Russian radiation incident on Saturday

Well, that's interesting news, via the New York Times: 

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.
The statement, though, shed little light on exactly what detonated on Thursday at the White Sea testing range. No use for the propulsion unit was mentioned, although President Vladimir V. Putin previously boasted that Russia has developed a nuclear engine for long-range missiles. And there was no explanation why the authorities in a nearby city had reported rising radiation levels for a brief period several hours later.
While the government has provided no full explanation of what happened, Rosatom’s statement suggested a mishap during a test of a new class of nuclear-engined weapons that Mr. Putin first spoke publicly about last year.... 

...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.”

I guess I don't really understand what this unit is, i.e. a missile powered with a nuclear reactor? What is the propellant?

[A brief Wikipedia search indicates that it may be a ramjet design:

The principle behind the nuclear ramjet was relatively simple: motion of the vehicle pushed air in through the front of the vehicle (ram effect), a nuclear reactor heated the air, and then the hot air expanded at high speed out through a nozzle at the back, providing thrust.]

I can't imagine the potential risk behind testing a missile with a nuclear reactor in it - and I thought running 1000 or 2000 gallon reactions had a lot of potential hazards!