Nuclear reactors have too low uranium enrichment to be capable of nuclear explosion, not even a detonation(explosion with supersonic velocity of propagation).
But the thermal power of uncontrolled fission may lead to mechanical explosion by pressurised gas or water vapour, or there can occur secondary chemical explosion by ignition of developed and escaped hydrogen.
RBMK-1000 reactor, used in Chernobyl, had nominal power 1000 MW. There were/are in use also RBMK-1500 and some designs were projected up to 2400 MW.
Explosion can be defined as very fast energy release, with extremely high power and very short time duration, with sharp begin and end.
Detonation is explosion propagating in exploding material by speed exceeding speed of sound in material. Additionally, explosion is not matter of total released energy but matter of way how it is released.
Nuclear detonation of fissionable material requires for this material to sustain runaway chain reaction of fast (x MeV ) neutrons.
2-3% 235U, additionally with fission products absorbing neutrons, scattered in small pieces across the big volume, cannot nuclearly detonate.
If it could, the detonation would not be small, but huge, temperature would reach some 100 millions K instead of few thousands and all power plant and close neighborhood would evaporate.
TNT explosive energy ( about 10% of coal energy ) is approx 4.2 MJ / kg. 3.2 GW of thermal power is equivalent cca 760 kg TNT/s. Equivalent of 75 ton TNT is released during less than 2 minutes of nominal reactor function.
Overheating of pressured casserole, just little bigger and dramatic for lifes of too many people.
That means, if the power of energy being released is just in range of few multiples of nominal power, being increased gradually and continually, it can hardly be consider as a nuclear explosion.
OTOH, the sudden reactor break can be, but as a mechanical explosion, even if based on originally nuclear energy.