[en] The conference is organized with intention to focus on specific aspects of usage of nuclear energy for electricity production in small and medium countries. Importance of international cooperation for the assessment of the nuclear option has been recognised by the International Atomic Energy Agency (IAEA). As a result of this recognition, the Conference is organized in co-operation with IAEA. Session topics reflect some current emphasis, such as country energy needs, accommodation on Kyoto restriction on CO₂ emission, new reactor technologies, operation and safety of the operating nuclear power plants. The conference also focuses on the exchange of experience and co-operation in the fields of fuel cycle, radioactive waste management, regulatory practices and liability and insurance for nuclear damage. All contributed papers are grouped in 10 sessions: Energy planning and nuclear option; Power reactors and technologies; Nuclear energy and environment; Operation and maintenance experience; Safety culture; Nuclear safety analyses; Reactor physics and nuclear fuel cycle; Radioactive waste management and decommissioning; Public relations; Regulatory practice and general papers.

[en] The Republic of Croatia has been building the Krsko Nuclear Power Plant and is participating in all the activities necessary for a successful operating of the plant now for almost thirty years. However, in the light of the nuclear energy renaissance it is necessary to prepare ourselves for new challenges, stricter criteria of safety and protection, respect the indispensability of continuous re-examination of safety of procedures and methods. The of Croatia has strictly committed herself to the nuclear energy programme development-CRONEP in accordance with the methodology of the International Atomic Energy Agency. Certainly, in the first moment till the possible decision on the building of nuclear power plant, it will be necessary to make an institutional framework and create human resources and such an infrastructure that will be able to, when the decision will be taken, support the project and realize it with maximal efficiency. We consider it the unique way in which it is possible to avoid what proved to be the weakness of some projects of nuclear power plants, that is missing a deadline and problems concerning financing that are intolerable taking into account the value of the investment. Likewise, since the Conference is dedicated to small and medium-sized electric networks or to small nuclear power programmes, it needs to be mentioned that except the largest facilities it should be promoted researching of nuclear power reactors of medium size whose development somehow falls behind in this moment Medium size reactors gives great advantages to smaller economies in technical and financial sense. From the current standpoint solutions of viability of nuclear programmes through re-processing of the spent nuclear fuel in new generation of power plants are discernible. Since today's technologies are sufficiently safe there is no need to wait with this development and fuel from one generation shall be re-processed into the fuel for the next generation of reactors. In the case of this closed fuel cycle, nuclear fuel reserves will be sufficient for some millennia. Therefore, in the European strategic technology plan the development of nuclear technology is included as one of seven areas of energy industry development to achieve deployment of cost-effective low carbon technologies. Instead of conclusion I would like to point out that only if nuclear power is used in a safe, technically justifiable and moral way it may become available to all and be a good answer to energy challenges of the modern world. Therefore it is necessary that each national nuclear energy programme has its full credibility.(author).

[en] Levelized unit of energy costs (LUEC) are calculated for New Nuclear, Gas and Coal Power Plants. Parameters needed for LUEC calculation are identified and standard internationally accepted LUEC methodology is used. Calculation is performed with the expectation that observed power plants are used as base load power plants. External costs originating from environmental impact are also included in the calculation. Sensitivity study of important parameters is performed.(author).

[en] The most important task of every utility operating a nuclear power plant is the continuously keeping of the desired safety and reliability level. This is achieved by the performance of numerous inspections of the components, equipment and system of the nuclear power plant in operation and in particular during the scheduled maintenance periods at re-fueling time. Periodic non-destructive in-service inspections provide most relevant criteria of the integrity of primary circuit pressure components. The task is to reliably detect defects and realistically size and characterize them. One of most important and the most extensive examination is a reactor pressure vessel in-service inspection. That inspection demand high standards of technology and quality and continual innovation in the field of non-destructive testing (NDT) advanced technology as well as regarding reactor pressure vessel tool and control systems. This article presents remote underwater contact ultrasonic inspection equipment and developed advanced nondestructive techniques for the examination of the defined sections (reactor welds). Eddy current method applies for clad surface examinations; visual inspection is used for examination of the vessel inner surface and advanced phased array ultrasonic technique applies for base material examination. The movement of probes and data positioning are assure by using new RPV tool concept that is fully integrated with NDT systems. The successful performance is attributed thorough pre-outage planning, training and successful performance demonstration qualification of chosen NDT techniques on the specimens with artificial and/or real defects. Furthermore, use of advanced approach of inspection through implementation the state of the art examination equipment significantly reduced the inspection time, radiation exposure to examination personnel, shortening nuclear power plant outage and cutting the total inspection costs.(author).

[en] Uninterrupted auxiliary system power supply in large power plants is a key factor for normal operation, transient states, start-ups and shutdowns and particularly during fault conditions. Therefore, there are many challenges in designing the main electrical system as well as the auxiliary systems power supply. Depending upon the type of fuel used and the environmental control system required, a thermal power plant may consume as much as 10% of its total generation for auxiliary power, while a nuclear power plant may require only 4 - 6% auxiliaries. In general, the larger the power generating plant, the higher the voltage selected for the AC auxiliary electric system. Most stations in the 75 to 500 MW range utilize 4,2 kV as the base auxiliary system voltage. Large generating stations 500 - 1000 MW and more use voltage levels of 6,9 kV and more. Some single dedicated loads such as electric driven boiler feed pumps are supplied ba a 13,8 kV bus. While designing the auxiliary electric system, the following areas must be considered: motor starting requirements, voltage regulation requirements, short-circuit duty requirements, economic considerations, reliability and alternate sources. Auxiliary power supply can't be completely generalized and each situation should be studied on its own merits to determine the optimal solution. Naturally, nuclear power plants have more reliability requirements and safety design criteria. Main coolant-pump power supply and continuity of service to other vital loads deserve special attention. This paper presents an overview of some up-to-date power plant auxiliary load system concepts. The main types of auxiliary loads are described and the electric diagrams of the modern auxiliary system supply concepts are given. Various alternative sources of auxiliary electrical supply are considered, the advantages and disadvantages of these are compared and proposals are made for high voltage distribution systems around the thermal and nuclear plant. Arrangements for DC supply systems and uninterruptable power supply (UPS) systems are also discussed.(author).

[en] Quality and safety assurance at implementation of nuclear power engineering projects is important and difficult task for realization. Many problems arise during this process, when many companies from different countries participate, with various kinds of activities and services provided. The scope of activities necessary for quality and safety assurance is therefore quite expanded and diverse. In order to increase the safety and reliability of Kozloduy NPP Plc (KNPP) Units 5 and 6, as well as to bring the units in conformity with the newest international requirements for quality and safety in the field of nuclear energy, a program for their modernization on the basis of different technical studies and assessments was implemented. The Units 5 and 6 Modernization Program of Kozloduy Nuclear Power Plant was composed of 212 modifications aimed to improve the safety, operability, and reliability of the Units. The Program was realized by stages during yearly planned outages since year 2002 to 2007, without additional outages. A major Program Objective was to extend the Units Life Time in at least 15 Years, under a continuous, safe, and reliable operation. The Modernization Program of Units 5 and 6 of the Bulgarian Nuclear Power Plant in Kozloduy was the first and for the time being the only one in the world, program in the field of nuclear power engineering, by which the full scope of recommendations for improvement of the Kozloduy NPP units was applied. The main goal of the National Electric Company, which is the Employer for the construction of new nuclear facility in Bulgaria, is after completion of all activities regarding construction of Belene NPP the plant to meet or exceed the requirements of the respective national and international quality and safety codes and standards, as well as the IAEA guidelines, as they are established. The objective of this report is to describe different aspects of the quality assurance according to the requirements of quality and safety standards at implementation of complex energy projects. Experience is shared on the basis of implementation of the Units 5 and 6 Modernization Program of Kozloduy NPP and the project 'Construction of Belene NPP'. Examples are given related to the definite quality improvement goals achievement in case of participation of big multinational and international companies.(author).

[en] The most important task of every utility operating a nuclear power plant is the continuously keeping of the desired safety and reliability level. This is achieved by the performance of numerous inspections of the components, equipment and system of the nuclear power plant in operation and in particular during the scheduled maintenance periods at re-fueling time. Periodic non-destructive in-service inspections provide most relevant criteria of the integrity of primary circuit pressure components. The task is to reliably detect defects and realistically size and characterize them. One of most important and the most extensive examination is a Steam Generator (SG) in-service inspection. That inspection demand high standards of technology and quality and continual innovation in the field of non-destructive testing (NDT) advanced technology as well as regarding SG inspection tools, control systems and techniques. This article presents overview of the system for SG examination with focus on bobbin and array probes eddy current of the tubes, ultrasonic examination of SG collector welds and service - mechanical plugging of the tubes.(author).

[en] Pressure locking and Thermal Binding are two different but related physical phenomena, which under certain conditions may prevent the opening of some types of Motor Operated Valves. US Nuclear Regulatory Commission (NRC) informed and gave guidance for preventing pressure locking and Thermal Binding (PL/TB) phenomena in GL 89-10 'Safety-Related Motor-Operated Valve Testing And Surveillance', and its Supplement 6. Further action has been required by issuing GL 95-07 'Pressure Locking And Thermal Binding Of Safety-Related Power-Operated Gate Valves', that required evaluation of operational configurations of all safety-related power operated gate valves for susceptibility to PL/TB. Corrective actions are required for susceptible valves that shall assure performance of safety function within plant licensing bases. NPP Krsko (NEK) first addressed this issue through the analysis of the MOV valves that were included in the NEK MOV program. This approach resulted with 31 valves in Krsko NPP being found as susceptible to TB, 21 of which to both TB and PL. Subsequently, a more detailed analysis was performed, which took into account realistic operational parameters and detailed evaluation of likely scenarios. This produced a list of 8 valves susceptible to Pressure Locking, and another 8 susceptible to Thermal Binding. Methods used and results produced are the subject of this paper.(author).

[en] Improvements to existing EddyOne eddy current analysis software are being presented. Those improvements are geared towards improved interaction between the software and ECT analyst by having a better and more featured user interface, while keeping some industry standard signal display norms intact to keep the familiar factor and ease the transition to the next generation of EddyOne. Improvements presented in this paper thus ease the transition to the new software by reducing training requirements for the existing analysts and for new analysts coming to the industry. Further, by utilizing modern technologies next generation of software is able to further reduce maintenance and deployment costs of the whole system for future to come.(author).

[en] During a severe accident in a PWR plant, core degradation and melting could result in slumping of the molten material to the lower head. The relocated material will be composed of ceramic (UO₂, ZrO₂) and metallic (Zr, Fe, Ni) materials whose chemical composition depends on the reactor design, the accident scenario, the core heat-up rate and availability of coolant during core degradation processes, thus, on thermal-hydraulic behaviour of the reactor coolant system (RCS). If the temperature of that mixture is higher than its liquidus temperature (2800-2900 K), the mixture will be in the liquid state. The mixture of molten core materials is known as corium.Production of decay heat will cause intense natural circulation. Although ceramic and metallic melts are immiscible, turbulent nature of the flow will prevent stratification, which might occur due to density difference, and result in negligible spatial temperature variations. The reactor pressure vessel (RPV) lower head wall in contact with the melt will be heated up depending on the energy balance between the heat flux from the molten pool and the heat losses on the outside RrPV wall surface. Cooling of the melt results in formation of a crust surrounding corium which acts as a heat insulator. Therefore, the wall temperature increase will be slowed down by the presence of the crust. Nevertheless, in the case of a large molten pool present in the lower head, there is a high probability of the wall melt-through or rupture.In the presented analysis, corium mass and composition corresponded to a prototypic PWR core. ANSYS code was used for the study of corium natural circulation (FLOTRAN) and for the analysis of the wall thermal behaviour.(author).