Thermal interaction during the Three Mile Island Unit 2 2-B coolant pump transient is considered as an explanation of the partial melting of the plenum assembly. The primary system pressure response is used to calculate the energy transferred from the reactor core to the coolant and the steam generation rate during the pump transient. The steam generation rate is then used to calculate the heating of the plenum assembly. An energy balance indicates that exothermic oxidation of Zircaloy by steam must have taken place. In calculations of plenum assembly heating, the thermal emissivity of steam is reduced to simulate the effect of hydrogen generated during the oxidation process. It is determined from the calculations that in the presence of an appreciable amount of hydrogen (∼60%), only the thin structures at the lower end of the plenum assembly would have melted, while the relatively thick structures would have been partially ablated. These results are consistent with the observed damage.