Three Mile Island Unit 2 Degraded Core Heatup and Cooldown Analysis

A two-dimensional finite element model was developed to simulate the Three Mile Island Unit 2 core heatup between 174 and 224 min and the subsequent cooling of the consolidated core region after the relocation of ≃25 tonnes of core material to the lower plenum of the reactor at 224 min. The model considered heat losses at the surfaces of the degraded core zone, core material melting, convective heat transfer within the molten pool, and decay heat reduction from the release of the volatile fission products. The results obtained from the model indicate that at least 17% of the consolidated core material must have been molten at 174 min in order to generate the ≃25 tonnes of core material that relocated at 224 min. The cooldown calculation indicated that as long as the core remained covered with coolant, the core configuration would remain thermally stable with pool cooldown beginning at ∼324 min after the initiation of the accident.