The containment cooling system (see Fig. II-6) includes design features for circulation and cooling of the RB atmosphere both during normal operation and following an accident.

During normal operation local air coolers (LACs) operate to remove heat from the RB atmosphere. Two fuelling machine vaults and steam generator enclosures are atmospherically isolated from the rest of the RB.

Following an accident large airflow paths are established by, interconnecting the fuelling machine vaults and steam generator enclosures with the rest of the RB, permitting natural circulation flows to mix the RB atmosphere, without the need for fans or other active components (i. e. LACs), .This feature allows hydrogen dispersion/dilution throughout the larger RB volume and prevents formation of regions of locally high temperature. Hydrogen control design features are provided to restrict the concentration of hydrogen to below the limit for deflagration or detonation.

If needed, post-accident pressure and temperature suppression is performed by a containment cooling spray system supplied from the RWS.

A combination of passive and active features are provided in the ACR-1000 for atmospheric hydrogen control; passive auto catalytic recombiners, and active igniters (or ‘glow plugs’) that limit the concentration of hydrogen in the RB atmosphere to below the threshold limit at which deflagration or detonation could occur.

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.

Nuclear reactors that implement natural circulation passive safety systems may produce large temperature gradients in their working fluid as a result of local cooling caused by emergency core coolant (ECC) injection or local heating caused by steam condensation or heat exchanger heat transfer. Thermal stratification arises because the low flow condition typically encountered in a natural circulation system greatly reduces the amount of fluid mixing that can occur. Examples of thermal stratification during ECC injection include the formation of cold plumes in the downcomer, and liquid thermal stratification in the lower plenum, cold legs and loop seals.

ECC injection into horizontal piping partially filled with steam also results in liquid temperature stratification. The cooler liquid condenses the steam forming a saturated layer of liquid water on top of the sub-cooled liquid layer. This saturated layer is at a higher temperature than the sub-cooled layer, resulting in a stratified temperature condition. The formation of the saturated layer may mitigate occurrences of condensation-induced water hammer (CIWH) events.

Liquid temperature stratification can also arise in passive safety systems such as the natural circulation driven core make-up tanks (CMT) and the large liquid-filled tanks that serve as the heat sink for reactor core or containment passive cooling systems. Steam vented into the large safety tanks condenses in the cold liquid producing hot rising plumes that form thermal layers at the free surface of the tank. Thermal layers having different temperatures grow with time to create a large temperature gradient in the liquid.

The most effective means of describing natural circulation in the HTS, Calandria and containment is to integrate their operation in response to a large break loss of coolant accident (LOCA). The first phase of a LOCA is the subcooled blow down phase. During this phase, high pressure subcooled liquid is vented from the break under choked flow conditions. The primary HTS pressure and liquid inventory will decrease. When a LOCA is detected, the following automatic actions are initiated:

• Shut down the reactor,

• Initiation of the ECC system,

• Open the RWS valves to HTS,

• Crash cooldown (rapid depressurization of the steam generators),

• Isolate the broken loop of the HTS from the intact loop, and

• Trip the HTS pumps.

II-3.1. Decay heat removal — broken loop

Following reactor shutdown, the HTS loops are isolated and crash cooldown (rapid depressurization of the secondary side of the steam generators) is initiated. This depressurizes the HTS and facilitates refill of the broken loop by either the ECC system, or by gravity from the RWS (as a back-up to ECC).

Heat is removed from the broken loop by the LTC system in recovery mode (see Section II-2). If the LTC system fails (low probability event), the fuel and fuel channels of the broken loop will heat up until the pressure tubes sag into contact the Calandria tubes, resulting in direct conduction of heat to the moderator (refer to Section II-2.1). The moderator cooling system, assisted by natural circulation in the Calandria, serves as a back-up heat sink, preventing further core damage.

If the moderator fails as a heat sink (e. g. if moderator make-up from the RWS fails), the fuel channels will fail. The shield vault contains a large volume of water that can cool and contain the damaged fuel channels inside Calandria and halt the progress of the accident. Inventory is allocated in the RWS to make-up inventory boiled off from the shield vault during this time. As long as make-up can be provided to the shield vault to cool the fuel, Calandria vessel integrity will be maintained and corium — concrete interaction will be prevented.

The cross connection between passive systems described in Sections 2 and 3 and phenomena identified in Section 4.1 can be derived from Table 2, namely considering the first and the last column. This is consistent with the second bullet under the heading of the present section as elements of aims of Section 4.

In addition, an effort has been made to homogenize the nomenclature adopted by different designers: having defined the type of passive safety system (for core decay heat removal and for containment cooling and pressure suppression) and the related (key) phenomena, in column 3 of Table 2. The passive safety systems are listed and named according to the nomenclature provided by the designers (Annexes I to XX).

In the column 2 of Table 2 the passive safety systems are foreseen in ‘various advanced water cooled nuclear power plants’ (part I of annexes of the present document, i. e. Annexes I to XIII) are distinguished from ‘integral reactor systems’ (part II of annexes of the present document, i. e. Annexes XIV to XX).

As in the case of the previous table, the content of Table 2 is self-standing and directly understandable including the supporting description provided in Sections 2 and 3 and in Annexes I to XX (as already mentioned). The following additional items should be noted:

• An attempt has been made to list in the column 2 all passive systems installed in the reactors described in Annexes I to XX. However, the current stage of the design and the different level of detail in the descriptions prevent the possibility of an imperfection-free list. This is particularly true in relation to the accumultors (first row in the table).

• The variety of definitions adopted by designers for passive systems is wider than what is considered here.