So, when the reactor is isolated from the turbine rapidly, pressure in the vessel rises rapidly, which collapses the water vapor, which causes a power excursion which is terminated by the Reactor Protection System.
12.
The lower portion of the reactor remained somewhat intact; the graphite neutron moderator was exposed to oxygen-containing air; heat from the power excursion in addition to residual heat flux from the remaining fuel rods left without coolant induced oxidation in the moderator and in the opened fuel rods; this in turn evolved more heat and contributed to the melting of more of the fuel rods and the outgassing of the fission products contained therein.
13.
In the Chernobyl disaster, the moderator was not primarily responsible for the primary event; instead, a massive power excursion during a mishandled test caused the catastrophic failure of the reactor vessel and a near-total loss of coolant supply, and as a result, the fuel rods rapidly melted and flowed together while in an extremely-high-power state, causing a small portion of the core to reach a state of runaway prompt criticality and leading to a massive energy release, resulting in catastrophic disassembly of the reactor core and the destruction of the reaction building.
