Don W. Miller

Probabilistic risk assessment modeling of digital instrumentation and control systems using two dynamic methodologies

The Markov/cell-to-cell mapping technique (CCMT) and the dynamic flowgraph methodology (DFM) are two system logic modeling methodologies that have been proposed to address the dynamic characteristics of digital instrumentation and control (I&C) systems and provide risk-analytical capabilities that supplement those provided by traditional probabilistic risk assessment (PRA) techniques for nuclear power plants. Both methodologies utilize a discrete state, multi-valued logic representation of the digital I&C system. For probabilistic quantification purposes, both techniques require the estimation of the probabilities of basic system failure modes, including digital I&C software failure modes, that appear in the prime implicants identified as contributors to a given system event of interest. As in any other system modeling process, the accuracy and predictive value of the models produced by the two techniques, depend not only on the intrinsic features of the modeling paradigm, but also and to a considerable extent on information and knowledge available to the analyst, concerning the system behavior and operation rules under normal and off-nominal conditions, and the associated controlled/monitored process dynamics. The application of the two methodologies is illustrated using a digital feedwater control system (DFWCS) similar to that of an operating pressurized water reactor. This application was carried out to demonstrate how the use of either technique, or both, can facilitate the updating of an existing nuclear power plant PRA model following an upgrade of the instrumentation and control system from analog to digital. Because of scope limitations, the focus of the demonstration of the methodologies was intentionally limited to aspects of digital I&C system behavior for which probabilistic data was on hand or could be generated within the existing project bounds of time and resources. The data used in the probabilistic quantification portion of the process were gathered partially from fault injection experiments with the DFWCS, separately conducted under conservative assumptions, partially from operating experience, and partially from generic data bases. The purpose of the quantification portion of the process was, purely to demonstrate the PRA-updating use and application of the methodologies, without making any particular claim regarding the specific validity and predictive value of the data utilized to illustrate the quantitative risk calculations produced from the qualitative information analytically generated by the models. A comparison of the results obtained from the Markov/CCMT and DFM regarding the event sequences leading to DFWCS failure modes show qualitative and quantitative consistency for the risk scenarios and sequences under consideration. The study also shows that: (a) the risk significance of the timing of system component failures may depend on factors that include the actual variability of initiating conditions of a dynamic transient, even within the nominal control range and (b) the range of dynamic outcomes may also be dependent on the choice of the assumed basic system-component failure modes included in the models, regardless of whether some of these would or would not be considered to have direct safety implications according to the traditional safety/non-safety equipment classifications.

A Practical Gamma-Ray Camera System Using High-Purity Germanium

A prototype gamma camera system has been constructed which is based on a high purity germanium detector fabricated with orthogonal strip electrodes. In this device, position sensitivity is obtained by connecting each contact strip on the detector to a charge-dividing resistor network. Excellent energy and spatial resolution have been achieved by cooling the resistor network to 77°K and by proper selection of noise filtering parameters in the pulse shaping amplifier circuitry. The significant advantage of employing this charge-splitting detector in a semiconductor gamma camera system is its electronic-readout simplicity, requiring only three amplifier channels to measure the energy and two-dimensional location of gamma ray events. A complete discussion of our investigation of the charge-splitting detector concept is presented with special reference to its potential use in the construction of a high resolution gamma imaging system having sufficient field of view and sensitivity for clinical utilization. Several orthogonal strip-electrode germanium detectors have been fabricated and evaluated experimentally in our laboratory. The most recent of these measures 2 cm × 2 cm × 5 mm thick and incorporates 10 contact strips on each surface which are spaced on 2 mm centers. The measured FWHM energy and spatial resolutions were 5.5 keV and 1.66 mm, respectively. Theoretical calculation of the magnitude of noise in the energy channel and comparison of these values to measured data shows that correlated noise cancellation significantly enhances the energy resolution in this type of charge splitting device.

Position Sensitive Gamma Ray Detectors Using Resistive Charge Division Readout

The analysis of an orthogonal strip, two-dimensional position sensitive high purity germanium gamma ray detector is discussed. Position sensitivity is obtained by connecting each electrode strip on the detector to a resistor network and the ends of the network to charge sensitive preamplifiers. The difference of the voltage pulses at the output of each preamplifier is proportional to the position at which the charge entered the resistor network and the sum of the voltage pulses is proportional to the energy of the detected gamma ray. The results of the analysis of the energy and spatial signal noise show that the position resolution is proportional to the square root of the filter amplifiers output pulse time constant and that for energy measurement the resolution is maximized at the filter amplifiers noise corner time constant. The electronic noise filtering system and experimental noise measurement results are presented.

Gamma radiation resistant Fabry–Perot fiber optic sensors

The Nuclear Regulatory Commission (NRC) in 1998 completed a study of emerging technologies that could be applicable to measurement systems in nuclear power plants [H. M. Hashemian et al., “Advanced Instrumentation and Maintenance Technologies for Nuclear Power Plants,” NUREG/CR-5501 (1998)]. This study concluded that advanced fiber optic sensing technology is an emerging technology that should be investigated. It also indicated that there had been very little research related to performance evaluation of fiber optic sensors in nuclear plant harsh environments, although substantial research has been performed on nuclear radiation effects on optical fibers in the last two decades. A type of Fabry–Perot fiber optic temperature sensor, which is manufactured by Fiso Technologies in Canada, is qualified to be a candidate for potential applications in nuclear radiation environment due to its unique signal processing technique and its resistance to power loss. The gamma irradiation effects on this type of sensors...

An Integrated Operator Advisor System For Plant Monitoring, Procedure Management, and Diagnosis

This paper reports on an integrated operator advisor system (OAS) built using generic task methodology. The operators activities of plant monitoring, data interpretation, procedure execution, and diagnosis have been implemented as the four generic tasks in the system. The OAS is capable of identifying the abnormal functioning of the plant in terms of threats to safety, preenumerated abnormal events, and deviations from normality. After the identification of abnormal functioning, the system will identify the procedures to be executed to mitigate the consequences of abnormal functioning and will help the operator by displaying the procedure steps and monitoring the success of actions taken. The system also is capable of diagnosing the cause of abnormal functioning. The diagnosis is done in parallel to the task to procedure execution.

Scintillating fiber optics for X-ray radiation imaging

Abstract Research to evaluate scintillating fiber optics (SFO) as radiation imaging detectors is described. The fundamental properties of the imaging detector, radiation detection efficiency, spatial resolution and signal-to-noise ratio (SNR), are measured. It has been concluded from this evaluation that the intrinsic detection efficiency, spatial resolution and SNR are superior to that observed in scintillating screens. The measured spatial resolution with 90 300 keV X-rays ranges between 100 and 200 mm and is independent of the SFO thickness. The overall detection efficiency is, however, considerably lower than that of scintillating screens. Since the detection of photons is the primary source of statistical uncertainty, the problem of detection has been solved by the use of light amplification with an image intensifier. Scintillating fiber optics with light amplification demonstrate a desired detection efficiency, and spatial resolution at higher energy superior to scintillating screens. The implications of this result is that SFO should demonstrate good spatial resolution and detection efficiency at higher energies. These measurements indicate that SFO do have good potential for high-energy photon radiation imaging.

Scintillating fiber optics and their application in radiographic systems

Research to evaluate scintillating fiber optics (SFOs) as X-ray radiation imaging detectors is presented. Scintillating fiber optics combines scintillating material with a dielectric waveguide. This configuration permits an increase, in thickness of the scintillating material without significant dispersion of light, therefore preserving spatial resolution with increased detection efficiency. This characteristic is advantageous for imaging of high-energy X-rays. A model based on the basic physics of SFOs and the Monte Carlo modeling of radiation was developed. The model provides a basis for analysis of the performance characteristics of SFOs. An experimental evaluation of the performance of SFOs as radiation imaging detectors is reported. In this study, two types of Tb/sub 2/O/sub 3/-based SFOs were studied using monoenergetic gamma rays and low- and high-energy X-ray sources. The study evaluated the overall detector efficiency by combining experimental measurements and analytical Monte Carlo methods. >

A Multi-Detector Germanium Gamma Ray Camera

The design, fabrication, and evaluation of a multidetector germanium gamma ray camera is described. The camera consists of a square array of four orthogonal strip high purity germanium detector elements which are 12 mm thick and have a 41 cm2 field of view. Position sensitive readout is achieved using the resistive charge splitting method. The measured FWHM system energy and spatial resolution at 140 keV are 5.5 keV and 2 mm, respectively. Comparative phantom and animal studies yielded radionuclide images with contrast and in-depth spatial resolution superior to that of state-of-the-art scintillation cameras.

Methodologies for the probabilistic risk assessment of digital reactor protection and control systems

Nuclear power plants are in the process of replacing the existing analog instrumentation and control (I&C) systems with digital technology. Digital systems distinguish themselves from other control and instrumentation systems mainly due to the presence of active software/firmware as well as hardware. The U.S. Nuclear Regulatory Commission policy statement on the use of probabilistic risk assessment (PRA) methods in nuclear regulatory activities encourages licensees to use PRA and associated analyses to support the licensing applications to the extent supported by the state-of-the-art and data. Before digital system reviews can be performed in a risk-informed manner, PRAs will need the capability to model digital I&C systems. The available methodologies for the reliability and risk modeling of digital I&C systems are reviewed with respect to their capability to account for the features of the digital I&C systems relevant to digital reactor protection and control systems, as well as the integrability of the resulting model into an existing PRA. It is concluded that the methodologies that rank as the top two with most positive features and least negative or uncertain features (using subjective criteria based on reported experience) are the dynamic flowgraph methodology and the Markov methodology combined with the cell-to-cell mapping technique, each with different advantages and limitations.

The noise analysis and optimum filtering techniques for a two-dimensional position sensitive orthogonal strip gamma ray detector employing resistive charge division☆

Abstract The analysis of an orthogonal strip, two-dimensional position sensitive high purity germanium gamma ray detector is discussed. Position sensitivity is obtained by connecting each electrode strip on the detector to a resistor network. Charge, entering the network, divides in relation to the resistance between its entry point and the virtual earth points of the charge sensitive preamplifiers located at the end of each resistor network. The difference of the voltage pulses at the output of each preamplifier is proportional to the position at which the charge entered the resistor network and the sum of the pulse is proportional to the energy of the detected gamma ray. The analysis of energy and spatial noise resolution is presented for this type of position sensitive detector. The results of the analysis show that the position resolution is proportional to the square root of the filter amplifiers output pulse time constant and that for energy measurement the resolution is maximized at the filter amplifiers noise corner time constant. The design of the electronic noise filtering system for the prototype gamma ray camera was based on the mathematical energy and spatial resolution equations. For the spatial channel a Gaussian trapezoidal filtering system was developed. Gaussian filtering was used for the energy channel. The detector noise model was verified by taking rms noise measurements of the filtered energy and spatial pulses from resistive readout charge dividing detectors. These measurements were within 10% of theory.