GENERAL APPROACH TO SITING AND DESIGN

Two ideal approaches may be used for siting and design of a nuclear installation, either a full deterministic approach or a full probabilistic approach. However, both approaches have drawbacks  

In conclusion, the engineering practice for research reactor design suggests that neither approach may be completely rehable nor effective, and therefore the following mixed approach is proposed, in agreement with Ref. [1]. This proposal is more in line with current practice in Member States, where a simplified risk informed probabilistic approach is used to clarify the use of a more code oriented deterministic design. 

The proposed sequence relies on some degree of conservatism in classification and design in order to avoid any further iteration on the design as a consequence of the radioactive dispersion analysis. The approach is more straightforward, even if it relies on engineering practice in the choice of the conservatism level to avoid loop back from the radiation doses to siting and design of the facility. 

The safety margin and conservatism (or robustness) are validated by the engineering experience and are driven by the design class method and by a series of deterministic assumptions at all phases of the siting and design process. Details of the multi-step approach are provided in Fig. 1. 

Step 2 The safety classification of structures, systems and components reflects the internal postulated events and external events as set forth in the safety analysis of the plant (box (3) of Fig. 1). The definition of the defence in depth levels and barriers [2], the application of the single failure criterion and the assessment of the potential for common cause failures are identified in box (2) of Fig. 1 [19], bearing in mind the categorization of the facility. Next is the evaluation of the need for emergency procedures, both on and off the site. This is followed by identification of the internal events to be considered as a consequence of an external event or as contemporaneous to an external event, and therefore of the safety functions to be maintained in case of an external event (e.g. cooling of radioactive material, reactivity control, confinement). 

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