III-17. Contact between molten core material and concrete in the reactor cavity will result in molten core-concrete interactions. This process involves the decomposition of concrete from core debris and can challenge the containment by various mechanisms, including the following:

(a) Pressurization as a result of the production of steam and non­condensable gases to the point of containment rupture;

(b) Transport of high temperature gases and aerosols into the containment, leading to high temperature failure of the containment seals and penetra­tions;

(c) Melt-through of the containment liner or the basemat;

(d) Melt-through of reactor support structures, leading to relocation of the reactor vessel and the tearing of containment penetrations;

(e) Production of combustible gases such as hydrogen and carbon monoxide.

Molten core-concrete interactions are affected by many factors, including the availability of water in the reactor cavity, the geometry and physical layout of the containment, the composition and amount of the core debris, the temperature of the core debris, and the type of concrete.

III—18. Potential longer term challenges to the containment involve slow releases of mass and energy, typified by the generation of decay heat and non­condensable gases. The risks associated with these specific challenges can be judged on the basis of probabilistic safety assessments and research studies on severe accidents relevant to the specific design of the plant. Generally, the effectiveness of any proposed design feature can be assessed by means of a combination of probabilistic safety assessment, best estimate models and computer codes, together with consideration of the effects of initial boundary conditions and uncertainties in the modelling.

III-19. The long term pressurization of the containment may also be affected by the availability or unavailability of containment sprays (or heat exchangers) and air coolers.

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