Accident Conditions to be Considered for 'Practical Elimination'. Appendix 4

A sudden mechanical failure of a single large component in the reactor coolant system (RCS) could initiate an event where reactor cooling would be lost in a short time and a pressure wave or a missile would damage the containment boundary. The defence in depth provisions would not be effective in such situation and an early large radioactive release would follow. This is a very exceptional type of initiating event for which safety systems and safety features are not designed for their mitigation and therefore it needs to be demonstrated that their likelihood would be certainly so low that they can be excluded, i.e. 'practically eliminated', from consideration. This is essential at least for the reactor vessel, which break would eliminate the capability of holding and cooling the core but also the likelihood of pressurizer and steam generator (SG) shell failure need to be shown to be extremely low, or alternatively it needs to be demonstrated that a failure of pressurizer or SG would not lead to unacceptable consequences to the containment