'Nuclear-powered' missile accident in Russia: What really happened?

Russian authorities have confirmed that the explosion involved "an isotope power source in a liquid propulsion system". There's nothing particularly new about the propulsion system—early ballistic missiles used a pressurized stream of liquid fuel and oxygen which, when ignited, expanded and rushed out of the bottom of the missile, propelling it in the opposite direction. The "isotope power source" part is new though. Radioactive isotopes are unstable atoms that release excess energy by emitting radiation. So if the missile is powered by isotopes this indicates the Russians have developed a mini-nuclear reactor—able to fit inside a missile—that is capable of using radiation to heat the liquid fuel for propulsion. This has never been achieved before. 

The exact details of the mini-nuclear reactor that may have been developed to power a Russian missile are not known, but there are a few potential types that may be used.  The key difference between a nuclear reactor used to generate energy and one that might be used to power a missile is the quantity of material required. The RBMK reactor that blew up at Chernobyl contained 200 tonnes of uranium dioxide fuel. A significantly smaller amount of fuel would be required—perhaps a few kilos at most—to lift a missile.

One possibility is what's known as a radioisotope thermoelectric generator (RTG). This converts heat from radioactive decay into electricity. Potential candidates for the fuel are...

• Plutonium-238, 4.8kg of which powered the Curiosity Rover on Mars, americium-241—widely used to power smoke detectors—and polonium-210, infamously used in the poisoning of Russian spy Alexander Litvinenko.
• Strontium-90, which emits both beta and gamma radiation in its radioactive decay, has been used in both American and Russian applications of RTGs in the past, including inside Russian lighthouses. Given the measured increase in gamma activity at nearby Severodvinsk, the latter is certainly plausible.

The second possibility is that the missile was powered by a nuclear thermal reactor. This is perhaps more likely given the authorities' description of the accident. These reactors could use the heat generated from radioactive decay to heat liquid hydrogen fuel. Such a system could theoretically use a solid uranium core, a liquid radioisotope core, or even gaseous uranium to power a missile in flight for long distances. However, none of these technologies have been proven, at least with regard to missiles, and it is not possible to guess the fuel type with any certainty, making the radiation in Severodvinsk difficult to explain.

Whatever the source of radiation, the release seems to be relatively small. To the layperson, 16 times above background rate may sound like a lot, but that background rate is tiny and relatively harmless—for instance the English county of Cornwall has three times the background rate thanks to naturally-occuring uranium-bearing rocks in the earth there.

https://techxplore.com/news/2019-08-nuclear-powered-missile-accident-russia.html

ΚΟΥΡΗΤΗΣ: Η... "ΠΡΟΙΣΤΟΡΙΑ" ΤΗΣ ΑΝΑΠΤΥΞΗΣ ΤΟΥ ΝΕΟΥ ΡΩΣΣΙΚΟΥ ΚΙΝΗΤΗΡΑ: https://isxys.blogspot.com/2012/10/breakthrough-in-quest-for-nuclear.html

ΜΑΛΟΝ Ο ΝΕΟΣ ΡΩΣΣΙΚΟΣ ΚΙΝΗΤΗΡΑΣ ΕΙΝΑΙ ΣΥΝΔΥΣΜΟΣ ΑΥΤΩΝ ή ΚΑΤΙ ΣΧΕΔΟΝ ΠΑΡΟΜΟΙΟ
https://isxys.blogspot.com/2013/06/nasa.html 
https://isxys.blogspot.com/2012/12/blog-post_6808.html
https://isxys.blogspot.com/2017/08/nasa-nuclear-thermal-propulsion.html
https://isxys.blogspot.com/2018/11/ionic-wind-propulsion-engineers-fly.html

ΠΟΛΥ ΕΝΔΙΑΦΕΡΟΝ ΣΧΕΤΙΚΟ ΜΕ ΕΠΙΠΛΕΟΝ ΣΥΝΔΕΣΜΟΥΣ:  https://isxys.blogspot.com/2016/03/blog-post_58.html