[en] Motor Operated Valves (MOV) are in many plant systems safety-related part of equipment. From their proper function depends response of a safety system and its essential function. Their proper maintenance and testing is an important part of every nuclear power plant surveillance and testing programs

[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] In present work, the behaviour during accident of main reactor Coolant Pump is described. Reactor coolant pump is essential part of primary coolant system of nuclear power plant. To know it's integrity is very important in computer analysis and under a hypothetical loss of coolant accident conditions. The form of a homologous curve is explained. The pump transient speed behaviour and structural evaluation of the system considering time varying loads is described. (author)

[en] The events in the nuclear industry are investigated from the license point of view and from the regulatory side too. It is well known the importance of the event investigation. One of the main goals of such investigation is to prevent the circumstances leading to the event and the consequences of the event. The protection of the nuclear workers against nuclear hazard, and the protection of general public against dangerous effects of an event could be achieved by systematic approach to the event investigation. Both, the nuclear safety regulatory body and the licensee shall ensure that operational significant events are investigated in a systematic and technically sound manner to gather information pertaining to the probable causes of the event. One of the results should be appropriate feedback regarding the lessons of the experience to the regulatory body, nuclear industry and general public. In the present paper a general description of systematic approach to the event investigation is presented. The systematic approach to the event investigation works best where cooperation is present among the different divisions of the nuclear facility or regulatory body. By involving management and supervisors the safety office can usually improve their efforts in the whole process. The end result shall be a program which serves to prevent events and reduce the time and efforts solving the root cause which initiated each event. Selection of the proper method for the investigation and an adequate review of the findings and conclusions lead to the higher level of the overall nuclear safety. (author)

[en] Nuclear Power Plant incidents or accidents are initiated very often by a component failure even if it is not regarded as a safety related component. Plant safety, reliability and cost effectiveness can be enhanced and what is most important, the hazards for the environment can be reduced significantly through appropriate use of some specific Early Failure Detection Method or System. In the present paper the current trends in Nuclear Power Plant Krsko and licensing aspects introducing monitoring and diagnosis systems are described, to improve NPP reliability and safety. Most of the nuclear power plants are currently under preparation of the program - a systematic approach to implement different Early Failure Detection Systems. Possible contents of these programs is described in detail in several technical papers. Usually, monitoring of specific nuclear power plants parameters, components and systems shall be considered as a tool of integrity. Reasons for monitoring are contained in regulatory requirements and in operation experience, on the other hand, the licensee is highly interested for safe and reliable operation. The operational events and failures are usually the first initiator for systematic approach to specific phenomena observation and its analysis. The Slovenian Nuclear Safety Administration follows the efforts of NPP Krsko in all above mentioned fields including training of personnel, adequate procedures related QA Program and experience from other NPP utilities and Regulatory Bodies. The licensing aspects contain all the specific steps as they are required for specific plane modification. As a guide, the methodology and guidelines from the US code 10CFR 50.59 for safety evaluation are taken into account. The safety review process is a complex activity of all departments at the Slovenian Nuclear Safety Administration. In the case of an unreviewed safety question an engineering evaluation and a thorough understanding of the design basis of the system are essential. In the conclusion the possibility of the early failure detection operator support systems introduction into NPP Krsko regarding existing systems is discussed. (author)

[en] Slovenian Nuclear Safety Administration (SNSA) division of nuclear and radiological safety inspection has developed systematic approach to their inspections. To be efficient in their efforts regarding regular and other types of inspections, in past years, the inspection plan has been developed. It is yearly based and organized on a such systematic way, that all areas of nuclear safety important activities of the licensee are covered. The inspection plan assures appropriate preparation for conducting the inspections, allows the overview of the progress regarding the areas to be covered during the year. Depending on the licensee activities and nature of facility (nuclear power plant, research reactor, radioactive waste storage, others), the plan has different levels of intensity of inspections and also their frequency. One of the basic approaches of the plan is to cover all nuclear and radiological important activities on such way, that all regulatory requests are fulfilled. In addition, the inspection plan is a good tool to improve inspection effectiveness based on previous experience and allows to have the oversight of the current status of fulfillment of planned inspections. Future improvement of the plan is necessary in the light of newest achievements on this field in the nuclear world, that means, new types of inspections are planned and will be incorporated into plan in next year.(author)

[en] In the present paper, the aspects of optimizing In-Service Inspection (ISI) is discussed. Slovenian Nuclear Safety Administration (SNSA) and its authorized organization for the ISI activities, Institute of Metals and Technologies, are actually permanently involved in the ISI processes of the nuclear power plant (NPP) Krsko. Based on the previous experience on the ISI activities, evaluation of the results and review of the new ISI program, the decision was made to improve recent regulatory and professional practice. That means, the conclusion was made to optimize the evaluation process of the ISI as a process. Traditional criteria, standards and practice gives good fundament for the improvements implementation. Improvements can be done on the way that the more broad knowledge about safety important components of the systems shall bee added to the basic practice. It is necessary to identify conditions of the safety important components, such as realistic stress and fatigue conditions, material properties changes due aging processes, the temperature cycling effects, existing flaws characterization in the light of the previous detection and equipment technique used, assessment of the measurement accuracy on the results etc. In addition to the above mentioned, risk assessment and evaluation of the whole ISI shall be done. To do this it is necessary to made risk evaluation, based on previous structural element probability assessment. Probabilistic risk assessment is important and one of the most powerful tools in the ISI optimization. Some basic work on the filed of the risk informed methods related to the nuclear safety components has been already done. Based on reference documentation, the most important steps in risk informed ISI are discussed: scope definition, consequence evaluation, failure probability estimation, risk evaluation, non-destructive examination method selection and possibilities of implementation, monitoring and feedback. Recent experience on the ISI in the nuclear world shows that such practice plays strong role in the ISI decision making process and has some measurable effects too. It is clear that classical criteria for the selection of ISI inspection locations are not completely in accordance with current measures of safety importance. Optimization shall give the operator of nuclear facility benefits on the resources and duration of the ISI inspections, on the other hand, the most important benefit for the nuclear safety administration shall be more effective control of the safety important components, aging management control and overall nuclear safety. (author)

[en] In the present paper the Leak Before Break (LBB) methodology is described on the way, as it was evaluated in Slovenian Nuclear Safety Administration in the process of the nuclear power plant (NPP) Krsko modernization process. In the recent decade, reviewed regulatory position regarding elimination the dynamic effects of the postulated primary coolant pipe ruptures in some countries was issued. The basis for such new approach are research achievements on different areas of science like metallurgy, fracture mechanics, dynamic analysis and testing of materials. By this new regulatory position the utility has the possibility to adopt LBB concept, but has to fulfill at least general prerequisites, described in standard review plan, where the basic principles and objectives of the evaluation process are described. World wide practice shows that more intensive and detailed evaluation is necessary as it is described in the standard review plan. The concerns and requests raising during evaluation of the consequences of the adopted LBB concept have generally some common points comparing regulatory experience of different countries which have already accepted this methodology or are currently under the process of evaluation. But nevertheless every nuclear power plant is unique, comparing specific material properties, dynamic analysis assumptions, safety analyses performed etc. One of the most important areas in the LBB evaluation process is reliable primary piping material evaluation. Several generic material studies were performed and the applicability of these studies for the justification of LBB has to be carefully performed. Elimination of the double ended guillotine break from the safety analysis has very strong impact to the safety analysis results and finally to the structures, systems and components too. Nuclear safety administration has made a lot of effort to make its position on this methodology clear. The expertise of the authorized institutions is in such process necessary. It helps to justify and assess different areas of the standard review plan which has to be fulfilled and needs more detailed analysis. In the addition to the standard scope of items to be reviewed during the LBB evaluation, some questions arise which has not been previously discussed neither evaluated on the plant specific basis. These questions and the way of their evaluation and justification is described in the paper. (author)

[en] The reactor pressure vessel flaws are affected by the service loadings. The consequence is fatigue crack growth. It is necessary to make an evaluation of the fracture toughness of the reactor pressure vessel material in the case of flaws presence, specially in the case of transients with pressurized thermal shock. The ASME Code in section XI prescribe the evaluation procedure for the fatigue crack growth in reactor pressure vessel. This evaluation is used for the residual life time of the reactor pressure vessel calculation. (author)

[en] In the present paper, the aspects of Risk Informed In-Service Inspection (RI-ISI) are discussed. Slovenian Nuclear Safety Administration (SNSA) and its authorized organization for the ISI activities, Institute of Metals and Technologies (IMT), are actually permanently involved in the ISI processes of the nuclear power plant (NPP) Krsko. Based on the previous experience on the ISI activities, evaluation of the results and review of the existing practice in nuclear world, the activities are started to asses the piping of systems in the light of probability of failure. This is so called Risk Informed approach. By the design established criteria, standards and practice gives good fundaments for the improvements implementation. Improvements can be done on the way that the more broad knowledge about safety important components of the systems shall bee added to the basic practice. It is necessary to identify conditions of the safety important components, such as realistic stress and fatigue conditions, material properties changes due aging processes, the temperature cycling effects, existing flaws characterization in the light of the previous detection and equipment technique used, assessment of the measurement accuracy on the results etc. In addition to this deterministic approach, the principles of risk evaluation methods should be used. NPP Krsko has, as practically majority of NPP's, probabilistic risk assessment (PRA) studies for all safety important systems and components. The methods and results from these studies can be efficiently used to upgrade classical deterministic results, based on which the in-service program as a whole is usually done. In addition to the above mentioned, risk assessment and evaluation of the piping shall be done, which is not covered by the existing PRA analysis. To do this it is necessary to made risk evaluation of the piping segments, based on previous structural element probability assessment. Probabilistic risk assessment is important and one of the most powerful tools in the ISI optimization. Some basic work on the filed of the risk informed methods related to the nuclear safety components has been already done. Based on reference documentation, the most important steps in risk informed ISI are discussed: scope definition, consequence evaluation, failure probability estimation, risk evaluation, non-destructive examination method selection and possibilities of implementation, monitoring and feedback. Recent experience on the ISI in the nuclear world shows that such practice plays strong role in the ISI decision making process and has some measurable effects too. It is clear that classical criteria for the selection of ISI inspection locations are not completely in accordance with current measures of safety importance. Optimization shall give the operator of nuclear facility benefits on the resources and duration of the ISI inspections, on the other hand, the most important benefit for the nuclear safety administration shall be more effective control of the safety important components, aging management control and overall nuclear safety.(author)