**5. Case study: Isolation condenser system (ISO)**

The isolation condenser system simply referred to as ISO is a standby highpressure system that removes residual and decay heat from the reactor vessel in the event of a scram signal in which the reactor becomes isolated from the main condenser, or if any other high-pressure condition exists. The schematic diagram is shown in **Figure 3**. The ISO system transfers residual and decays heat from the reactor coolant to the water in the shell side of the isolation condenser resulting in steam generation (SG). The steam generated in the shell side of the isolation condenser is then vented to the outside atmosphere. During the normal operation, the ISO system is in standby mode. During the standby mode, the steam isolation valves (VS1 and VS2) are open because the condenser tube bundles are at the reactor

**Figure 2.** *Proposed framework of the system.*

pressure. The condensate is built in the condenser and condensate by returning pipe. The condensate is stopped from a return back to the reactor by closing the condensate return valve (VC2). The condensate valve (VC1) is open at the stand-by condition and vent valves (VV) at main steam lines normally open to vent noncondensable gases from ISO. The makeup water must be provided to prevent uncovering the condenser tubes that are the combination of firewater and condensate using makeup water valve (VW) normally closed at standby mode. The water inventory on the shell side of the condenser will provide heat removal for between 20 and 90 minutes depending on the plant design, at which time makeup water must be provided to prevent uncovering the condenser tubes. On the shell side of the condenser, the water inventory will be provided for the heat removal between 20 to 90 minutes. At which time water makeup has to be provided to prevent uncovering the condenser system tubes (**Figure 3**).

**Figure 3.** *Schematic diagram of isolation condenser system.*

*Reliability Analysis of Instrumentation and Control System: A Case Study of Nuclear Power… DOI: http://dx.doi.org/10.5772/intechopen.101099*

The ISO system may be initiated manually, or automatically initiated on high reactor pressure or low reactor pressure. On the initiation of ISO, one of the condensate return valves (VC2) opens and the vent valve (VV) gets closed. The steam flows from the reactor vessel to steam isolation valves (VS1 and VS2). The steam gets condensed in condenser tube bundles and condensed steam returns to the reactor vessel (VC2 and VC2) with help of a recirculation pump. The boiled-off water is replaced by the condensate transfer system or the firewater system. The ISO system is designed in such a way that, the system automatically gets isolated from the reactor pressure vessel in the event of a system pipe break. All the valves are closed automatically (VS1, VS2, VC2, VC2, and VV) in the event of low differential pressure exceeds three times the normal flow value. This isolation will mitigate the loss of water inventory. The ISO system instrumentation and control consists of initiation and containment isolation circuitry [18]. These circuits provide different functions, both of which are important to system reliability. The entire system is operating in a closed-loop manner.
