The TRIGA Mark II research reactor at the Jo & zcaron;ef Stefan Institute is a key facility in the field of nuclear research, characterized by its versatility and applicability in a wide range of scientific disciplines. This document highlights its operational history, contributions to nuclear safety, education and various scientific applications, including advances in reactor and radiation physics, neutron activation analysis, environmental science and even contributions to the fight against the COVID-19 pandemic. It highlights the reactor's significant role in fostering international collaborations, improving computer modeling techniques for nuclear research, and providing invaluable educational experiences. The great versatility and applicability of the JSI TRIGA reactor is emphasized by its adaptability to various research needs and its ability to enable groundbreaking studies in both fundamental and applied sciences.

The paper focuses on the Jozef Stefan Institute (JSI) TRIGA Mark II research reactor's operational highlights from 2023. Firstly, some essential operating performance indicators are presented and compared to the ones from the previous years. In 2023, we operated less than in years 2022 and 2021. However, more than 570 hours of operation represents the average from the last 10 years. Research work performed in the last year is described. The research campaign involving CEA has continued from the previous years. Installation of a new irradiation facility called a water activation loop was fully completed. In the field of education and training, we continued all standard activities. In 2022, we faced a failure of all three thermocouples inside the fuel element. The paper presents several options on how to re-establish measurement of fuel temperature using two independent fuel elements.

After the calendar year, operating performance indicators for Jozef Stefan Institute TRIGA Mark II research reactor are analysed. After several years of increasing yearly operating hours, a significant decrease is observable last year due to the pandemic of the SARS-CoV-2 virus. However, the number of irradiated samples was larger than the year 2019, indicating that the reactor was operated more efficiently. Furthermore, reactor personnel tried hard to adjust the situation. Since September 2020 we can offer all exercises designed for education and training purposes in remote option. In that way, our reactor became much more accessible, especially for students who cannot afford to travel to Slovenia.

After the calendar year is over, operating performance indicators for Jozef Stefan Institute TRIGA Mark II research reactor are analysed. For the second year in a row, there was significant increase in operating hours of the TRIGA reactor and total received dose by the operating staff. The main reason for that are research campaigns for foreign users which number is growing year by year. Such campaigns are not important just to serve as an excellent reference but since customers are from abroad, campaigns are important to keep our budget positive. This is reflected in improved service received not just by foreign researches but also by domestic users.

TRIGA type research reactors, even relatively new ones, are originally equipped with rather obsolete irradiation pneumatic transfer systems. Therefore, the irradiation system of Slovenian TRIGA Mark II system was renewed in 2015 to improve the overall quality of irradiations. This year, the system was upgraded to allow for automated short irradiations under more thermalized neutron flux. The modernization includes also a so-called "triangular" channel, allowing for in-core irradiation of samples up to 5 cm in diameter and a horizontal channel allowing for irradiations of objects under homogeneous neutron flux in the length of over 60 cm.

The International Atomic Energy Agency (IAEA) has implemented a detailed E-learning on-line course in neutron activation analysis (NAA). Existing books and guidance documents on the concepts and execution of NAA are out of date and not sufficient anymore to ensure the transfer of current knowledge on its practice. The overall objective of the E-learning tool in NAA is to realize a 'living book', summarizing the basic concepts and providing practical information on the implementation of the methodologies which can be more easily updated than a common book, allowing also for visualization using contemporary media.

After the calendar year is over, operating performance indicators for Jozef Stefan Institute TRIGA Mark II research reactor are analysed. The numbers for year 2018 show significant increase in operation which can be seen from indicators like operating hours or total received dose to the operating staff which doubled from year 2017 to year 2018. The main reason for that are several interesting research campaigns during which characterisation of newly developed gamma and neutron detectors was done. Such campaigns are not important just to serve as an excellent reference but since customers are from abroad, campaigns are important to keep our budget positive and therefore reactor in good shape.

Jozef Stefan Institute (JSI) TRIGA Mark II research reactor has been in operation for more than 52 years. Its safety performance indicators (SPIs) have been systematically monitored and thoroughly analysed during the last 10 years, to ensure and further improve its safe operation. In the paper, first SPIs for the year 2017 are presented and compared with the ones from previous years. In the second part of the paper, main achievements related to the reactor maintenance and reactor utilization are presented, for the years 2017 and 2018.

The k 0 method of neutron activation analysis (k 0 -NAA) has become a well-established analytical tool for elemental analysis of various samples since its introduction in mid-1970s.To strengthen its development, applications and use, a series of regular workshops dedicated to this particular analytical method has been introduced twenty-five years ago, held in Astene, Belgium

The flipped classroom approach is being developed for teaching of basic course on analytical radiochemistry. The objective of the course is to provide basic knowledge and practice on the principles of analytical radiochemistry. Video and other teaching materials are being developed to utilize them in flipped classroom. More than 50 modular videos will be produced. They are designed in modular way, which enables to arrange them in different fashion, providing teacher greater flexibility of delivering specific topic either with focus on the radionuclide or specific steps, such as sample preparation techniques, radionuclide separation techniques, preparation of counting sources, radionuclide measurements etc. Such approach also enable using materials for different target audiences such as in regular classes, summer schools, tailor made training courses and workshops. The structure of the course, modular videos and methodology used to produce them is presented and discussed. Modular videos, forming part of flipped classroom are posted at Videolectures.net portal (http://videolectures.net/meet-cinch_courses/).

Analytical radiochemistry of neutron-activated samples, as practiced during past decades at the Jožef Stefan Institute Ljubljana, is outlined. The paper reviews achievements made in both elemental analysis and analyses of long-lived radionuclides, in variety of sample types. The presented analytical procedures include application of diverse chemical separations, multiple irradiations of samples, use of various nuclear reactions and detection modes for particular measurands, and determination of elements that are difficult to be determined by neutron activation analysis (NAA). Useful practical applications of neutron-activated tracers for chemical yield determinations, as unique feature of radiochemical NAA in comparison with non-nuclear analytical methods, are also addressed.

Selected natural radionuclides from the uranium decay chain were determined in radish, savoy and rocket grown in uranium mill tailing contaminated growth substrates. The data obtained were used in calculation of dose assessment. Based on the results obtained for activity concentrations of four radionuclides ( 238 U, 230 Th, 226 Ra and 210 Pb) in vegetables, cultivated in contaminated soil, the committed effective doses, along with the maximum values for adults, were estimated. Statistical analyses of linear correlation among calculated annual effective ingestion dose and uranium mill tailings showed to be highly significant.

Environmental concern due to plant accumulation of natural radionuclides is a major concern in uranium mining areas. To evaluate the risk associated with the transfer of radionuclides to edible plants, the uptake of 238U, 226Ra, and 210Pb by Chinese cabbage (Brassica rapa L. subsp. pekinensis (Lour.) Hanelt) grown in soils contaminated with uranium-mill tailings (UMT) was investigated. Test plants were grown under controlled conditions in substrate composed of soil and UMT in different ratios. Activity concentrations of 238U, 226Ra, and 210Pb in substrate, leaves, and roots were measured and the concentration ratios determined. Soil characteristics were determined, since they directly affect bioavailability of radionuclides. Concentration ratios of 238U, 226Ra, and 210Pb in leaves varied from 0.001 to 0.006, 0.024 to 0.172, and 0.004 to 0.011, respectively, and in roots from 0.020 to 0.126, 0.015 to 0.241, and 0.033 to 1.460, respectively. Concentrations of 238U, 226Ra, and 210Pb in leaves and roots were found to correlate with the amount of 238U, 226Ra, and 210Pb in the substrate. A higher amount of 226Ra accumulated in aboveground parts (57–877 Bq kg−1 d. m. for leaves) compared to 238U (0.6–4.7 Bq kg−1 d. m. for leaves) and 210Pb (8–53 Bq kg−1 d. m. for leaves), which were mainly stored in the roots. The relationships between the amount of radionuclides in plants and soil characteristics and their role in radionuclide uptake are discussed and critically evaluated.

A survey is given of the installation, applications and main achievements of the k 0 method of neutron activation analysis as implemented at the Jožef Stefan Institute, Ljubljana/Slovenia, during 30 years of its continuous use. Main scientific achievements include international dissemination of its application, by contributions to processes of certifying new reference materials and improvements in better assessment of its measurement uncertainty.

Initiation and main achievements in applying neutron activation analysis (NAA) at the TRIGA Mark II reactor of the Jozef Stefan Institute during its 50 years of continuous operation are reviewed from a historical perspective, with emphasis on the features specific for NAA. The development of radiochemical neutron activation analysis for characterising reference materials is described in detail in the first part. Several approaches specific for the method such as the determination of long-lived radionuclides, the simultaneous determination of several elements using double irradiation and the internal standard method are presented in more detail. Several interesting approaches such as combination with liquid scintillation counting detection and combination with radiometric measurements are outlined. Then, the development of instrumental neutron activation analysis and in particular the so-called k(0) - NAA method is presented and discussed. The contribution towards methodological development of uncertainty assessment is outlined, and the importance of the nuclide-specific and neutron fluence-specific approach is emphasised. Throughout the paper, the importance of neutron activation analysis and contributions of measurements results obtained by this method to characterising (candidate) reference materials is shown.

A novel method for determination of (210)Pb activity concentration using a liquid scintillation counter (LSC) in environmental samples is presented. After radiochemical separation of (210)Pb on Eichrom Sr Resin column, the decay product (210)Bi starts to in-grow and interfere with the (210)Pb during measurement with LSC. Instead of eliminating this interference, a novel method utilises (210)Bi in-growth to improve the detection efficiency and subsequently to lower the minimum detectable activity (MDA). This allows for substantial reduction of the MDA compared to conventional methods.

The pneumatic transfer system (PTS) of the Jožef Stefan Institute’s TRIGA Mark II research reactor was modernized. The main goals of the PTS’s renewal were to improve the overall quality of the irradiations, to increase the level of safety and to improve the timing precision. To lower the costs and to facilitate the licensing process, this upgrade was made with the minimum possible intervention with respect to the existing tubing connections. The upgrade involves only two new mechanical components: a terminal station and an automated capsule loader. A similar solution can be applied to any TRIGA-type PTS.

Nuclear components are under strict supervision of operators and safety authorities. The Reactor Centre of the Jožef Stefan Institute decided to make an inspection of its TRIGA Mark II research reactor to verify the conditions for long-term future operation within the on-going periodic safety review. Two main inspection methods were used: ultrasonic and visual inspection. Ultrasonic inspection was selected to prove that there is no significant reduction of wall thickness anywhere in the tank. The inspection confirmed that the reactor tank has not been degraded or corroded. In the future such inspection will take place every 10 years within the periodic safety review in order to monitor every 10 years the reactor tanks condition.