Well, Asymetrical Dimethylhydrazine (usually referred to as just hydrazine) is definetely a recognized rocket fuel. It's a very stable coumpound at just about any temperature while in liquid form, but becomes highly explosive in gaseous form at the proper intermix with oxygen.
In a rocket, it is usually kept in liquid form in the tank, with liquid oxygen as the fuel intermixer. Both are injected in the engine's bell at low pressure, and so vapourize instantly, and are ignited to propel the rocket. Cut the flow of either and the engine shuts down.
From the description in the articles above, I can't figure out how any nuclear product could play a role.
This is way over my paygrade, but I note that the Americans seem unsurprized, so they may already have an inkling of the science behind all this.
Purely as a speculation here: In a hydrazine fueled rocket, the LOX is much more voluminous then the hydrazine, so using such engine for a hypersonic, or even regular, cruise missile would make the damn thing pretty big. In the atmosphere, we obviously have oxygen as part of the composition of gaz available everywhere, but it is difficult to extract it fast enough to maintain the intermix properly in a hydrazine fueled engine. That is why even in the atmosphere, those engine cary their own LOX. Again, I am speculating here, but perhaps the Russians have been working on a reaction with nuclear isotopes as catalysts to extract oxygen more efficiently and thus, at a sufficient rate to provide the engine with oxygen without the need for onboard storage of LOX. That would be a big breakthrough in atmospheric rocket engines.
I note here, however, that if that is the case, and when you consider the radiation that just resulted from this accident, I would hope that this propulsion is only considered for nuclear armed missile (becasue causing a little bit more radiation when you nuke someone is not an issue), not for classic explosives used for conventional attacks - where the irradiation of an area much larger than the explosion would result from any use.