Sludge removal is performed on two steam generators (SG’s) at the Krško Nuclear Power Plant (NEK) during every outage. SG’s are a meeting point of five major plant systems: Reactor Coolant System (RC) on the primary side and four systems on the secondary side – Auxiliary Feedwater System (AF), Blowdown System (BD), Main Feedwater System (FW), and Main Steam System (MS). Sludge removal activities take place on the secondary side of the SG’s on the top of the tube sheet. It always consists of classical Sludge Lancing (SL) which is done by spraying water at different angles (30°, 90°, 150°) between the tube gaps in the steam generator tube bundle with a pressure of around 220 bars. Another method is Inner Bundle Lancing (IBL) which means spraying water directly inside the tube bundle with a traveling lance tape with a spraying nozzle at the end. Water is sprayed at an angle or directly on the top of the tube sheet with a robot-guided manipulator which is placed inside a steam generator. The manipulator and therefore the spraying action is controlled by an operator and at times it is fully autonomous to provide the highest protection measures possible. Another method of sludge removal which was for the first time utilized in 2019 at the Krško site was Chemical Cleaning (CC) of both SG’s. During this process, a chemical was injected into the SG’s through the BD system and periodically pumped between the two SG’s to create a dynamic flow and maximize the cleaning effect. To achieve the best results, a constant temperature of the chemicals had to be maintained at all times. Upon completion of chemical cleaning, a rinsing phase was followed to remove any post-treatment chemicals. After all sludge removal activities, a televisual inspection (TVI) of the top of the tube sheet was performed to access the hard sludge area and to search for potential foreign objects in the SG’s. If for instance an object of importance during TV inspection is found, an attempt to retrieve it would usually take place. Other methods of sludge removal such as upper bundle flushing or ultrasonic cleaning have not been implemented in NEK thus far. Since the power plant uprate in May 2000, NEK conducted SL on both SG’s every outage also starting with IBL in 2013 and 2015, and the same method was used in the 2018 outage. During the outage in 2019, all three methods (SL, IBL, and CC) have been utilized with the main purpose to extend the full load operation of the plant, preventing and/or stopping denting processes in the SG’s from occurring, reducing and stopping the build-up of hard sludge area to increase/sustain efficiency and remove foreign objects found in the SG’s. SG’s U-tubes are a barrier between the primary side coolant and the secondary side of NEK and the environment. Therefore, it is crucial to keep the highest level of integrity of the U-tubes because any leak could potentially mean a release of radioactive material into the atmosphere. This paper describes the purpose and workflow of sludge removal activities in the outage of 2019 in NEK.

The purpose of this paper is to establish a supervisory battery management system which collects measurements from the installed battery storage units and enables various battery management functions. In addition, an HMI user interface was designed to allow the user to monitor and control the batteries. The batteries communicate with the PLC via Modbus TCP/IP protocol. The communication enables the exchange of measured power and state of charge values between the PLC and batteries, as well as setting the desired setpoint values. The PLC and HMI communicate via PROFINET. The battery storage units are installed in Smart Grid Laboratory (SGLab) at the University of Zagreb Faculty of Electrical Engineering and Computing.

The safety of nuclear power plants has always been one of the most important security issues in the industry in general. Numerous standards, techniques, and tools have been developed to deal specifically with the safety of nuclear power plants – one has specialised probes, robotized systems, electronics, and software. Although seen as a mature (or slowly evolving) industry, this notion about nuclear safety is a bit misleading – the area is developing in many promising new directions. Some recent global events will speed up this development even more. On the other hand, the industry is currently going through digital transformation, which brings networking of devices, equipment, computers, and humans. This fourth industrial revolution promises speed, reliability, and efficiencies not possible up until now. In the NDE sector, new production techniques and traditional manufacturing lines are getting to be lights-out operations (near-total automation). The same is most probably going to happen with the safety inspections and quality insurance. Robotics and automation are improving worker safety and reducing human error. The well-being of inspectors working in a hazardous environment is being taken care of. Most experts agree that the digitalization of NDE offers unprecedented opportunities to the world of inspection for infrastructure safety, inspector well-being, and even product design improvements. While the community tends to agree on the value proposition of digital transformation of NDE, it also recognizes the challenges associated with such a major shift in a well-established and regulated sector. The work presented in this paper shows a part of the project that aims to develop a modular ultrasound diagnostic NDE system (consisting of exchangeable transducers, electronics, and acquisition/analysis software algorithms), for applications in hazardous environments within nuclear power plants. The paper will show how the software part of this system can reach near-total automation by implementing various deep learning algorithms as its features and, then, testing those algorithms on laboratory samples, showing encouraging results and promises of online monitoring applications. Furthermore, future general prospects of this technology are discussed, and how this technology can affect the well-being of nuclear power plant inspectors and contribute to overall plant safety.

During their lifespan all the primary equipment installed in power system is subjected to transients. Therefore, it is necessary to adequately dimension and protect the equipment. However, in the networks with high share of renewables, power electronics, cables or near GIS, the failure can occur even though good practice of insulation coordination are fulfilled. In this paper high frequency power transformer model is presented. Such model is based on measurements of admittance matrix and is adequate for simulation of fast front transient in EMTP like software. Additionally, simulations of lightning and switching overvoltages are given for the observed power transformer unit.

The paper presents the RELAP5/mod3.3 analysis of natural circulation cooldown with one inactive loop for Nuclear Power Plant Krško (NEK). The aim of the analysis is to determine the limiting cooldown rates during operator recovery actions to minimize the effect of flow stagnation in inactive loop. Since this is typical asymmetrical transient, the RELAP5/mod3.3 NEK model with split reactor vessel model was developed (models of the reactor vessel and core were axially divided in two parts) and used for this analysis. The several transients of cooldown, with one inactive loop, for different time after shutdown (different decay heat) were performed. The extreme conservative assumptions were applied for the analyses, i.e. the complete loss of feedwater (FW) and auxiliary feedwater (AF), including turbine driven (TD) AF pump, and the cooldown has started after the SG is completely dry (inactive). The analyses show that the cooldown rate shall be significantly reduced, and, based on the results the procedure ES-0.2 “Natural Circulation Cooldown” was modified.

Being a facility with potential for radioactive release, any nuclear power plant (NPP) is, over its operating life time, permanently subject to numerous safety reviews with different scopes and objectives. The reviews may be initiated and implemented by various stakeholders, including regulators, utilities or industry. Some of them are, by their nature, general and extensive in terms of different safety areas or safety attributes which are covered. An example of such a review is a Periodic Safety Review (PSR) which is promoted by the International Atomic Energy Agency (IAEA) and a number of national safety authorities in Europe and worldwide. The others may, depending on the objective, be targeted at particular safety area (e.g., ageing management or equipment qualification or safety analyses). Both of the mentioned cases (single general review or multiple targeted reviews over a time period) can generate an inventory of observations (“findings”) or “issues” which need to be addressed but may be very different in their nature and implications, as well as in benefits or resources associated with their resolutions. For some issues a resolution may be straightforward. For others, it may require a feasibility study and identification of options for possible resolution. Also, in some cases the resolution is simply a “must” (e.g., discrepancy from licensing basis) while in some other cases it may be a matter of balance (e.g., effectiveness of maintenance program). Furthermore, while some of the issues may be directly related to operational safety (e.g., non-compliance with single failure criterion or aging-related degradation of safety features), for some others the link to operational safety may not be explicit (e.g., comparison of safety bases against the newly emerging methodologies or issues observed with regard to so called “soft factors”). The paper discusses types of different observations or issues which may come from general or targeted safety reviews and outlines some basic principles for their comparison and prioritizing with regard to possible safety impacts, which is many times needed for the purpose of developing an action plan for safety improvements.

The Electricity Market Act stipulates that the distribution system operator is encouraged to use flexibility, including participation in congestion management in distribution network in coordination with transmission system operator, to increase efficiency, develop the distribution system and promote energy efficiency measures. DSO can access flexibility in one or more of the following ways: market-based procurement of flexibility services, distribution network tariffs, flexible (non-firm) connection agreements, rules based (regulated) approach, in combination or separately. The categories are not necessarily mutually exclusive and the inherent regulatory incentives and implemented measures may overlap. Member States and national regulatory authorities should, therefore, carefully evaluate the interactions when implementing new forms of access to flexibilities or when enhancing existing ones. The paper reviews the mentioned possibilities of using flexibility in the distribution network.

The paper presents the importance of the grid inertia constant for the frequency stability of the future high-res low-inertia power systems. Since more and more renewable energy sources (RES) are being connected to the power system via frequency converters, the grid inertia constant is reduced. This issue can be mitigated by applying appropriate control mechanisms through which RES can provide virtual inertia and provide rotating reserves for primary frequency control. The concept of a virtual synchronous generator for providing virtual inertia is elaborated, as a solution to the presented problems. By applying virtual synchronous generators, RES can provide support for frequency control during disturbances almost like conventional synchronous generators. The influence of virtual inertia on the stability of the Croatian power system was analyzed using a battery energy storage systems (BESS) with a control mechanism that enables its participation in frequency control.

This paper analyses possible synergies between demand response flexibility programmes and energy savings delivered by households. In the framework of the energy transition, European Union (EU) directives are endorsing energy consumers to become full-fledged participants of the energy market, mostly via independent aggregator intermediaries. The flexibility aggregators have a very arduous role in collecting, optimising and settling aggregated flexibility delivered from heterogenous sources on the energy market. Novel business models incorporating both flexibility and energy savings opportunities from households’ consumers could deliver revenue diversification for flexibility aggregators and support them in overcoming technical and motivational challenges for activating consumers in the energy market. This paper discusses the main pillars for a sustainable flexibility aggregator business model which sums up the potential for flexibility placement on energy, ancillary services and energy savings markets. The main challenge identified in this work are the requirements for programme establishment, allowing the recognition and proper interpretation of energy savings triggered by short-term events and obtained by an aggregator via explicit demand response actions. This paper proposes possible solutions for a joint venture of a flexibility and energy savings aggregator, thus alleviating possible data collection problems. Collaborative efforts have been recognised in the establishment and maintenance of information and communication technologies and infrastructure, therefore facilitating continuous monitoring and verification of flexibility programmes which are able to deliver energy savings.

The aim of this paper is to point out the importance of early diagnostics and assessment of the condition of the electric motor in increasing the reliability and availability of the plant. Electric motors are unavoidable elements in every production process. As the complexity of the process increases, the number of electric motors involved in it increases. By increasing the number of electric motors, the possibility of cancelling and interrupting production increases. It is therefore very important to monitor the situation and detect defects at an early stage so that timely repair or replacement can be planned, without affecting the production process. Modern diagnostic methods that can be used both during the operation of the electric motor, and with the dismounted and dismantled electric motor are presented in the paper. With good organization and testing schedule and a combination of static and dynamic diagnostic methods, the reliability and availability of the plant can be significantly increased. With increased reliability and availability, a reduction in maintenance costs is achieved. In this article, some examples from the author's practice are given, when serious engine defects were discovered. These cases confirm the value of predictive testing to increase reliability of electric motor.