In the second episode of the mini tv series Chernobyl it was said that a steam explosion because of the below would generate a 3 to 4 megaton explosion plus spread fallout all over Europe. The fallout part is plausible but would the energy generated be that great.
The 1986 Chernobyl nuclear disaster in the Soviet Union was feared to cause major steam explosion (and resulting Europe-wide nuclear fallout) upon melting the lava-like nuclear fuel through the reactor’s basement towards contact with residue fire-fighting water and groundwater. The threat was averted by frantic tunneling underneath the reactor in order to pump out water and reinforce underlying soil with concrete.
Two floors of bubbler pools beneath the reactor served as a large water reservoir for the emergency cooling pumps and as a pressure suppression system capable of condensing steam in case of a small broken steam pipe; the third floor above them, below the reactor, served as a steam tunnel. The steam released by a broken pipe was supposed to enter the steam tunnel and be led into the pools to bubble through a layer of water. After the disaster, the pools and the basement were flooded because of ruptured cooling water pipes and accumulated firefighting water, and constituted a serious steam explosion risk.
The smoldering graphite, fuel and other material above, at more than 1200 °C, started to burn through the reactor floor and mixed with molten concrete from the reactor lining, creating corium, a radioactive semi-liquid material comparable to lava. If this mixture had melted through the floor into the pool of water, it was feared it could have created a serious steam explosion that would have ejected more radioactive material from the reactor. It became necessary to drain the pool.
Once the bubbler pool gates were opened by the Ananenko team, fire brigade pumps were then used to drain the basement. The operation was not completed until 8 May, after 20,000 metric tons of highly radioactive water were pumped out.