| Summary: | The effect of water ingress in two pebble bed high temperature gas-cooled reactors
i.e. the PBMR-200 MWthermal and the PBMR-400 MWthermal were simulated and
compared using the VSOP 99/05 suite of codes.
To investigate the effect of this event on reactivity, power profiles and thermal
neutron flux profiles, the addition of partial steam vapour pressures in stages up to
400 bar into the primary circuit for the PBMR-400 and up to 300 bar for the PBMR-
200 was simulated for both reactors. During the simulation, three scenarios were
simulated, i.e. water ingress into the core only, water ingress into the reflectors only
and water ingress into both the core and reflectors. The induced reactivity change
effects were compared for these reactors.
An in-depth analysis was also carried out to study the mechanisms that drive the
reactivity changes for each reactor caused by water ingress into the fuel core only,
the riser tubes in the reflectors only and ingress into both the fuel core and the riser
tubes in the reflectors.
The knowledge gained of these mechanisms and effects was used in order to
propose design changes aimed at mitigating the reactivity increases, caused by
realistic water ingress scenarios. Past results from simulations of water ingress into
Pebble Bed Reactors were used to validate and verify the present simulation
approach and results. The reactivity increase results for both reactors were in
agreement with the German HTR-Modul calculations. === Thesis (MSc (Engineering Sciences in Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013
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