The Lenin reactor could have been part of a special program to have a secret sodium fast reactor. Soviet vanguard, “call it Lenin !”. Perfect for Pu and Na24, especially as Na24 production would need to be close from NATO because of short half life. Na24 is a perfect neutron source with light elements (deuterium or tritium in water for instance : 2000 neutrons per Curie with heavy water, 1000 times more than Po210). 

This could explain the “tunnel effect” : in fact no tunnel effect but simply the gammas of Na24 are too powerful for most Geigers when close by. Progressive reduction mean that it’s seen farther from the core.

The positive void coefficient means it is perfect for taking off part of the generated Pu without shutting down and being seen with satellite infrared cams. Pressure bars of RBMK design confirm intentionality (design used to ensure injection of moderating bars as a protection against the high risk of criticity excursion because of sodium).

This is certainly why Russians today have the best sodium fast reactor, the BN800. Sodium and the positive void coefficient are the perfect explanation for the violence of the blast and length of fire.