[en] The diagnostics of a phase state of the coolant under the normal operation of the reactor and at the pressure drop in it has been developed. The diagnostics is based on the registration and the analysis of temperature noises of the coolant. It ensures the control for appearing of a steam-gas mixture and for the level of the coolant in the reactor. At present the diagnostics system is being tested at NPP 'Kozloduy' in Bulgaria. (author)

[en] Petri Net models (PN) and Fault-Tree Analysis (FTA) are employed for the purpose of reliability analysis of the spallation neutron source SINQ. The monitoring and shut-down system (SDS) structure is investigated using a Petri-Net model. The reliability data are processed using a Fault-Tree model of the dominant part. Finally, suggestions for the improvement of system availability are made. (author)

[en] Periodic oscillation in measured temperature fluctuation was observed near to surface of a heated rod in certain heat transfer range. The frequency of the peak found in power spectral density of temperature fluctuation and period estimated from the cross correlation function for two axially placed thermocouples change linearly with linear energy (or surface heat) production. It was concluded that a resonance of such surface (inlet) temperature oscillation with the pole of the reactor transfer function can be responsible for power oscillation in BWR and PWR, thus instability is not solely due to reactor transfer function. (author)

[en] Nuclear power industry has a quite long tradition in on-line diagnostic of mechanical components and a considerable effort was put in developing diagnostic systems which are able to detect arising mechanical problems at an early stage. Computers are increasingly exploited to provide higher level information on process behaviour such as: early indication of the process deviation from normal conditions; rapid identification of the cause of any disturbance; prediction of the evolution of a disturbance; operator aid through computerized help. Following the recommendation of Several Member States to strengthen the activity in this field two divisions of IAEA established in 1995 the International Task Force on Nuclear Power Plant Diagnostics. The scope of the task force cover both technological developments and safety/licensing aspects of diagnostics. This report contains papers presented at the last in the series of Technical Committee Meetings on the Diagnostic Systems in Nuclear Power Plants organized in the framework of International Task Force

[en] Substantial progress could be reached both in system technics and in application of digital acoustic monitoring systems for assessing mechanical integrity of reactor primary systems. For the surveillance of PWRs and BWRs during power operation of the plants, acoustic signals of Loose Parts Monitoring System sensors are continuously monitored for signal bursts associated with metallic impacts. ISTec/GRS experience with its digital systems MEDEA and RAMSES has shown that acoustic signature analysis is very successful for detecting component failures at an early stage. Methods for trending and classification of digital burst signals are shown, experience with their practical use will be presented. (author)

[en] Recent development work at ISTec/GRS has been directed to more automation of surveillance techniques by utilization of the technological progress and existing tools. Neural nets, fuzzy techniques and rule-based methods were investigated for application in feature classification and automated identification of anomalies. First applications were aimed at classification of useful patterns and suppression of non-relevant signal components in order to avoid false alarms (e.g. in acoustic monitoring) and at signal validation under normal and disturbed plant conditions. Other on-going projects are aimed at the application of the successful methods to other surveillance tasks such as on-line assessment of sensor behaviour and ageing phenomena of instrumentation. The paper gives an insight in the intelligent analysis techniques and highlights their potential use for other surveillance tasks in nuclear power plants. (author)

[en] The integrity and safety of all nuclear power plant systems and components is guaranteed by the high requirements to quality assurance during all phases of design, fabrication, construction and operation. Many of the countries operating nuclear facilities, introduced advanced, sophisticated diagnostic systems for continuous monitoring safety important process parameters. The licensee should perform an assessment of the existing diagnostic systems, often supplied by the original design, their reliability and the need for the introduction of the additional monitoring/diagnostic systems. The operating experience should be taken into account and the assessment of the further needs. On this field has to be made on the results of PSA studies. In addition to the cost benefit analysis the evaluation of the new diagnostic systems in the light of nuclear safety should be also made. Experience, gained from the utilities, which have already installed this kind of the equipment should be very useful. Introducing new diagnostic systems will require often a safety assessment of the necessary modifications. Licensing process should be based on the existing nuclear legislation with certain additional requirements. (author)

[en] Nuclear Power Plant (NPP) surveillance and fault diagnosis systems in Dutch Borssele (PWR) and Dodewaard (BWR) power plants are summarized. Deterministic and stochastic models and artificial intelligence (AI) methodologies effectively process the information from the sensors. The processing is carried out by means of methods and algorithms that are collectively referred to Power Reactor Noise Fault Diagnosis. Two main schemes used are failure detection and instrument fault detection. In addition to conventional and advanced modern fault diagnosis methodologies involved, also the applications of emerging technologies in Dutch reactors are given and examples from operational experience are presented. (author)

[en] A special type of instability occurred in the Swedish BWR Forsmark 1 in 1996. In contrast to the better known global or regional (out-of-phase) instabilities, the decay ratio appeared to be very high in one half of the core and quite low in the other half. A more detailed analysis showed that the most likely reason for the observed behaviour is a local perturbation of thermohydraulic character, e.g. a density wave oscillation (DWO), induced by the incorrect positioning of a fuel assembly (an 'unseated' assembly). In such a case it is of large importance to determine the position of the unseated assembly already during operation such that it can be easily found during reloading. The subject of this paper is to report on development and application of methods by which the position of such a local perturbation can be determined. Two different methods that support and complement each other were used. First a visualisation technique was elaborated which expedites a very good qualitative comprehension of the situation and which can be useful for the operators. It also gives an important basis for the application of the localisation algorithm. Second, a quantitative (algorithmic) localisation method, suited for this type of perturbation, was elaborated. This latter takes noise spectra from selected detectors as input and yields the perturbation position as output. The method was tested on simulated data, and then applied to the Forsmark measurements. The location of the disturbance, found by the algorithm, is in accordance with independent judgements for the case, and close to a position where an unseated assembly was found during refuelling. (author)

[en] Monitoring systems used in nuclear power plants have made remarkable progress over the past four or five years. Development has followed the trends and changes in philosophy for the purpose of monitoring systems in nuclear power plants: They are no longer expected to fulfill only safety tasks, the plant personnel require information on which to base condition-oriented maintenance. A new generation of monitoring and diagnostic systems has been developed by Siemens recently. This new generation, called Series '95, is PC-based. An overview is given for the KUeS '95 loose parts diagnostic system, the SUeS '95 vibration monitoring system, the FLUeS leak detection system and the SIPLUG valve diagnostics system. The objectives behind the development of these new systems are both safety-related and economic. The new systems improve the reliability and quality of monitoring techniques and incorporate better detection and diagnostic capabilities. Progress has also been made in automation of the systems so as to reduce routine work, give higher sensitivity for the monitoring task and reduce the scope of maintenance. (author)