Level 1 Low Power and Shutdown PSA

Two nuclear power plants (NPPs) are in operation in Slovakia equipped with WWER440/V213 type reactors. The Jaslovske Bohunice V2 NPP has two reactors in operation, the Mochovce NPP has also two reactors in operation and another two reactor units are under construction which will be given into operation in 2013. Full power and shutdown level 1 and level 2 probabilistic safety assessment (PSA) as part of the plant safety report were performed for these plants by the RELKO PSA team. The role of PSA for NPPs is an estimation of the risks in absolute terms and in comparison with other risks of the technical and the natural world. Plant-specific PSAs are being prepared for the NPPs and being applied for detection of weaknesses, design improvement and backfitting, incident analysis, accident management, emergency preparedness, prioritization of Research & Development and support of regulatory activities. There are three levels of PSA, being performed for full power operation and shutdown operating modes of the plant:  Level 1 PSA: The dominant accident sequences leading to the core damage are identified and the core damage frequency is calculated. The strengths and weaknesses of the safety systems and procedures to prevent the core damage are also provided as results.  Level 2 PSA: The ways in which radioactive releases from the plant can occur are identified and the magnitudes and frequency of release are calculated. Detailed analyses of the containment are performed. Safety measures are proposed to minimize the release of radioactive materials into the environment after a severe accident.  Level 3 PSA: The public health and other societal risks such as contamination of land or food are estimated. Damage to people (number of fatalities, the number of injured, reduction of life expectancy) and damage to property (loss of agricultural products and of natural resources, destruction, the cost of relocating the population and decontaminating effecting areas, etc.) are identified and safety measures are proposed to be implemented to minimize the risk. The Nuclear Regulatory Authority does not require the level 3 PSA for NPPs in Slovakia, however, the performance of analyses is strongly recommended. There are two basic types of the plant outage: unplanned maintenance outages due to the repair of the components and planned refuelling outages. The differences are in:

Outboard-traverse flame migration in an annular profile, combustion gas pathway of a Siemens aero-derivative turbine engine can be detected with a dual immersion thermocouple. This solution is applicable for Oil & Gas operators using gas turbines fueled by natural gas. The typical flame profile within the annular combustion gas flow path is disturbed by introducing poor quality fuel into the engine. The skewed or outward bias flame profile in turn causes severe overheating of the hot section components around the outer radius of the annular combustor exit wall covering a number of hot section components. This ultimately causes accelerated components deterioration and failure to meet its target design life. Consequently, resulting in rejection of these components and increasing the life cycle cost of their asset operations. The introduction of the dual immersion thermocouple allow us to detect outward bias flame pattern using the exhaust gas temperature profile during operation and warn the operator of this condition via software alarm and trip provision. By implementing a means of detecting outward bias flame patterns, the operator will be made aware of this condition and can then take means to resolve this matter by first, allowing to optimize hot section components boundary limits and saving overhaul costs and second, avoid unplanned maintenance outages due to hot section premature failures.

ABSTRACTEnvironmentally assisted cracking (EAC) is a potential threat to the safety and integrity of water-wetted components in operating water-cooled nuclear power plants. Two forms of EAC are commonly distinguished, depending on the form of loading contributing to damage: stress corrosion cracking (SCC) and corrosion fatigue. A number of instances of in-service degradation due to EAC have occurred in operating plants worldwide, often leading to unplanned plant outages. Understanding the causes of EAC is essential to minimise the loss of plant availability due to its occurrence and to avoid the possibility of catastrophic failure, for example, if a crack grew to a critical size in a major pressure boundary component. This paper will describe some examples of these phenomena in the main materials of construction of pressure boundary and other critical components in pressurised and boiling water reactors (BWRs). Over the last several decades, substantial research programmes have been carried out in a number of laboratories worldwide, aimed at further understanding of the processes leading to EAC to manage occurrences in plant and minimise future failures. Selected areas of research on EAC in light water reactor environments are discussed. Corrosion fatigue in low-alloy pressure vessel steels was the subject of considerable attention in the 1980s and early 1990s because of its potential threat to pressure vessel integrity and the publication of data, suggesting that there is a major influence of environment on fatigue crack growth in some laboratory tests. The author’s research provided insight into the conditions under which the major environmental effects occur and contributed to the development of an ASME Code Case for pressurised water reactor (PWR) conditions which provided a means of screening based on steel sulphur content and loading conditions. More recently, the research focus in this area has moved to austenitic stainless steels, again providing support to Code Case development and furthering mechanistic understanding. A recent review of knowledge gaps for EPRI provides a basis for future research on environmentally assisted fatigue and will inform the development of new assessment methodologies. A key area of the current study concerns differences in loading conditions between specimens in laboratory tests and plant components subject to transient loading. In the case of SCC, stainless steels have shown the greatest propensity to cracking in BWRs, while Alloy 600 has been a major cause of in-service failures in PWRs, both on the primary side, as recognised by Coriou in the early 1960s, and in secondary environments where a number of different corrosion-related failure processes have been identified. High-strength alloys, such as Alloy X-750 used for fastener applications, have also caused failures in both reactor types. For austenitic materials, SCC susceptibility is enhanced by irradiation, resulting in failures in core internals components. Ferritic stainless steels also undergo SCC under some specific circumstances but are generally more resistant than the lower chromium austenitic materials.

Owners of existing generating units can increase revenue generation through the utilization of a systematic approach to improve availability, reduce unplanned outages and maintenance costs as well as improve system design. This approach yields a reduced unit cost of operation through higher dispatch rates and capacity factors. It is beneficial to generating units in both regulated and competitive markets. An underpinning of this approach includes benchmarking, which identifies whether maintenance spend and unit availability tracks that of similar populations. Benchmarking also uncovers unbalanced application of maintenance resources based upon historical and projected future equipment failure rates and their impact on EFOR and economic availability. This data is enhanced through a detailed FMECA, which pinpoints areas of focused investment based upon an equipment’s impact due to failure. Knowledge gained through benchmarking, FMECA and best practices is applied in adjusting maintenance strategies. The design of the generating unit is evaluated to determine if there are single points of failure or other similar constraints that contribute to increased outages or derates. Additionally, operating history is reviewed against original design criteria and then overlaid onto failure rates and equipment histories to identify operational changes that can improve availability. This paper discusses the process successfully utilized recently at generating facilities in the US.

This study focused on the design of a battery pack cooling channel based on a Tesla Model S electric car. This study aimed to achieve a balance between cooling efficiency and pressure drop while maintaining safe and optimal operating temperatures for the batteries. A cooling channel design similar to the basic type employed in the Tesla Model S using 448 cylindrical Li-ion batteries was considered. Consequently, important parameters, such as the maximum temperature and temperature difference in the battery cells in a module, as well as the pressure drop of the coolant, were analyzed. In addition, the characteristics of the temperature changes in each cooling channel shape were investigated. The temperature limit for the battery in a module and the temperature limit difference were set to 40 °C and 5 °C, respectively, to evaluate the performance of the cooling system. Further, the effects of discharge rates (3C and 5C), cooling channel shapes (counter flow and parallel types), and coolant inlet velocities (0.1, 0.2, 0.3, and 0.4 m/s) on battery thermal management were analyzed. The results revealed that the parallel type channel yielded a lower pressure drop than the basic type channel; however, it was not as effective in removing heat from the battery. In contrast, the counter flow type channel effectively removed heat from the batteries with a higher coolant pressure drop in the channel. Therefore, a multi-counter flow type cooling channel combining the advantages of both these channels was proposed to decrease the pressure drop while maintaining appropriate operating temperatures for the battery module. The proposed cooling channel exhibited an excellent cooling performance with lower power consumption and better heat transfer characteristics. However, relatively minimal differences were confirmed for the maximum temperature and temperature difference in the battery module compared with the counter flow type. Therefore, the proposed cooling channel type can be implemented to ensure the optimal temperature operation of the battery module and to decrease system power consumption.