**2.1. AP600/AP1000 Passive Residual Heat Removal systems (PRHR)**

Figure 1 presents a schematic that describes the connections of the primary system passive safety systems.

**Figure 1.** Passive Safety Systems used in the AP600/AP1000 Designs

The AP600/AP1000 passive safety systems consist of:


26 Nuclear Power – Practical Aspects

the case of systems including pumps. Moreover, there are considerable uncertainties associated with factors on which the magnitude of these forces and counter forces depends (e.g. values of heat transfer coefficients and pressure losses). In addition, the magnitude of such natural driving forces depends on specific plant conditions and configurations which could exist at the time a system is called upon to perform its safety function. All these

Consequently, a lot of efforts have been devoted mostly to the development of consistent approaches and methodologies aimed at the reliability assessment of the T-H passive systems, with reference to the evaluation of the implemented physical principles (gravity, conduction, etc.). For example, the system fault tree in case of passive systems would consist of basic events, representing failure of the physical phenomena and failure of activating devices: the use of thermal-hydraulic analysis related information for modeling the passive

The efforts conducted so far to deal with the passive safety systems reliability, have raised an amount of open issues to be addressed in a consistent way, in order to endorse the proposed approaches and to add credit to the underlying models and the eventual reliability figures, resulting from their application. In fact the applications of the proposed methodologies are to a large extent dependent upon the assumptions underlying the methods themselves. At the international level, for instance, IAEA recently coordinated a research project, denoted as "*Natural Circulation Phenomena, Modelling and Reliability of Passive Systems"* (2004-2008), [2,3], while another coordinated research project on "*Development of Methodologies for the Assessment of Passive Safety System Performance in Advanced Reactors"* (2008-2011) is currently underway: while focus of the former project has been the natural circulation and related phenomena, the objective of the latter program is to determine a common analysis-and-test method for reliability assessment of passive safety system performance. This chapter provides the insights resulting from the analysis on the technical issues associated with assessing the reliability of passive systems in the context of nuclear safety and probabilistic safety analysis, and a viable path towards the implementation of the research efforts in the related areas is delineated as well. Focus on these issues is very important since it is the major goal of the international research activities (e.g. IAEA) to strive to reach a common consensus about the different proposed approaches. The chapter is organized as follows: after an overview on passive safety systems being implemented in the design of innovative reactors and an introduction on the main components of Probabilistic Safety Assessment approach, at first the current available methodologies are illustrated and compared, the open issues coming out from their analysis are identified and for which one of them the state of the art and the outlook is presented; the relative importance of each of them within the evaluation process is presented as well.

**2. Passive systems implementation in advanced reactor designs** 

Several advanced water cooled reactor designs incorporate passive safety systems based on natural circulation, as described in [2,3]: some of the most relevant design concepts for

aspects affect the thermal-hydraulic (T-H) performance of the passive system.

systems should be considered in the assessment process.


The PRHR implemented in the Westinghouse AP1000 design consists of a C-Tube type heat exchanger in the water-filled In-containment Refuelling Water Storage Tank (IRWST) as shown in the schematic given in Figure 2. The PRHR provides primary coolant heat removal via a natural circulation loop. Hot water rises through the PRHR inlet line attached to one of the hot legs. The hot water enters the tube sheet in the top header of the PRHR heat exchanger at full system pressure and temperature. The IRWST is filled with cold borated water and is open to containment heat removal from the PRHR heat exchanger occurs by boiling on the outside surface of the tubes. The cold primary coolant returns to the primary loop via the PRHR outline line that is connected to the steam generator lower head.

Reliability of Passive Systems in Nuclear Power Plants 29

ICS, the IC condensate return valve is opened whereupon the standing condensate drains into the reactor and the steam water interface in the IC tube bundle moves downward below

the lower headers.

**Figure 3.** Isolation condenser arrangement

**2.3. ESBWR Passive Containment Cooling System (PCCS)** 

The PCCS is a passive system which removes the decay heat released to the containment and maintains the containment within its pressure limits for design basis accidents such as a LOCA. The schematic of the PCCS is shown in Figure 4. The PCC heat exchangers receive a steam-gas mixture from the Dry Well (DW), condense the steam and return the condensate to the RPV via the Gravity Driven Cooling System GDCS pools. The non condensable gas is vented to the Wet Well (WW) gas space through a vent line submerged in the Suppression Pool (SP). The venting of the non condensable gas is driven by the differential pressure between the DW and WW. The PCCS condenser, which is open to the containment, receives a steam-gas mixture supply directly from the DW. Therefore, the PCCS operation requires no sensing, control, logic or power actuated devices for operation. The PCCS consists of six PCCS condensers. Each PCCS condenser is made of two identical modules and each entire PCCS condenser two-module assembly is designed for 11 MWt capacity. The condenser condenses steam on the tube side and transfers heat to the water in the IC/PCC pool. The evaporated

**Figure 2.** AP1000 passive residual heat removal systems (PRHR)
