The engine was to be installed in a Burevestnik cruise missile, promised by Russian President Vladimir Putin, with "unlimited range".
A deadly explosion at a missile test site last week may have been caused by a failed test of a nuclear-powered cruise missile, although Russian Federation has yet to say what its engineers were working on at the time of the blast.
The men were buried in Sarov, a city known for nuclear research, Bloomberg reports, explaining that experts suspect that what blew up might have been a compact nuclear reactor.
Some reports suggested that previous tests of the Burevestnik missile had been conducted on the barren Arctic archipelago of Novaya Zemlya and the Kapustin Yar testing range in southern Russian Federation before they were moved to Nyonoksa.
The accident caused a radiation spike in the port city of Severodvinsk, Russia. It also said that "three of our colleagues received injuries and burns of varying severity" and revealed that "the tragedy occurred during the period of work related to the engineering and technical support of isotopic power sources in a liquid propulsion system". "Not good!" Trump said, using the North Atlantic Treaty Organisation alliance's name for the Burevestnik.
The scientists and engineers "tragically died while testing a new special device", Alexei Likhachev, the head of Rosatom, said at the funeral Monday.
He said: "Our further work on new weapons that we will certainly complete will be the best tribute to them".
He said similar developments are "also happening in many countries", adding the institute's work serves both civilian and military ends. The Americans past year. also tested a small-scale reactor.
Anxious residents surrounding the accident zone have since stocked up on iodine, used to reduce the effects of radiation exposure. Sarov's administration said the experts died while performing a task of national importance.
"The testers involved in this are of course the national heroes and we always believed that they are the elite of the federal nuclear centre".
Valentin Kostyukov, head of the nuclear centre, which is part of Rosatom, said the test had been preceded by a year of careful work and a state commission was investigating what went wrong.
The nuclear experts battled to control the situation, but were unable to prevent the accident, Mr Kostyukov said. He said they would be posthumously awarded with top medals.
Interest in Chernobyl, the world's worst nuclear disaster, has resurged since the release this year of an HBO miniseries on the events surrounding the 1986 explosion and meltdown of the nuclear plant of the same name in Soviet Ukraine.
In the article, he concludes that the weapon tested last week was likely the Burevestnik and argues that an escalating arms race between the USA and Russian Federation could lead to more nuclear accidents.