Break preclusion approach. Expertise and safety reference document 2019

French Pressurized Water Reactors are designed accordingly to the defense-in-depth principle. Safety approach considers equipment failure notwithstanding design and manufacturing requirements that are applied to prevent their failure. Difficulties to limit the consequences of some PWR pieces of equipment break have led not to postulate the corresponding event in safety assessments. Then, absence of realistic and demonstrable provisions to limit the consequences according to current knowledge and the techniques available is justified by implementation of particularly stringent requirements for design, manufacturing and both initial and in-service reinforced monitoring aiming at guaranteeing prevention of these pieces of equipment break: this is the 'break preclusion' approach. Regarding pressurized water reactors, these cases of 'break preclusion' are related to large components of the main primary system and main secondary systems. Concerning pipework, 'break preclusion' approach can only be considered for main primary pipes and main secondary steam pipes: 'break preclusion' is then a design choice. According to ASN guide no. 22 produced jointly with IRSN, this choice may be considered acceptable only on the basis of a justification of major advantages for safety. The possibility to apply 'break preclusion approach' shall be assessed at an early stage of the design considering that this choice is structuring for the design.