• Severe Accident Phenomena
• Application of In-Vessel Retention Strategy
• International Reactor Innovative and Secure (IRIS)

The objective of this study is the evaluation of most suitable heat removal system for IRIS (between the AP600 and AP1000 solutions) in order to ensure In-Vessel Retention success. The quantification of margin to failure due to thermal stresses is calculated by using the Risk Oriented Accident Analysis Methodology (ROAAM), already used for AP600 and AP1000 calculations (Reference 6,7).
The ROAAM program is a semi-deterministic approach that evaluates the structural and thermal challenges to the vessel during the relocation of corium to the lower head. For AP600 and AP1000 power plants, the ROAAM applications concluded that the limiting challenge to the vessel integrity is the thermal loading produced during the steady-state heat transfer to the lower head wall after complete debris relocation to the lower plenum. Based on these results, reactor vessel failure in the AP600 and AP1000 was considered to be physically unreasonable (under supposed conditions).
Also for IRIS the thermal loading after complete debris relocation has been supposed the limiting challenge to the vessel integrity. Application of ROAAM code, carried out in this work, showed that, for IRIS, margin to the critical heat flux is larger than for AP1000, and then, the vessel failure in a severe accident can be supposed physically unreasonable.