.	Basic principles
•	Challenges and risks
•	Operation objectives
•	Definition of safety
•	Defense in depth
•	Three safety functions
•	Development of safety
•	Summary
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4.1 Definition

The Three lines of defense that are normally provided for each failure envisaged are the following:

• Prevention
• Monitoring
• Means of actions

However, all three lines of defense are not always possible. For instance, there are no means available for mitigating the consequences of rupturing the reactor or pressure vessel. In this case, other lines of defense must then be reinforced.

Consequently, there are multiple lines of defense to provide for "prevention" such as the following:

• Choice of materials
• Safety margins in the design basis
• Extremely stringent manufacturing verifications

There are also multiple lines of defense for "monitoring" such as:

• Non-destructive testing during full and partial inspections
• Expert appraisal of irradiated specimens to monitor the ageing of the pressure vessel material

The lines of defense must be as reliable and as independent as possible.

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