge. But a society willing to accept this complex as necessary is probably going to be persuaded, by open and fair argument or by subterfuge, to accept contamination at levels we might not be willing to see OTL, in the name of national security.
Evolving air-breathing open cycle nuke jet engines into closed cycle power plants using helium, nitrogen or CO2 for working gases, and cooling those down for further cycles, ought to raise the safety level since the working gas is after all isolated.
But it still leaves open the question--why do you think these engines would on the whole work out to be cheaper to make and operate than OTL PLWRs, and would this include safety features at least equivalent to these and, to keep net contamination levels down, if at all possible superior?
Would the reactor core physics be such that there is more inherent safety against runaway reactions? Or merely equivalent to in LWR where we've seen amply that is something that has to be actively controlled?
A jet engine by its nature tends to run at a constant power level, since the thrust requirement of an airplane tends to fall within a tight limited band. Indeed I suspect nuclear jets, or alternatively turbofans or turboprops, would be more limited than normal jet engines. A combustion jet inherently tends to produce more power and thrust at sea level--where thrust is most needed for takeoff and landing--and be naturally "throttled" to lower steady outputs by the thinner air of the stratosphere where they are designed to cruise. There is simply more oxygen available per cubic meter at sea level than up there, although speed and ram effects offset falling density a bit. The reaction rate is governed by the available oxygen. Not so a nuke plant--there it is separately controlled by a deliberate choice of controls, and the higher power at sea level needed might not correspond closely with the higher cooling rate the denser air provides. Higher power is needed to move that air enough to get the necessary high thrusts for takeoff or landing, and that higher power here comes from higher rates of fission, hence higher temperatures in the core elements, which can only be checked by higher rates of cooling, but even when so damped still involve steeper temperature profiles within the condensed liquid and solid elements involved. The name of the game is to move that higher power flux into the denser air for higher thrust. But it may be quite unwise to attempt to design the reactor elements for this wide power band; instead it might be necessary to restrict power output to levels appropriate for sustained thrust at altitude. At sea level, this probably means higher thrust even so, because the denser air will absorb the heat at lower temperature (therefore more efficiently) but this heat though power available suffers from lower thermodynamic efficiency is moving more mass and thus thrust is higher. But only moderately so compared to full power at full temperature.
Fortunately a nuclear core heated jet offers another option for bursts of higher thrust--none of the oxygen running through the core is involved in chemical combustion yet, all of it is available for that in the exhaust from the core. It would be possible to inject fuel at that point and boost the thrust with after burning. This works well enough on normal jets because they operate fuel-lean in the cores, to limit temperatures there to something feasible materials can handle in the turbine; on a nuke jet the boost is higher due to more oxygen being available as well as the exhaust being pre-heated by the nuke core. Compared to combustion at higher pressure it is inefficient and so afterburners guzzle down fuel, but here it is only needed briefly for takeoff and landing and possibly for operational bursts of thrust in flight, as with my proposal to launch a missile from a parabolic trajectory. Or on a transport or AWACS plane seeking to evade an enemy attack perhaps. Moderate reserves of fuel for these purposes can also serve as secondary radiation shielding; if and when they are used up, it will be during some kind of vital maneuver that probably foreshadows the plane coming down for a landing soon after, and essential reserves for landing maintain some of the shielding level.
I'd think these cores in airplane jet engines would be designed to burn a lot of fissionable stuff up relatively fast and require frequent replacement, perhaps being designed for one nominal mission, and with each landing the cores are removed, shunted off for refurbishment or disposal, and new ones inserted. Redesigning for closed cycle gas generators might therefore involve heavier cores that burn at a lower percentage rate for say monthly or annual replacement instead. Still, a lot of fresh cores full of highly refined nuclear fuel are going to be circulating around the nation using them, at risk of being hijacked or stolen by chicanery for bad actors to abuse.
The airplane jet engine by the way I suppose might be designed somewhat differently than a typical turbojet. In the latter, the plan is, take air in, compress it with a compressor set driven by a turbine, combust and expand the gas to drive that turbine, and exhaust the jet with remaining heat energy for thrust. In a turbofan or turboprop one uses a more powerful turbine and exhausts considerably less jet thrust for a given air/fuel consumption rate, but uses the extra power beyond what the compressor needs to drive a fan or prop which moves a larger mass flow at a lesser exhaust speed, but overall the thrust is multiplied, at the cost of a heavier and more complex engine, in particular making more demands on the turbine which is trying to extract power from a very hot combustion exhaust flow. For the nuke, I suspect a reconfiguration is in order--make no attempt to extract any mechanical power from the reactor exhaust, including perhaps after burning enhancement, instead use auxiliary cooling loops from the reactor core to drive a turbine to provide the necessary compression. Then again I suspect the nuclear core exhaust might be cooler than combustion jet exhaust and perhaps it is more sensible to use a conventional turbine in this stream? But not doing so would eliminate an extra worry about wear and tear on a turbine that might be drawing power from slightly radioactive air! The radioactivity might cause structural weakening of the turbine despite lower operating temperatures.
Anyhow, I'd be very interested on any remarks you might have as to why the air-cooled or closed-cycle gas cooled reactor cores might have inherently better negative feedback control of fission for steady and controllable (at any rate, reliably shut-down-able) reaction rates.
For airborne use, a light exposed reactor core might seem desirable, but the pesky matter of crash survival is relevant too. With the liquid metal heat exchanger design the RAND study I read proposed for a subsonic turbofan, how that worked was clear enough--the reactor core is essentially a "solid" mass. Not really solid since the sodium mix coolant has to be liquid (and how it starts again from a cold shutdown where the sodium presumably freezes into solid metal is not so clear, perhaps it is kept from ever freezing, or auxiliary heating elements pre-melt the stuff, or a slow startup cycle can reliably melt it all before any of it needs to circulate, or something). Anyway it is condensed, presumably then a hard emergency shutdown definitively stopping the chain reaction combined with a seal-off of the heat exchanger flow and a certain degree of heat sink margin allowing core temperatures to peak well below softening the shell, with the core all contained in shielding/containment designed to tolerate a very hard impact, turns the whole thing into one dense sphere of essentially solid metal that doesn't crack upon crashing--I suppose it might be concussed or a bit squashed to ruin it as an operational unit, but it might not leak.
What can be done for lightweight air-core reactors to make them too immune to shattering and scattering their hot innards to the four winds?
If the answer is "not much really, without making it so heavy it can't fly" then perhaps this is why the actual nuclear airplanes remain a pipe dream--but one believed in long enough to develop light efficient gas core units for surface work?
What sort of containment would gas core light reactors need? Can they be designed for inherent assured shutdown if the gas loop is breached?
Having seen later comments of yours I have more yet to say, but I'll post this now!