Severe accident assessment: Results of the reactor safety research project VAHTI

The report provides a summary of the publicly funded nuclear reactor safety research project "Severe Accident Management" (VAHTI). The project has been conducted at the Technical Research Centre of Finland (VTT) during the years 1994-96. The main objective of the project was to assist the severe accident management programmes of the Finnish nuclear power plants, but the project scope also included thermal hydraulic development. The project consisted of several subprojects. The thermal hydraulic subprojects have validated models of the Advanced Process Simulation Environment, APROS. APROS thermal hydraulic description has been extended to plant shutdown conditions. The project has also contributed improvements in APROS containment modeling for severe accident simulation and calculation of tests done with the PACTEL thermal hydraulic facility. The RELAP5 code has been used to support APROS development. A computer code PASULA has been developed for analysis of core melt-pressure vessel interactions. The code has been applied to pressure vessel penetration analysis. The results showed the importance of the nozzle construction to the pressure vessel failure mode. A creep model and possibility to simulate porous debris have been added into the PASULA code. Cooling of a degraded core and BWR recriticality aspects have been studied as joint Nordic projects with a set of severe accident codes. The core was estimated coolable if maximum cladding temperature at the beginning of reflooding was below 1800 K. Reflooding could not terminate accident progression in the lower head after dryout. Reflooding of a core partly free of control rods gives a power peak of a high amplitude, but short duration. Aerosol experiments have been conducted with the AHMED test vessel. The experiments were successful, achieving homogeneous temperature and humidity. 17 aerosol test were performed using NaOH, CsOH, CsI and Ag aerosol species. The data have been used by developers of aerosol models. A CSNI calculation exercise has been arranged based on the AHMED tests.