The effect of airborne hygroscopic matter on aerosol behavior in severe nuclear power plant accidents: Dissertation

The growth of hygroscopic particles due to steam condensation has been studied.Sophisticated and simplified models for particle growth have been compared to see what sort of approximations can be used in the safety analyses of severe LWR accidents.A model suitable for evaluating condensational growth in containment conditions has been developed and incorporated in the NAUA containment aerosol program.The hygroscopic growth has been studied experimentally and the results of the NAUA-HYGROS computer program developed here have been compared with the experimental results.For heat and mass transport the classical Mason equation compared well with the more accurate solutions at the expected thermal-hydraulic conditions in the containment atmosphere.For applications with short residence times at high humidities, where the Mason equation is not strictly valid, a more accurate solution for mass and heat transfer to the particle has been developed.A conservative model for the dissolution of multicomponent particles at high relative humidities has been developed and included in the NAUA-HYGROS code.The development of NAUA-HYGROS was closely related to the LACE experiments, where the behavior of hygroscopic CsOH and non-hygroscopic MnO aerosols in a 852 m³ containment vessel was studied.The code results agreed well with the measured values. However, the difficulty in measuring the steam partial pressure with the accuracy needed for the code input was recognized.The comparison of the results from the Modular Accident Analyses Program (MAAP) with the LACE experimental results showed the invalidity of the simple model used.In MAAP it had been assumed that the hygroscopic particles and the gaseous phase are in immediate equilibrium and that all particles are of equal size.An experimental set-up for measuring the behavior of hygroscopic particles in a flow type reaction chamber was developed and constructed.The time development of initially dry CsOH and NaOH particles was measured at different relative humidities.The most critical parameters affecting the particle size were found to be the relative humidity and the residence time in the chamber before sampling.Theoretical calculations with the NAUA-HYGROS code showed that during travel through the chamber, the particle size change can be attributed to hygroscopic growth and sedimentation.