A scoping study on a confinement approach to a PBMR type plant was performed to quantify its performance in terms of retention and thermal-hydraulics in case of a postulated breach in the Helium Pressure Boundary. This paper introduces a simulation strategy for accident analyses of a PBMR confinement. Based on two accident sequences characterized by Helium Pressure Boundary breaches, a small (AOO) and a large (DBA) break, the estimates from the ASTEC and CONTAIN codes are presented and compared to check their capabilities and consistency. The results obtained indicate that both codes predict very similar thermal-hydraulic responses of the confinement both in magnitude and timing. As for the aerosol behaviour, both codes predict that most of inventory coming into the confinement is eventually depleted on the walls and only about 1% of the aerosol dust is released to environment. The cross-comparison of codes states that largest differences are in the inter-compartmental flows and the in-compartment gas composition. Concerning the capabilities of the codes, CONTAIN has shown to be more robust in dealing with the injection of large aerosol mass flow and with a very small injection of fission products. ASTEC however has an ad-hoc model for aerosol retention in filters.

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