MAAP5 ENHANCEMENTS FOR FUKUSHIMA DAIICHI ACCIDENT SIMULATION

摘要

This paper describes the planned MAAP5 code enhancements performed under the MAAP5 enhancement project. The main purpose of the project is to provide prediction of the location and state of core materials in Fukushima Daiichi - reactor units 1, 2, and 3. The key areas of MAAP5 enhancements are (1) BWR reactor coolant system thermal-hydraulics, (2) BWR core melt progression, (3) BWR lower plenum models, (4) Traveling in-core probe (TIP) tube failure model, and (5) Debris coolability and MCCI model. Improved BWR thermal hydraulics model for pressure and water level calculations and new refined RCS nodalization as well as salt and boron tracking calculations will be implemented. All potential core relocation paths to lower plenum have been examined and model enhancements for core melt progression have been identified. The enhanced lower plenum model includes radial and axial nodalization of the lower plenum debris bed taking into consideration the presence of a forest of CRD tubes which can behave differently in different radial zones. The CRD tubes in each radial zone will also axially interact with the nodalized stratified debris bed. A new mechanistic model for TIP tube failure that opens flow paths from the RCS to the containment will be implemented. A more realistic semi-2 -dimensional MCCI modeling approach for BWR pedestal sumps with improved convective heat transfer has been identified.