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[en] In recent years, much data and experience concerning the degradation of SG tubes accumulated. Because of regular full length inspection of all active tubes in steam generators, a huge amount of inspection records is now available. A computerized database was developed by Reactor Engineering division to accelerate the management of about 100.000 records. Initially, the data base was designed to support the development and decisions related to the plugging criteria for damaged tubes. In this paper, two prevailing groups of data are statistically analyzed: (1) the axial cracks in expansion transitions and (2) ODSCC at tube support plates. Both of them caused a vast majority of repaired tubes (e.g., plugs and sleeves). The main results of the analysis include the trends in number of defects, distribution of defect sizes, defect progression (growth) and expected life of a detected defect. Based on the results of statistical analysis, some predictions are given concerning the remaining lifetime of the steam generators. Those predictions may contribute to the optimum performance of the plant until the replacement of steam generators. (author)
[en] Since the companies Siemens AG and Framatome S.A. joined their experience and efforts in the field of steam generators replacements and formed a consortium in 1991, the following projects were performed in 1995: Ringhals 3, Tihange 3 and Asco 2. Further projects will follow in 1996, i.e., Doel 4 and Asco 2. Currently, this European consortium is bidding for the contract to replace the steam generators at the Krsko NPP and hopes to be awarded in 1996. An overview of the way the Consortium Siemens and Framatome approaches SG replacement projects is given based on the projects performed in 1995. Various aspects of project planning, management, licensing, personnel qualification and techniques used on site will be discussed. (author)
[en] Different concepts for the interim storage of spent fuel arising from operation of a NPP are discussed. We considered at reactor as well as away from reactor storage options. Included are enhancements of existing storage capabilities and construction of a new wet or dry storage facility. (author)
[en] The functionality the equipment important to safety is deteriorated during its service due to ageing and harsh environment conditions. Since the environment is a potential for common cause failures, the purpose of Environmental Qualification (EQ) is to demonstrate the capability of safety-related equipment to perform its safety function in aged conditions and under extreme conditions after design bases event (DBE). EQ is one of the steps in licensing process according to US regulatory documents and standards (10CFR50.49, RG 1.89, NUREG-0588, IEEE-323). This paper presents the efforts in establishing the EQ program in the Krsko nuclear power plant. (author)
[en] The possible impact of the non-uniform hydrogen distribution and the consequent fast burning should be assessed for LWR containments. Usual approach is to use integral severe accident codes or in some cases CFD codes to calculate detailed hydrogen distribution and to assess potential for hydrogen ignition. The problem is lack of spatial details in severe accident codes and long running time of CFD codes together with their inability to address situation where discharge of hydrogen is together with steam-water mixture. It was decided to use containment code GOTHIC to calculate response of the containment and hydrogen distribution for mass and energy releases calculated with MAAP 4.0.5 code for characteristic NPP Krsko severe accident scenarios. Standard 10-volumes NPP Krsko containment will be used in calculation assuming first lumped volumes and then their generalization to 3D space.(author)
[en] Whiskers phenomenon and their growth on pure tin layers have been known for many years. Whiskers are hair-like, electrically conductive, single-crystal structures that may grow to be several millimetres long. Tin Whiskers was identified in the late 1940s, during WWII due to detrimental short circuits and arcing in electrical devices, eventually causing the failure of devices. Whiskers growth mechanism remains unknown after more than 60 years of research. In recent years, concerns have been escalating, especially in the sector of high-reliability products. The elimination of lead in electronic equipment due to government regulations (RoHS) has increased the risk of failures due to the formation of tin whiskers in electronic devices because the use of pure tin (Sn) presents a high risk of tin whiskers caused failures in electronics, particularly those demanding high level of reliability. Consequently, JEDEC standard No.201 provides general guidelines for electronic products defined as Class 3, i.e. 'Mission/Life critical applications such as military, aerospace and medical applications'. In such products, pure tin and high tin content alloys are not acceptable. However, in commercial electronics segment, which utilizes the majority of electronic components, product life cycles are often measured in months. In contrast, high-performance electronics products typically have product life cycles that are measured in decades and therefore are much more susceptible to potential long-term threat of tin whiskers. Many of electronic failures are attributed to tin whiskers, ranging from space and military applications through medical, industrial applications and applications in nuclear power plants (NPP). Signal transients, parametric deviation or equipment failure can be linked to whiskers shortening. At Krsko Nuclear Power Plant, tin whiskers inspection, cleanliness and other planned maintenance activities on built-in electronic equipment are being performed during outage period. Preventive maintenance of storage equipment is being performed periodically through 'ISM' (In-Storage Maintenance) program. Despite preventive and corrective maintenance, equipment could fail to function. Tin whiskers are almost invisible to human eye and require magnification and lighting to be seen. Inadequate lighting technique and incorrect angle of inspection or magnification could cause tin whiskers to be overseen. In these cases, operator's experience with whiskers and other electronic inspections are crucial factors. At Krsko NPP maintenance department, the goal is to decrease tin whisker failure risk on electronic equipment by planning and performing preventive maintenance actions, applying recommended mitigation technics and monitoring operational experience. (author).
[en] We have already made a big step into new millennia and with it there is no more dilemma about presence of computers and internet in our lives. Almost all modern facilities struggle with this new dimension of information flow and how to use it to their best interest. But there is also the other side of the coin- the security threat. For nuclear power plants this threat poses even greater risk. In addition to protecting their trade secrets, personal data or other common targets of cyber attacks, nuclear power plants need to protect their digital computers, communication systems and networks up to and including the design basis threat (DBT). As stated in U.S. Nuclear Regulatory Commission (NRC) Regulatory Commission Regulations, Title 10, Code of Federal Regulations (CFR), section 73.1, 'Purpose and Scope' this includes protection against acts of radiological sabotage and prevention of the theft or diversion of special nuclear material. The main purpose of this paper is to explore the NRC Regulatory Guide (RG) 5.71 and its guidance in implementing cyber security requirements stated in NRC 10 CFR, section 73.54, 'Protection of Digital Computer and Communication Systems and Networks'. In particular, this section requires protection of digital computers, communication systems and networks associated with the following categories of functions: · safety-related and important-to-safety functions, · security functions, · emergency preparedness functions, including offsite communication, and · support systems and equipment which, if compromised, would adversely impact safety, security, or emergency preparedness functions. This section requires protection of such systems and networks from those cyber attacks that would act to modify, destroy, or compromise the integrity or confidentiality of data or software; deny access to systems, services or data; and impact the operation of systems, networks, and equipment. This paper will also present some of experiences from implementing these regulations in NPP Krsko. (author)
[en] Dosemeters were placed at 70 locations in and off site for a period of 4 months. The results were divided into 4 dose ranges (in μGy): (i) 170-220, (ii) 220-270, (iii) 270-320, and (iv) > 320. Group 1 includes areas close to the plant fence; the doses are due to terrestrial radiation (asphalt, ...). Group 2 includes the majority of data. Group 3 involves areas at 50-75 m from the waste storage facility or equipment access hatch; and Group 4 concerns the equipment access hatch and the radioactive waste storage facility. (P.A.)
[en] This paper describes Krsko experience and problems that had to be resolved with RTD Bypass Elimination project (RTDBE). Following RTD bypass manifold isolation valve leak in 2008, Krsko decided to perform RTDBE modification on reactor coolant system narrow range temperature measurement system. The installation was performed during Outage 2013. Soon after the plant returned to power, newly configured measurement channels showed that OPΔT reactor trip was oversensitive to spikes caused by auxiliary relay operation in the cabinets nearby. The solution was to reconfigure OPΔT trip filtering constants to filter out short-term spikes in the signal. After almost full operating cycle of trouble-free operation, RTD failures started occurring on reactor coolant system cold leg, which was caused by the high frequency vibrations (3-5 kHz) induced by reactor coolant pumps. To resolve RTD failures, Krsko ordered re-design of the RTDs to add robustness and specific qualification in high-frequency vibration operating environment. Improved RTDs were installed in Outage 2016 and were operating one full cycle with minor deviations. (author).