PLANT PROTECTION AND INSTRUMENTATION SYSTEM

The Safety Protection Subsystem of the Plant Protection and Instrumentation System (PPIS) is that portion of the PPIS which performs lOCFRlOO-related functions. It includes the reactor trip Instrumentation hardware and associated system sensors which are used to detect abnormalities in the plant... 

Signals to the Plant Protection and Instrumentation System (PPIS) and the NSSS Control Subsystem (NCS) are supplied by neutron detectors. During power operation, the neutron flux levels are monitored by detectors located in wells between the reactor vessel and the concrete cavity wall. These detectors are distributed symmetrically around the reactor vessel at about the core midplane. During low power operation, starting up, shutting down, and while shut down, the neutron flux levels are monitored by source-range detectors, located in selected side reflector elements near the bottom of the active core. 

Monitoring neutron flux and providing signals to the Plant Protection and Instrumentation System (PPIS). 

The PIUS plant is also provided with instrumentation systems, protection, logic, and actuation systems for reactor shutdown, residual heat removal, containment isolation, etc. in a similar way as present-day LWR plants. Their importance for ensuring safety is significantly reduced in a PIUS plant. The equipment of these instrumentation, monitoring, protection, and actuation systems is separated from that of other systems and located in separate, physically well protected compartments at the bottom of the reactor building. The reactor protection system (RPS), with a two-out-of-four coincidence logic, has the task of initiating power level reduction, reactor shutdown or reactor scram when reactor process parameters exceed set limits, in order to prevent further departure from permissible conditions. 

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

It is important that personnel understand how to achieve safe operation, but not at the exclusion of other important considerations, such as reliability, operability, and maintainability. The chemical industry has also found significant benefit to plant productivity and operability when SIS work processes are used to design and manage other instrumented protective systems (IPS), such as those mitigating potential economic and business losses. The CCPS book (2007) Guidelines for Safe and Reliable Instrumented Protective Systems discusses the activities and quality control measures necessary to achieve safe and reliable operation throughout the IPS lifecycle. 

In this way, the fault tree can be quantified, which makes this technique very powerful for the reliability analysis of protection systems. The prerequisite is the availability of statistical reliability data of the different devices and instruments that is often difficult to obtain for multi-purpose plants, where devices can be exposed to very different conditions when changing from one process to another. Nevertheless, if the objective is to compare different designs, semi-quantitative data are sufficient. 

A fault tree for a process plant can be built up by starting with the fault tree for the unprotected system (the demand tree), to which is added branches representing protection by the process operator and/or instrument systems. ... 

Chernobyl, on the contrary, is an example of what can happen if a completely opposite principle is applied, that to do only what is necessary for safety. In RBMK reactors, like the Chernobyl reactor, the safety margins were not stringent enough. For example, the plant had a containment system for the primary circuit but it was only partial the reactor itself, and in particular the fuel channel heads, were not included in it. The designers thought that it was sufficient only to install protective monitoring instrumentation. Figure 3-4 shows the containment for a typical 900 MWt PWR and the Chernobyl reactor containment. 

Safety Series Nos 50-SG-D3, Protection System and Related Features in Nuclear Power Plants (1986) 50-SG-D8, Safety-related Instrumentation and Control Systems for Nuclear Power Plants (1984) and Safety Standards Series No. NS-G-2.2, Operational Limits and Conditions and Operating Procedures for Nuclear Power Plants (2000). 

The instrumentation system provides all necessary information for operation of the plant, various signals for displaying, recording, controlling, protecting, annunciating, and the alarm function for the equipment and operational systems. It includes the in-core instrumentation... 

The instrumentation and control systems in the PHWR plant include a variety of equipment intended to perform display, monitoring, control, protection and safety functions. The concepts presented form the basis for the system design and development. General guide-lines followed are ... 

The simplicity of the GT-MHR is reflected in its instrumentation systems Total visibility of plant conditions, and all control and protection actions are provided with a relatively low number of instrumentation channels. Clear and concise operator information is provided. The passive safety of the reactor, its slow response, and the neutronic transparency and the absence of phase changes in the gas coolant eliminate many of the human factors complications found in other reactors. 

The instrumentation and control system consists of the reactor protection system, engineered safety features opmtion system, plant control system and reactor monitoring system. The reactor protection syston has two diverse shutdown ems. The engineered safety features are the decay heat removal system (PRACS) and the containment system. 

Designing the Nuplex 80+ instrumentation and controls to incorporate semi-automated and on-line testing features for the Plant Protection System and on-line monitoring of fluid and electrical systems, thus enhancing the detection of sabotage. 

The first two layers are described in section 3.1 and 3.2. Apart fi om these control layers the plant is protected against excursions outside the operating boimdaries by an Alarm Management system and an Instrumented Protective System (IPS). The Alarm Management system warns the operators to take manual action in case the plant moves outside the allowed operating window. The IPS system is fully independent fi om the control and alarm system and can automatically shut down (parts ol) the plant in a safe manner. 

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