During the April 1986 accident in the 1000-MW(e) RMBK Chernobyl reactor, the water in the cooling tubes of the graphite-moderated reactor was allowed, through operator error, to boil into steam and cause a supercritical, run-away chain reaction. The resulting energy excursion resulted in the destruction of the reactor containment and a large amount of the fission products in the fuel elements were released to the environment as the reactor containment ruptured. Explain why the boiling in the cooling tubes led to supercriticality.