.	Safe Design
•	Two approaches
•	Acceptable risks
•	Events accounted
•	Application to these risks
•	Prevention of failures
•	Protection against events
•	Summary
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The deterministic approach

The approach used in the design of nuclear power plants was initially deterministic. It consisted of listing the events which could lead to the dispersal of radioactive material into the environment so as to establish "upper boundary" accidents. In other words, accidents representing the worst cases in terms of consequences for the environment. Estimates were made of these upper boundary accidents and the defense in depth concept was used to limit the risks to acceptable levels.

The deterministic approach was subsequently complemented with the probabilistic approach.

The probabilistic approach

Probabilistic Safety Assessments (PSAs) make it possible to assess the probability of accident scenarios. The probabilistic approach allows for the probability of success or failure of the safety functions designed to mitigate the consequences of the initiating events to be determined based on the frequency of occurrence of initiating events (transients, breaks, steam generator tube ruptures etc.). The probability of success or failure depends on the reliability of the systems as determined from the experience acquired in operation and the reactions of operators during simulator tests.

It is therefore possible to calculate the probability of each accident scenario leading to core meltdown (level 1 PSA) or even to releases into the environment (Level 2 PSA incorporates the probabilities of success or failure of the "containment" function). This method has revealed certain weak points in the design, which have since been corrected. It has shown the need to allow for events which were not taken into consideration during the initial design, such as the total loss of electrical power. This is why additional provisions such as the H procedures (out of design basis procedures), or complementary procedures, were introduced.

The incorporation of severe accidents (with core meltdown) has resulted in the development of additional resources in the form of the U (ultimate) procedures.

The H and U procedures are described in Chapter 4: Safe Operation (What?).