Thermal-hydraulic phenomena and related parameter ranges that characterize the passive systems do not differ, in general, from phenomena that characterize the systems equipped with active components. This is specifically true for transient conditions occurring during safety relevant scenarios.

In other words, one can say that friction pressure drops or heat transfer coefficients are affected by local velocity and void fraction and not by the driving force that establishes those conditions, e. g. gravity head or centrifugal pump. The same can be repeated for more complex phenomena like two phase critical flow or counter-current flow limiting.

Thus, a large number of thermal-hydraulic phenomena that are expected to occur in passive systems during accident are classified in the OECD/NEA/CSNI documents ‘separate effect’ (SE) and ‘integral effect’ (IE) reported as references 19 and 20, hereafter. However, specific layout of passive systems and combination of parameter ranges brought the need of expanding the original list of phenomena in the same references 19 and 20. This was done in reference 21, where, mainly the passive systems proposed at the time of issuing of the report (1996) were considered.

The ‘expanded’ OECD/NEA list of phenomena for passive systems was up-graded in IAEA CRP on Natural Circulation Phenomena, Modeling and Reliability of Passive Safety Systems that Utilize Natural Circulation, considering the recently proposed passive systems by the industry. The description of the individual phenomena is given in the Appendix for the sake of completeness.

The identification and characterization of additional (i. e. with reference to the original SE and IE lists) phenomena for passive systems is presented in Table 1, which includes two main columns, other than the first column with numbering, which is consistent with the phenomena numbering in reference 19:

• Column 2: phenomena identification;

• Column 3: phenomena characterization based upon the individual phenomena description in the Appendix, considering the key layout of systems described in Sections 2 and 3.

The content of Table 1 is self-standing and directly understandable including the supporting description provided in the Appendix (as already mentioned). However, the following additional items should be noted:

• Specific geometry configurations or range of variations of affecting thermal-hydraulic parameters justify the presence of phenomena at rows 2, 5, 6 7 and 14 in both the present list and the list in reference 19. This is specifically true in the case of phenomenon 6 (natural circulation) that is expected to occur whenever a gravity environment exists.

• Natural circulation is also at the origin of the Core-make up Tank performance, phenomenon at row 15 in Table 1. However, the simultaneous presence of stratification in the tank, the possible condensation with level formation inside the tank, the specific loop connection and the values of boundary and initial conditions, suggest the consideration of a separate phenomenon.

• The phenomenon at row 3 is a containment related phenomenon: the phenomena discussed in the (OECD/CSNI) report at reference 22 should be connected for completeness with the present one.

• The phenomenon at row 9 is also relevant for characterizing the phenomenon at row 1: geometry peculiarities and boundary conditions suggest keeping two separate phenomena in the present list.

• The accumulator behaviour is at the origin of an individual phenomenon considered in reference 17. Furthermore, the accumulator performance is driven by gas pressure. However, other than the ‘passive nature’ for the component behaviour, similarity in geometrical configuration and in the ranges of variations of relevant parameters suggested to consider ‘accumulator behaviour’ together with ‘gravity flooding’.

• The list of relevant thermal-hydraulic aspects in the 3rd column of the table can be expanded consistently with descriptions in Sections 2 and 3, and, also, in the Appendix: an effort to make fully comprehensive or exhaustive, the contents of the information in this column has not been attempted.