thet's wny it unlikely be used this way
PWRs were and are widely used... that is what the explosions they talk about in nuclear reactors are.
Otherwise the nuclear fuel rods would just keep getting hotter and eventually melt and flow downwards in a puddle of self heating liquid... getting hotter and hotter...
they have experimented with liquid metal coolants, but the problem is that if you turn them off and the coolant hardens there is no flow to keep the reactor elements cool, so when you turn it on it overheats and melts down...
again unlikely liquid metal too.
Liquid metal would problematic because you would either need to keep it running all the time to keep the coolant metal in liquid form or it would need to be loaded into the missile in liquid form before use... and would need to continue running all the time to prevent the coolant going hard...
Having said that you could operate a nuclear reactor at the base you store these missiles at and pump the heated coolant into the missiles just before launch... some sort of catapult launch, or more likely solid rocket fuel booster to get it airborne while its reactor heats up to full temperature with the liquid propellent pumped from the rear of the jet engine through to the front in a lattice where the airflow comes through... meaning the heating system is hottest in the rear of the engine and slightly cooler at the front where it would be tapped off to go back through the reactor... which means assuming perhaps a 1.600 degree C heat as it leaves the reactor in the rear of the jet engine to perhaps 1,000-1,200 degrees at the front of the jet engine as it leaves the engine and reenters the reactor.
This base could be located in the middle of nowhere in Russia surrounded by all sorts of air defence systems etc hidden inside an entire mountain range in the Urals... when the missiles are launched they could fly in circles and climb and accelerate to enormous altitudes and then practically head off in any direction... they could take days to reach their targets... even weeks... In fact as long as the facility is hardened and kept secret it could continue to launch missiles repeatedly... monitoring communications and any surviving satellites to find targets worth hitting... a fast neutron breeder reactor could continue to create weapons grade fuel and production should be relatively simple... being a rocket launched ramjet... you don't need turbine blades or any fancy stuff...
Space thug need ion propulsion, that is completely unrelated technology.
High energy gas made energetic by super heating, though in the case of an Ion engine the manipulation with a magnetic field would also boost performance, but I suspect a lot of overlap in technologies...
It can be last stage of a space rocket, but that needs hydrogen as reaction mass, again, unrelated technology.
No reason why a nuclear powered jet engine operating in the atmosphere would need to operate with hydrogen... previously Xenon has been used for Ion engines but it is expensive... I have seen programmes in Russia using Nitrogen... which is super cheap.... 70% of the earths atmosphere is nitrogen and it is inert so there are no storage problems... well liquid nitrogen would be a useful way to store large amounts so there is an issue with very low temperatures.
Ironically a nuclear powered ramjet would be using 70% nitrogen anyway and at higher altitudes the concentration of oxygen is reduced... not that that matters for a nuclear ramjet.
I think the existence of this missile means Russia wants to keep the start.
There is no way of Russia knowing what the US will or will not sign, or will abide by or will rip up and ignore... most of these new systems started development when the ABM treaty was torn up by the Americans.
IMHO regardless if Russia wants to keep it (unlikely as US dont want any reductions + never reject ABMs) or not Russia surely wants to keep strategic balance. Since there is more resources on Us side Russia must go innovative, asymmetric way.
The Americans might want to reduce further, but not include UK and French nukes and also keep their ABM systems (and build a new one with South Korea and Japan), so it might be in Russias interests to let it expire and then use their new breeder reactors to massively increase their stock of nuclear weapons to force the US to the negotiating table with less of a chip on its shoulder and a bit more respect for Russia.
With breeder or fast neutron reactors making fissile material (for fuel and for weapons) becomes much easier and cheaper, so spent fuel rods that would other wise be nuclear waste can be enriched into fuel or weapons grade material...
Water H2O is thermally stable and will not dissociate into hydrogen and oxygen (and various combinations of the two) in any significant way until temperatures are over 2000 deg C.
Perfectly correct, but when under pressure a PWR pressurised water reactor does not operate at 100 degrees C at normal atmospheric pressure... they normally operate at much higher pressures and also much higher temperatures to increase the energy in the steam component... but even then you wont have problems unless there is an issue and you have a meltdown where temperatures get hot enough to melt the uranium fuel rods... at which temperatures any water in the system is a real explosion risk.
Don't try this at home... look up mythbusters and fat fire... where cooking oil is heated to several hundred degrees C... throw water on that and the water instantly converts to steam, spreading the super hot burning oil in a matter state conversion from liquid water to gas steam instantly... very much a form of explosion... which spreads the burning oil in a huge fireball as it spreads the oil into the air where it gets more oxygen and burns more fiercely.
In common sense terms you would expect the water to put out the fire but when transformed into steam its effect is to spread the fuel into the air and add more available oxygen near the fire to the burning oil fuel. The hydrogen and oxygen in the steam don't come in to it, the temperature is not high enough to release them, but a reactor meltdown literally adds fuel in the form of hydrogen gas and oxygen gas and enormous heat... really big boom... with water vapour as the product of the combustion ironically...
There is a failure mode when one or more channels blocked, and the reactor core meltdown and ejected with the gas trail.
There can be many failure mode that releasing reacotr core material to the enviroment.
To be able to turn the engine on and off it would need control rods... rather than dumping cores it would make more sense to deploy control rods to reduce the reaction.
I think there will be a final, few days long flight of a few missile, and afterwards they will launch them only if the launch place is radioactive contaiminated anyway.
I would say testing it under a wing of the Il-76PP would be enough... they could wrap it in all sorts of containment shields and have probes to measure radiation signatures and other parameters too.
If they can minimise or eliminate the radiation they could develop UAVs for long range patrols around Russia... never leaving Russian airspace... they could test it for years...
Of course no amount of testing can prove something to be infallible....