The New York Times' Russia Confirms Radioactive Materials Were Involved in Deadly Blast discusses the potential nuclear aspects of the recent missile test and references several time the possibility of development and testing of nuclear propulsion in missiles.
MOSCOW — A mystery explosion at a Russian weapons testing range involved radioactive materials, the authorities admitted on Saturday, as the blast’s admitted death toll rose and signs of a creeping radiation emergency, or at the least fear of one, grew harder to mask.
In a statement released at 1 a.m. Saturday, Russia’s nuclear energy company, Rosatom, said five employees had died, in addition to the two military personnel previously confirmed dead, as a result of a test on Thursday morning involving “isotopic sources of fuel on a liquid propulsion unit.”
“A bright memory of our comrades will forever live in our hearts,” the statement said.
Later it says:
Novaya Gazeta, an independent Russian newspaper, interpreted the presence of Rosatom nuclear engineers at the test site as confirming “the version that the military could have been experimenting with the newest rocket with a nuclear power unit.”
One new weapon Mr. Putin had discussed was a globe-spanning cruise missile called Burevestnik or the Petrel, named for the far-flying seabird. It would have an unlimited range thanks to a nuclear propulsion unit, he said. Mr. Putin said the device had already been tested.
“Russia’s advanced arms are based on the cutting-edge, unique achievements of our scientists, designers and engineers,” Mr. Putin said in the 2018 speech. “One of them is a small-scale heavy-duty nuclear energy unit that can be installed in a missile.”
Nuclear propulsion would use a thermonuclear source of some type to heat a material of some type to extremely high temperature and channel it though a nozzle. Often liquid hydrogen is mentioned.
Based on known principles of nuclear propulsion and any other reputable and possibly recent sources of information what is most likely to be the basis of the suspected and advertised Russian nuclear rocket propulsion? For example, would it be possible to be a fission reactor where the thermal output can be turned up and down, or a radioisotope that's "hot" all the time, or something more exotic such as the isomer source discussed in answers to How did Northrop Grumman propose to make the Global Hawk nuclear powered??