Regional Basic Professional Training Course in Korea

Pressurization of the containment
❙ 931 ❙
16. In‐Plant Accident Management
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.
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.
16.4.3.3. Loss of containment integrity [3, 7]
The loadings which can fail the containment are classified into two groups in terms of the
timing of containment failure: early and late. Hydrogen combustion‐detonation, steam
explosion, direct containment heating (DCH) were identified as the energetic processes
which could fail the containment early. The longer‐term gas producing MCCI, which
would pressurize the containment and the lack of coolability were identified as the
processes, which could fail the containment later.
Such distinction is needed to taking into account the natural processes such as deposition
of the fission product aerosols that remove most of them in the containment atmosphere
within approximately 4 hours. Therefore, a late release (after 24 hours) may be 4 to 5
orders of magnitude smaller, than that for the early failure of the containment. In this
context, the USNRC requires that for the new reactors the containment should maintain
its integrity for a minimum period of 24 hours following the onset of core damage, and
that following this 24‐hour period the containment should continue to provide a barrier
against the uncontrolled release of fission products.