Dmitry Orlov Came Up With A Hypothesis...

 ... about the nature of Oreshnik. He makes only one debatable assumption about the mass of MIRV--it is most likely not 1.5 ton but less, but since the last time I did anything electrodynamics related was about 30 years ago, LOL, I still think that it is very interesting to see this POV. 

Around a century ago it was noticed that when a fast-flying lead bullet or shell hits steel armor, it releases an amount of heat many times greater than the kinetic energy of the projectile (which is half the mass times its velocity squared, E_k=1/2 mv^2) — enough heat to burn a hole right through a steel plate. The reason for this anomaly is the same: electron inertia.

 

The binding energy in the crystal lattice of metals is approximately twice as large as that released during the explosive oxidation of TNT. At first glance, the explosion should not be much larger than that produced by a conventional explosive — about twice as large. The difference is that the time it takes to release this energy is hundreds of times shorter than during the chemical oxidation reaction in TNT and the energy from it is much more concentrated. Because of this, the destructive power of a Coulomb explosion can be 1000 times greater than that of a conventional explosive. Of course, this is not a nuclear explosion: no atomic nuclei are in any way damaged during this experiment. But it is comparable in its effects, which are much greater than what can be achieved using TNT.

 

For a bit of perspective, consider that 1 kg of uranium-235 can in theory (if every single atom of it undergoes nuclear fission) result in an explosion equivalent to 20 million kg of TNT. In reality, uranium is never enriched to 100% U-235 (anything above 90% is considered weapons-grade) and only a few percent of the U-235 have time to participate in a nuclear chain reaction before the whole contraption blows up. More realistically, a 1 kg nuclear charge is equivalent to about a million kg of TNT (or 1 kiloton). Meanwhile, 1 kg of metal in a Coulomb explosion will release energy equivalent to about a thousand kg of TNT (or 1 tonne). Nevertheless, these are still huge numbers.

And now we can address the question of what Oreshnik most likely is. Here is what has been publicly announced about it:

Warhead temperature: 4000ºC

Speed: Mach 10 (2.5–3 km/s)

Mass of warhead: ~1.5 tonnes

Perusing Dmitry Mendeleev's Periodic Table of Elements we find just one candidate for warhead metal: tungsten. It melts at 3422ºC and boils at 5555ºC. Taking the mass of the warhead (which, we assume for the sake of simplicity, consists entirely of a single shaped piece of tungsten) at 1,500 kg, it produces the equivalent of 1,500,000 kg of TNT or 1,5 kilotons — a respectable amount for a small tactical nuke.

It is behind the paywall, but Dmitry gave me permission to quote him, so here it is. Join Dmitry's Boosty account.