Introduction
Throughout the last four decades, control and instrumentation systems employed in nuclear power reactors have gone via a process of firm progression. During the 1950s to the 1980s, the emphasis was on having basic, obvious protection and control techniques. by use of usual analog systems that have proven instrumentation. Recently, the industry has put in place digital and smart sensors, digital communication and electronic systems which have however been limited to the usual plant systems. Many changes have taken place in I&C apparatus used in the non-nuclear industrial areas where benefits of computer and digital based systems are highly recognized. Nuclear I&C systems hold up much behind the ones found in non-nuclear industries. The enlargement of this gap is exacerbated by lack of a good nuclear construction program in most industries, and this has detached financial incentives of I&C manufacturers in developing environmentally and technically competent products more especially for the nuclear power plants (Yoshikawa, & Zhang, 2011).
Equipment Capable and Qualified to Reach Nuclear Standards
Equipment capable and qualified to reach nuclear standards is now becoming common. Although, this current practice of where qualified analogy technologies are used and migrating to digital processing under a controlled surroundings will be always the norm for quite some time. Computer and digital based equipment are being introduced in the nuclear plants although its use in protective systems has led to some heavy assessment by regulators. Important issues of software verification, digital product qualification, software reliability, safety function classification and system validation have been resolved and considered in an acceptable manner.
Instrumentation
There is a big range of instrumentation in a nuclear plant, as from electronics and specialized detectors to conventional systems intended for protection and control of key components like turbo generators, pumps, and burners. The present position is discussed below in a number of specific topics.
Nuclear Instrumentation
There was a need to examine the neutron flux against the full array of reactor power, that's about 14 decades ago. This is presently achieved through three sets of apparatus each covering almost a third of this range with roughly one decade of an overlay. The ranges are recognized as start-up, transitional and full power level. It is normal to use different electronics and detectors for every range. In the past few developments have taken place in detector technology. However, methods of getting neutron flux from detector physics, analysis techniques, and the measured signal are well proven. Materials that are used in detectors have however changed for example cobalt to platinum so as to utilize new manufacturing techniques, give higher reliability and reduce dose burden (Krishnaswany, 2013).
Typical instruments include:
- Fission chambers
- Ion-chambers
- Helium detectors together with their pulse counting, logarithmic and linear electronics.
Experience has clearly portrayed the neutron power instrumentation to be sensitive when the power is low hence causing several reactor trips to US plants, Harns, Zion and Robinson plants, after so long outages. The problems originate from signal noise raise-up and calibration difficulties. Some old meters are highly susceptible to radio-frequency interference (RFI) and electromagnetic interference (EMI) from portable cellular phones and transceivers leading to reactor trips.
Process Instrumentation
This instrumentation covers a broad range of activities that are needed to control and monitor both the balance and reactor of a plant. Process instrumentation covers the way from a sensor by initiation and displaying all events. It varies depending on the needs of a plant (Ramamoorthy, 2014).
The process sensors comprise of humidity detectors, leak and moisture detectors, gas detectors, flow elements including the nozzle, annular and orifices, transmitters, temperature detectors, conductivity cells, smoke and fire detectors, and motion sensors. The process monitors on the other hand comprise of data loggers, signal and vibration noise monitors, seismic instrumentation, samplers, analyzers, and video cameras. Other process instruments basically for display an indication include chart recorders, power supplies, switches, indicators, meters, and gauges.
Controllers and Actuators
There is a substantial range of actuation and control technology that is used in plants. This comprises of analog devices mainly in pneumatic, electrical and electronic, and mechanical technologies. These technologies are costly to replace and maintain. As a result, a move away from normal electrical and pneumatic, for example, the programmable electronic devices. This tendency has not been much established for actuator technology since physical limitations like the environmental and component size constraints have prevented the introduction of the modern technology (Nero, 2012).
Introduction of the electronic controllers especially PLCs, in most instances, has been followed with the introduction of data concentrators and smart sensors, for example, multiplexors. These reduce lead to reduced cable requirements by easing the testing burden.
Finally, moving from single to multi-term has been the most important change in the control system. Unclear logic control has been introduced also like that for the feed water control in the startup of Fugen reactor found in Japan.
Control Systems
Control Philosophy
The nuclear power plants traditionally have been using the closed loop like the Proportion and Integral control system for continuous processes and combinatorial logic mainly for drive control. Consistency is achieved by the use of redundancy together with the usage of breakdown revealing design and testing of those items that are not in a continuous operation. Even as base load operation used to be a custom for the nuclear power plant, changing power supply rule and overcapacity, especially in the western world, bring implementation of the load following regimes. As a result, provision to give back to Xenon poisoning and the associated effects is necessary (Chapin, 2013).
Digital Control Equipment
Digital control equipment that is located away from a harsh plant environment has been fruitfully used to execute complex control policies for the reactor control systems from early 1970's. The systems are normally implemented like a fault-tolerant architecture so as to allow a continued plant function in case failures are incurred.
In the years of 1960 and 1970, a typical approach to the operation of computer control, mainly, was using the central double abandoned computer control systems and not the reactor control plant (Das, 2008). In 1980, PLCs were widely used for programmable controllers. Currently, simple functions are handled by the apparent choice of technology, even though for integrated and complex system control, distributed control systems along with the networking capability are being preferred. The digital control systems are used highly in the UK as original equipment to mechanize the operation of AGRs. CANDU reactors make extensive use the digital control comprising of reactor power. The digital computer systems are also installed in Bruce to replace the analog control for turbine governors and standby generators. Related changes have been considered in USA and Germany.
Moving to digital systems can be seen tedious since the approaches to operating, maintenance, design and technical support by digital equipment may be rather different from those of relay and analog technology (Yoshikawa & Zhang, 2011). Exceptional care ought to be taken, especially during upgrades, to make sure that on hand functions, interface devices and sensors are simulated in the modern system. In case changes are essential, the whole staff is required to be made aware. There are several well-established issues, for example, the digital systems have specific difficulty interfacing by spring loaded portable manual switches to locate servo motors and position of switches that have signals at the finish of travel particularly for a motorized valve.
Nuclear System Control
This system comprises of heat transport system measures for pressure, temperature, flow, and inventory. Depending on the reactor type, control systems of boiler parameters for level, flow and pressure, moderator parameters for level and temperature, and other minor nuclear processes should be there. However a majority of these controls are founded on the analog control, the digital control systems in many countries have been also successful. In AGR and CANDU, digital reactor systems have been installed as the original equipment. The programmable controllers used in Germany help improve performance and also authorize automatic burn up control through integrated Xenon simulation, optimize main power distribution, and minimize demineralized water and boron consumption regarding Xenon transients (Ramamoorthy, 2014).
There are several proposals for the replacement of present analog systems with PLCs on the older reactors in the UK, to follow plants whose systems were first installed as the original equipment.
Convention Control
The regulation of non- nuclear segment of the plant together with the turbo generator, condenser cooling, the feed service water and other minor systems are among the earliest to exploit the digital technology. Performance and optimization of such systems is a significant matter in plant economics. Digital control is also regular with the recent strategies of system surveillance and plant monitoring hence playing a very important role in the practical system changes so as to improve performance (Das, 2008).
Other early control systems have become absolute and they replaced with plug-in companionable replacements which match the operation of current controllers. Although, the operator interface is generally different and care should be taken to avoid complications to the controller functionality and the operator interface. The flexibility that is provided with these systems can be used in up rating the station electrical generation minus changing the performance of the reactor thermal.
Reactor Regulating System Programs
The regulating system is composed of a high level of immunity to minute process upsets, together with measurement failures. Many checks are done in the programs to make sure that all defective signals have been discarded. Just in case there is a loss of signal or even the entire set, substitute measurements are used. Although, it is important to derate this reactor simply because of imperfect flux shape or limited information (Krishnasway, 2013).
Power Measurement and Calibration
The regulating system controls heavy reactor flux shape and level, by reducing or increasing the level of light water in zone controllers to change or equalize the powers inside power zones. Spatial flux control should be able to avert xenon-induced instabilities with other space reliable perturbations (Kalvet & Kattel, 2010).
The total power reactor which is determined by a mixture of Inconel zone detector signals and ion chamber signals. The intersect takes place at around 10% of the full power. Since neither measurement is complete, flux signals are calibrated continuously against the reactor power measurements with regard to thermal signals. Absolute measurements are of less importance since spatial control system always acts to make the measurements equal. Single flux measurement cannot however exactly represent average power of a region in the core since there are local flux disturbances for example refueling. Hence, the calibration of Inconel detector signals is dined continuously by flux mapping routine.
Flux Mapping Routine
This routine brings readings distributed all through the reactor core together and computes the finest fit of this particular data against flux modes that are expected for a certain core configuration. Flux mapping gives a correct estimate of the average zone flux at each power zone. These particular estimates are present once after every 2 minutes and delay the neutron flux for roughly 5 minutes. The routine discards individual detectors that have readings which disagree with the other detectors.
Demand Power Routine
This routine gives reactor power set point based on demands that are from the setback routine, the operator, and the steam generator. At high power, steam generator curriculum dictates reactor power changes so as to give reactor-follows-turbine kind of control. At low power, at operator's caution, the set point of a reactor power is controlled manually using the keyboard (Chapin, 2013).
All the reactor power set point differences are restricted to the control program up to safe upper limits. A variation limiter protects power set point to reach above 5% of the actual power so as to prevent chances of a large increase in power at excessive rates.
Reactivity Mechanism Control
The key method of instant reactivity control is through varying liquid level in zone controllers. Usually, adjusters are always fully inserted, control absorbers completely withdrew and standard liquid zone control section level is around 30% to 50%.
If a shortage of positive reactivity occurs, the adjusters should be driven out by a specific sequence. This is shown by a low zone controller point (Nero, 2012). In case it is a negative reactivity, mechanical control observers should be driven in with one bank each time. This is also shown through high zone controller level.
Mechanical control absorbers and adjusters are driven with a speed relative to the power error to reduce, in small power errors, the shim’s rate of reactivity that ought to be cancelled with the zone controllers.
The program withdraws all shutdown rods automatically unless: the reactor is tripped, all rods are completely out, measured log rate is so large, and the power error is large or automatic control absorbers not in the core.
Setback Routine
This routine monitors several parameters and minimizes reactor power-on time in a ramp style when a certain parameter exceeds the specified operating limits. The power level where the setback stops or that rate where the reactor power is minimized will be suitable for each parameter.
Step-back Routine
The step-back routine minimizes reactor power also, but it does not operate gradually as in the setback routine. It instead drops mechanical control absorbers when either partly or fully into the reactor, leading to a rapid power reduction.
Adjuster Control Logic
The logic enables the operator to either select the manual or the automatic mode of the adjuster control. The logic ends the drive motor after it is fully out or in as established with the signal one among the two potentiometers in the mechanism. The logic also provides information regarding the condition of the system to the operator and generates terror signals in case faults occur (Das, 2008).
Mechanical Control Absorber
This logic is almost similar to one of the adjusters only that it includes control circuits and power supplies to the clutches. The clutches enable mechanical control absorber parameters to be reduced in the core so as to achieve a fast step-back. The circuit of this clutch reduces mechanical control absorber parameters once a step-back is required.
Conclusions
Use of computer systems in plants by operators is becoming common. Operators and systems which have advanced human performance in diagnosis and monitoring are now being implemented at the nuclear power plants in the whole world.
Defective and liberal digital systems have been proven to be effective and hence control the technology in nuclear plant applications. The controllers use surplus signal validation methods and microprocessors to provide a wide range of algorithms that have higher reliability and more optimized than the preceding analog controllers.
References
Chapin, D. M., National Research Council (U.S.), National Research Council (U.S.), & National Research Council (U.S.). (2013). Digital instrumentation and control systems in nuclear power plants: Safety and reliability issues. Washington, D.C: National Academy Press.
Das, S. (2008). Functional fractional calculus for system identification and controls. Berlin: Springer.
Kalvet, T., & Kattel, R. (2010). Creative destruction management: Meeting the challenges of the techno-economic paradigm shift. Tallinn: Praxis Center for Policy Studies.
Krishnaswamy, K. (2013). Power plant instrumentation. Place of publication not identified: Prentice-Hall of India.
Nero, A. V. (2012). A guidebook to nuclear reactors. Berkeley: University of California Press.
Ramamoorthy, M., International Federation of Automatic Control. International Federation of Automatic Control, & IFAC Symposium on Automation and Instrumentation for Power Plans. (2014). Automation and Instrumentation for Power Plants: Selected Papers from the IFAC Symposium, Bangalore, India, 15-17 December 1986. Oxford: Published for the International Federation of Automatic Control by Pergamon Press.
Yoshikawa, H., & Zhang, Z. (2011). The progress of Nuclear Safety for Symbiosis and Sustainability [recurso electrónico]: Advanced Digital Instrumentation, Control and Information Systems for Nuclear Power Plants.
