In a severe accident in a CANDU reactor, disassembly of the core could produce a bed of coarse debris at the bottom of the calandria that would eventually transition into a pool of molten corium. During this process, it may be possible for small amounts of molten core material to contact the calandria wall. The transient heat flow through a calandria wall suddenly contacted by molten Zr or corium is analyzed with a finite element model. Ablation of the wall at its inner surface and the temporary increase in heat flux through its outer surface are calculated for various boundary conditions. Model calculations are compared to observations of the ablation and temperature of a stainless steel plate sprayed by prototypic corium in the Cesium Aerosol Generation-4 or CAGE-4 experiment.