Probably a nuclear powered hypersonic missile.
? last August ?
Of a type of hypersonic most likely.
On August 8, Russia's Ministry of Defense reported that the explosion at the test site in the Arkhangelsk region occurred during the tests of a liquid-propellant jet engine....the explosion occurred during the test of a rocket with a radioisotope element in the engine. "The tests were held on the offshore platform. After the tests were completed, the rocket fuel ignited, and an explosion followed," the department said.