5.44. Highly energetic severe accident conditions with the potential for damaging the containment should be virtually eliminated for new plants. Reliable depressurization of the reactor coolant system to prevent the ejection of molten core material and core debris and direct containment heating should be ensured as an accident management measure for existing and new plants.

5.45. The interaction of molten core material with water can cause highly energetic events (e. g. steam explosions; see para. III-9 of Annex III). There is an international consensus that in-vessel interactions of this type are unlikely to cause a containment failure, however, and that therefore no specific provisions are necessary. The effects of ex-vessel steam explosions are plant specific and are more difficult to predict. Therefore, for a specific plant design, if it cannot be shown that the threat associated with a steam explosion is low, special care should be taken in defining accident management provisions to balance the risk of a steam explosion with the necessity to cool the molten core material.

5.46. The combustion or deflagration of hydrogen, which would be potentially damaging to the containment systems, should also be dealt with by means of prevention (see also paras 6.22-6.27).

5.47. In the course of a postulated severe accident, the residual heat must be removed to prevent damage to the containment. Since the various cooling systems may not be operable, guidelines for the management of severe accidents should be developed for existing plants to help restore adequate core cooling and to reach a controlled state (paras 6.28-6.34). To this end, all possible means should be considered, including the unconventional use of safety systems and other plant equipment. If (probabilistic) analyses show that the risk of containment overpressurization is still too high for existing plants, the installation of a filtered containment venting system to prevent irreversible damage to the containment and uncontrolled releases of radioactive material should be considered.

5.48. For new plants an energy management system should be incorporated as the primary means of meeting the Level II acceptance criteria for structural integrity for loads derived from the pressures in the containment during accidents, as discussed in para. 6.10. In severe accidents, the systems for energy management in the containment and their support systems (the cooling water systems and power supply systems) should be independent of the systems used to prevent melting of the core. If this is not the case, the design of the containment should provide a sufficient period of time for measures to recover failed systems for energy management so as to be able to guarantee the operability of the energy management system under severe accident conditions. Venting systems should not be necessary for new plants.