Nuclear Instrumentation

Nuclear Instrumentation

Nuclear power plant instrumentation and control, or I&C, consist of equipment that control and ensure the safety of nuclear power plants by obtaining data from sensors monitoring the status of parameters such as temperature, pressure, and level; form and isolate these sensor signals; display and process the sensor data on recorders, indicators, and the plant computers; and issue commands to controllers, safety logic circuitry, or safety actuation systems. The I&C systems of nuclear power plants depend primarily on the measurement of temperature, pressure, level, and flow, and to the monitoring of neutron flux. These measurements are made using numerous sensors installed throughout the plant. The Engineered Safety Features or ESF systems are part of a collection of systems jointly called the Safety-Related Distributed Control and Information System or Q-DCIS. The Emergency Core Cooling System or ECCS encompasses the Automatic Depressurization System or ADS, the Gravity-Driven Cooling System or GDCS, the Isolation Condenser System or ICS, and the Standby Liquid Control System or SLCS. These four systems together ensure the safety of the nuclear power plant when other systems fail.

The Automatic Depressurization System or ADS is located within the Nuclear Boiler System or NBS. The ADS depressurizes the reactor so that the low-pressure GDCS can provide make up coolant to the Reactor Pressure Vessel or RPV (ESBWR Design Control Document, Tier 2, Chapter 7, Instrumentation and Control Systems, 2007). Through the following safety related requirements, the ADS instrumentation and controls or I&C detect reactor low water levels, which signal when emergency actions are required; automatically actuate the Safety Relief Vales or SRVs and Depressurization Valves or DPVs when Level 1 is reached; actuate the SRVs and DPVs in sequence and in clusters to achieve the required depressurization, this is to prevent the pressure from going down either too slow or too fast depending on the current temperature and level; render no more than one valve inoperative for any single failure, the system was engineered to complete its function with any one valve not working; ensure physical and electrical separation and isolation between safety related classes and from non safety related circuits and equipment, this is so that non safety related circuit and equipment may be de-energized/secured without compromising the ability of the plant to operate safely and protected from casualties ; and Indicate the status of SRV and DPV in the Main Control Room (MCR).

The ADS controls and instrumentation meet non safety-related requirements such as no single control or instrumentation failure inadvertently opens an SRV or a DPV. For the same reason the system is designed to function with any one fault in the system, it is also designed such that one faulty detector or control circuit will initiate an unintentional actuation of the system, and ADS-parameter alarms are provided in the MCR.

The ADS encompasses ten SRVs, eight DPVs, and the related Instrumentation and Controls or I & Cs (Hashermian, 2010). The SRVs are nitrogen operated solenoid actuated relief valves (ESBWR Design Control Document, Tier 2, Chapter 5, Reactor Coolant System and Connected Systems, 2007). Nitrogen is the pressurized gas that pushes the valve open or shut and a solenoid in an electronically controlled valve sort which controls the nitrogen to the ADS valve. The DPVs are electrically operated squib valves (Limited, 1972). These valves are separated into clusters and rise in sequence when necessary. The solenoid gets its input straight from the level instrumentation in the plant but can be overridden manually. Automatic actuation of the ADS takes place when the RPV water reaches Level 1. Four wide range RPV water level transmitters are used to detect Level 1. These transmitters are separate from those used for the Reactor Protection System or RPS functions and different from those used for the Diverse Protection System or DPS wide range level transmitters (Boiling Water Reactor, 2009). Basically, the ADS level detectors provide no input to any other system. When achievement of Level 1 has become aware of Group 1, which contains five SRVs, they are opened to initiate RPV pressure reduction, followed by Group 2 after a time suspension, which contains the other five SRVs. The series carry on with groups of DPVs, each opening after additional time suspensions. This chronological procedure diminishes the water loss as a result of liquid swell in the RPV when its pressure is hastily decreased. Swell is a term used to describe the phenomenon of a rapid increase in boiler level when there is a rapid increase in steam usage, like what occurs when the SRVs open. The safety-related Video Display Units, or VDUs, in the MCR offer a display arrangement permitting the operator to by hand open each SRV and each DPV separately, using the primary SSLC/ ESF logic function. Each display uses an 'arm/fire' arrangement necessitating at least two purposeful operator actions. Operator use of the 'arm' portion of the display triggers a plant alarm (Desai, 1993). The two manual opening schemes from SSLC/ESF and from DPS are diverse, allowing the 'arm/fire' configuration to prevent an accidental actuation of the SRVs and DPVs.

The ADS actuation logic is applied in four (SSLC/ESF) classes, each of which can make a Level 1 trip vote. Each of the class trip votes is shared with the other classes. Normally, each of the four classes make a two-out-of-four trip decision from the four class votes; however, the entire SSLC/ESF system has a bypass control such that any single class of sensors can be removed from the two-out-of-four decision process, so that the remaining three classes operate with a two-out-of-three trip decision (ESBWR Design Control Document, Tier 2, Chapter 7, Instrumentation and Control Systems, 2007). A minimum of 2 votes from the logic circuits required to initiate the ADS system in case one class gives a false indication such as during calibration, 2 out of 3 votes is in case one of the four classes is out of commission. The design allows only one division at a time to be bypassed, and can be used to facilitate either maintenance or calibration activities. The four class water levels and their trip set points are endlessly supervised for uniformity by the N-DCIS plant computer functions or CPF. An inconsistency results in an alarm. Each class of the SSLC/ESF has two trains of two-out-of-four trip logic, except for the DPV logic with has three trains, to support the requirement that single class failures cannot result in inadvertently opening any ADS valve (SRV, or DPV). Each commencing logic has entry to one channel of wide-range level sensing for the Level 1 trip result.

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