**7. Final remarks**

Based on the analysis of the critical aspects related to the open points discussed in the previous section a qualitative analysis, on the basis of the author's opinion, reported in Table 5 below aims at identifying for each of the above items both the criticality with respect to the passive system reliability assessment process, in terms of the relative importance and the existing advancement, according to Table 6 which ranks the relative level of both the importance and progress.


**Table 5.** Importance analysis

54 Nuclear Power – Practical Aspects



uncertainties modeling.

circulation start-up

**7. Final remarks** 

These characteristics of a high level of uncertainty and low driving forces for heat removal purposes justify the comparative evaluation between passive and active options, with respect to the accomplishment of a defined safety function (e.g. decay heat removal) and the generally accepted viewpoint that passive system design is more reliable and more

Here are some of the benefits and disadvantages of the passive systems that should be

Passive systems must be designed with consideration for ease of ISI, testing and

Reliance on "low driving forces", as a source of uncertainty, and therefore need for T-H

 Licensing requirement (open issue), since the reliability has to be incorporated within the licensing process of the reactor. For instance the PRA's should be reviewed to

 Need for operational tests, so that dependence upon human factor can not be neglected. Time response: the promptness of the system intervention is relevant to the safety function accomplishment. It appears that the inception of the passive system operation, as the natural circulation, is conditional upon the actuation of some active components (as the return valve opening) and the onset of the conditions/mechanisms for natural

 Reliability and performance assessment in any case. Quantification of their functional reliability from normal power operation to transients including accidental conditions needs to be evaluated. Functional failure can happen if the boundary conditions deviate

 Ageing of passive systems must be considered for longer plant life; for example corrosion and deposits on heat exchanger surfaces could impair their function. Economics of advanced reactors with passive systems, although claimed to be cheaper,

The question whether it is favourable to adopt passive systems in the design of a new reactor to accomplish safety functions is still to be debated and a common consensus has not yet been reached, about the quantification of safety and cost benefits which make nuclear power more competitive, from potential annual maintenance cost reductions to safety system response.

Based on the analysis of the critical aspects related to the open points discussed in the previous section a qualitative analysis, on the basis of the author's opinion, reported in

from the specified value on which the performance of the system depends.

must be estimated especially for construction and decommissioning.

 No external power supply: no loss of power accident has to be considered. No human factor, implying no inclusion of the operator error in the analysis. Better impact on public acceptance, due to the presence of "natural forces". Less complex system than active and therefore economic competitiveness.

economical than active system design has to be discussed [27].

maintenance so that the dose to the worker is much less.

determine the level of uncertainty included in the models.

evaluated vs. the correspondent active system.


**Table 6.** Grade rank for importance and advancement analysis

It is clear that he worst case is characterized by "high "and "low" rankings relative respectively to the importance and the advancement aspects, thus making the correspondent item development a critical challenge.

Based on this, the results of this qualitative analysis show the relevance relative to the uncertainties and the comparison between active and passive, as most critical points to be addressed in the application of the PRA to the evaluation of the passive system performance assessment. This allows the analyst to track a viable R&D program to deal with these issues and limitations and to steer the relative efforts towards their implementation.

### **8. Conclusions**

Due to the specificities of passive systems that utilize natural circulation (small driving force, large uncertainties in their performance, lack of data…), there is a strong need for the development and demonstration of consistent methodologies and approaches for evaluating their reliability. This is a crucial issue to be resolved for their extensive use in future nuclear power plants. Recently, the development of procedures suitable for establishing the performance of a passive system has been proposed: the unavailability of reference data makes troublesome the qualification of the achieved results. These procedures can be applied for evaluating the acceptability of a passive system, specifically when nuclear reactor safety considerations are important for comparing two different systems having the same mission and, with additional investigation, for evaluating the performance of an active and passive system on a common basis. The study while identifying limitations of the achieved results or specific significant aspects that have been overlooked has suggested areas for further development or improvements of the procedures:

Reliability of Passive Systems in Nuclear Power Plants 57

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Future research in nuclear safety addressing this specific topic relevant to advanced reactors should be steered towards all these points in order to foster and add credit to any proposed approach to address the issue and to facilitate the proposed methods endorsement by the scientific and technical community.
