The first 174 min of the Three Mile Island Unit 2 accident comprise the first two phases of the accident, starting from full-power operation and ending with severe fuel damage without recovery actions. Operator actions and plant initial and boundary conditions for this time period were developed by EG&G Idaho to provide a standard input for benchmarking severe accident codes. These standard plant parameters and accident boundary conditions were used with MAAP 3.0B to simulate the accident progression up until 174 min, when the first core recovery attempt was made. All the inputs were taken from the published package, and only nominal values of the modeling parameters were used. Excellent agreement with most data is observed for most of the simulated period. This simulation effort reveals two key phenomena that require attention during severe accidents:

  1. hydrogen generation and transport in the primary system
  2. flow through the pressurizer surge line when the hot legs are voiding.
Specific models for these phenomena are key to the successful simulation of the accident.