This review of severe accidents that can result in air ingress into the Reactor Coolant System (RCS) investigates both late phase sequences with lower head failure and mid-loop operation sequences. Effects due to the air ingress into the vessel are investigated, focusing on the thermal-hydraulic phenomena in such plant conditions and highlighting the role of the natural circulation both inside the vessel and between the vessel and the other main components of the RCS. Chemical phenomena related to air ingress are then analysed, mainly addressing the mechanisms and the effects of the interactions between air and Zry cladding, air and UO2 fuel interactions, and the influence of fuel oxidation on the fission products (FP) release. Finally, considering that the primary reason of interest on air ingress into the RCS during a severe reactor accident is in the release and the subsequent transport of ruthenium, whose radiotoxicity is high for both short term and long term accident consequences, some considerations have been carried out about the Source Term modifications due to sequences evolution in “air rich” instead of traditional “steam rich” conditions. This also considers the increase in the ruthenium inventory inside the core, calculated by specific ORIGEN-2 and CASMO analyses, for high burn-up fuel elements, because of the current trend in the nuclear industry is for higher burn-up in order to reduce fuel cycle costs.
Current Understanding of Air Ingress into the Vessel and Influence on Core Degradation Progression
S. Paci;
1997-01-01
Abstract
This review of severe accidents that can result in air ingress into the Reactor Coolant System (RCS) investigates both late phase sequences with lower head failure and mid-loop operation sequences. Effects due to the air ingress into the vessel are investigated, focusing on the thermal-hydraulic phenomena in such plant conditions and highlighting the role of the natural circulation both inside the vessel and between the vessel and the other main components of the RCS. Chemical phenomena related to air ingress are then analysed, mainly addressing the mechanisms and the effects of the interactions between air and Zry cladding, air and UO2 fuel interactions, and the influence of fuel oxidation on the fission products (FP) release. Finally, considering that the primary reason of interest on air ingress into the RCS during a severe reactor accident is in the release and the subsequent transport of ruthenium, whose radiotoxicity is high for both short term and long term accident consequences, some considerations have been carried out about the Source Term modifications due to sequences evolution in “air rich” instead of traditional “steam rich” conditions. This also considers the increase in the ruthenium inventory inside the core, calculated by specific ORIGEN-2 and CASMO analyses, for high burn-up fuel elements, because of the current trend in the nuclear industry is for higher burn-up in order to reduce fuel cycle costs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
