Conference Agenda

The TRIGA Mark II research reactor at the Jožef Stefan Institute (JSI) in Ljubljana achieved its first criticality on 31 May 1966 and has played a central role in the development of nuclear science, technology, and safety culture in Slovenia since then. It is a pool-type, light-water-cooled thermal research reactor with a maximum power of 250 kW, a maximum thermal neutron flux of approximately 2×10¹³ cm⁻²s⁻¹, and a 20% enriched U-ZrH fuel core. On the occasion of its 60th anniversary, this paper presents an overview of its evolution, current activities, and future perspectives.

During its early decades, the reactor was predominantly used for radioisotope production for medical and industrial applications, as well as for neutron radiography. In 1991, a comprehensive reconstruction was carried out, including the replacement of the grid plates, control rods, and instrumentation, and the loading of fresh 20% enriched fuel. The spent fuel was returned to the United States in 1999. Following the reconstruction, the reactor utilisation transitioned to research, education and training. Current activities focus on reactor physics benchmark experiments, verification and validation of computational codes (MCNP, Serpent, OpenMC, TRIPOLI, RAPID, TRIGLAV) and nuclear data, neutron activation analysis, dosimetry, and the recently established research programmes in radiation-catalysed chemistry and high-energy water activation (KATANA facility). Over 500 scientific articles have been published using the reactor between 1966 and 2024.

In recent years, the TRIGA reactor has become an important international hub for irradiation experiments, particularly in the development of radiation detectors. It serves as an unofficial reference facility for testing detectors used in high-energy physics, including for the ATLAS experiment at CERN, where it supports both R&D and the quality assurance programme for silicon sensor production for the High-Luminosity LHC upgrade. A long-standing collaboration with the French Alternative Energies and Atomic Energy Commission (CEA) has established the reactor as a reference platform for detector characterisation, neutron and gamma dosimetry, and the validation of measurement techniques in well-characterised mixed neutron-gamma fields.

Education and training are among the key pillars of reactor utilisation. The TRIGA reactor has evolved into one of the leading small research reactors in Europe in this field, supporting the education of students, nuclear professionals, and reactor operators. In addition to domestic users, it regularly hosts participants from various countries, including France, Italy, Sweden, and Saudi Arabia, as well as partners within the ENEEP (European Nuclear Experimental Educational Platform) and EERRI (Eastern European Research Reactor Initiative) frameworks. Since 1989, the Nuclear Training Centre (ICJT) at JSI has trained 321 future control room operators for the Krško Nuclear Power Plant through 20 courses, with experiments conducted at the TRIGA reactor. Public outreach activities at ICJT have hosted more than 200,000 visitors since 1993, a substantial share of whom have also visited the reactor.

Operation of the reactor is conducted under high safety standards, comparable to those of nuclear power plants, and includes periodic safety reviews performed every ten years. The current operating license is valid until 2034. The third periodic safety review is being prepared and is expected to support continued operation until at least 2043. In parallel, the VERONICA project (Versatile European Reactor fOr Neutron Irradiation and nuClear reseArch) is underway at JSI to develop technical guidelines and a substantiated basis for a future decision on a successor research reactor in Slovenia.

After 60 years of operation, the TRIGA reactor remains a modern, flexible, and internationally recognised research infrastructure, contributing significantly to nuclear science, education, and technological development in Slovenia and across the European research reactor community.

Across Europe, nuclear energy is experiencing renewed momentum through life extensions of existing reactors, deployment of Generation III+ plants, development of Small Modular Reactors, and increasing investments in advanced reactor technologies. Alongside these developments, research infrastructure remains an essential enabler for education, training, materials research, fuel development, isotope production, regulatory preparedness, and industrial innovation.

For the Nordic countries, this discussion has a particular relevance. Following the closure of facilities such as the Studsvik R2 reactor and the Halden reactor, the region no longer possesses domestic reactor-based irradiation infrastructure despite a long and internationally recognized history in nuclear research and development. At the same time, new demands are emerging from industry, academia, healthcare, regulators, and future nuclear programs.

Rather than beginning with a predefined technology concept, the Nordic Association for Nuclear Acceleration (NA²) initiated a structured stakeholder-driven process to better understand long-term regional needs and priorities. During 2026, a comprehensive stakeholder consultation was conducted involving more than 500 individually contacted experts, organizations, and decision-makers from the Nordic countries and the wider European nuclear community.

The consultation explored strategic priorities, competence requirements, isotope needs, research demands, supply-chain readiness, governance expectations, and potential collaboration models. The results indicate that stakeholders consistently place competence development, education, knowledge retention, medical applications, and industrial capability building among the highest priorities. At the same time, respondents identify governance, financing, implementation capacity, and regulatory preparedness as key success factors for future infrastructure initiatives.

This paper presents the methodology and key findings of the consultation and discusses how these insights are being translated into the early stages of a Nordic research infrastructure feasibility process. Particular attention is given to stakeholder engagement, collaborative governance, Human Factors Engineering (HFE), Man-Technology-Organization (MTO) principles, and the role of research infrastructure as a long-term capability platform rather than solely a technical facility.

The intention is not to propose a replacement for existing European initiatives, but to contribute to the ongoing discussion on how regional capabilities can be strengthened in a manner that complements and supports the broader European nuclear ecosystem. The lessons learned may be relevant to other organizations and countries facing similar questions regarding competence continuity, infrastructure renewal, and long-term nuclear capability development.

Nuclear research reactors, such as Material Testing Reactors (MTRs), support existing nuclear power plants and future generations of reactors thanks to studies on material and fuels under representative and extreme conditions, including accelerated aging and accidental conditions. Due to the aging of the Material Testing Reactor (MTR) park, the Jules Horowitz Reactor (JHR), a new research reactor with unequaled performance in Europe, is currently under construction at the CEA Cadarache center in the south of France. At its nominal power of 100 MW_th, it will provide a fast neutron flux (5.5 × 10¹⁴ n.cm^-2.s^-1 for E > 1 MeV), high accelerated aging (up to 13 displacements per atom per year), and consequently, significant nuclear heating rate (20 W.g^-1 in Aluminum). Since 2009, this reactor has led to new research programs between AMU and CEA, thanks to the LIMMEX joint laboratory. These research programs aim to improve online measurements of crucial nuclear quantities, such as the nuclear heating rate, which is essential to size and interpret experiments conducted in experimental channels of the JHR.

A primary objective is to design and develop innovative instrumentation and measurement methods for online nuclear heating rate measurement via non-adiabatic calorimeter. One associated challenge corresponds to the expansion of the measurement range from very low to high nuclear heating rate while focusing on several key aspects: the miniaturization of the calorimeters, the optimization of the metrological performances (sensitivity, linearity, response time, etc.), the development of new calibration and measurement methods and the measurement of the associated thermal properties as a function of the temperature. A new research program, in the framework of the LIMMEX laboratory, called MICRO-CALOR, began in 2020 to design and study miniaturized single-cell calorimeter integrating thin-film heating element (patented by AMU and the CEA). A step-by-step scientific approach starting from laboratory conditions was adopted to characterize, qualify and validate the use of the new thin-film heating element under real conditions. First, a high sensitive single-cell CALORRE calorimeter (integrating a classical heater) was specifically designed for irradiation in the dry Triangular Irradiation Channels (TIC) of the JSI TRIGA reactor (Slovenia). It was successfully tested under laboratory conditions and qualified in the TIC in 2023 and 2025. Very low values of nuclear heating rate (< 0.1 W.g^-1 in Aluminum) were measured for the first time in this channel. Then, the calorimeter was optimized to integrate an innovative thin-film deposited heating element and was used to validate the behavior of this new heater under real conditions thanks to other irradiation campaigns in the TIC in 2024 and 2025.

The paper will outline the design of the first single-cell CALORRE calorimeter and its optimization, the characterization of the innovative heater, its integration into a second optimized single-cell CALORRE calorimeter, the comparison of the response of the two calorimeters under real conditions and the validation of the innovative-heater characteristics after three irradiation campaigns in the JSI TRIGA reactor.

The first part of the paper will present the JSI TRIGA reactor core conditions and the TIC. The second part will present the design and key features of the first calorimeter integrating one classical heater (size, material composition, instrumentation and assembly), the experimental set-up, the operating protocol used for the calibration under laboratory conditions and the measurement method used to determine the nuclear heating rate. The main metrological characteristics of the calorimeter (temperature, linearity, response time, sensitivity, etc.) will be given and compared with those analytically calculated using 1-D thermal model. The measurement of the nuclear heating rate in the TIC (at the median plane) as a function of the reactor power will be presented. Finally, a comparison will be made between the in-pile experimental results and 3-D thermal numerical calculations providing important feedback on the global performances of the calorimeter.

The third part will focus on the validation of the innovative heating element inside the second calorimeter. First, the design and the fabrication of this new heating element fabricated using the serigraphy technique will be described. Then, its specifications and experimental characterization (chemical, electrical, and thermal) out-of-calorimeter will be detailed. After the integration of this heater inside the second calorimeter, electrical characteristics (heater resistance) and calibration curves of the calorimeter obtained before and after the different irradiation campaigns in the TIC at the median plane were analyzed. Thanks to this analysis, the validation of the behavior of the new heater will be shown.

The last part will be dedicated to experimental comparisons. Based on the heater validation, the nuclear heating rate was quantified using two measurement methods (including one using the new validated heating element). These results will be compared with results obtained in the TIC with the first calorimeter and will be discussed.

Neutron radiography has become an increasingly important diagnostic tool, among other areas, in the field of cultural heritage due to its non-invasive nature and its capability to investigate the internal structure of historical artefacts without causing damage. The technique enables the identification of different constituent materials and their spatial distribution within an object, providing a suitable level of resolution and contrast for the study of complex archaeological and artistic samples.

Neutrons provide enhanced contrast for light elements, especially hydrogen, which is often associated with corrosion products, mineralization phases, organic residues, or moisture content, rendering this technique uniquely compatible with X-rays. This complementary behaviour makes neutron radiography especially valuable for the investigation of metal–mineral interfaces and for the characterization of degradation and conservation processes in cultural heritage artefacts.

Neutron radiography facilities utilize scintillator-detectors, imaging plates, and high-resolution cameras, along with advanced image-processing techniques, in order to obtain higher spatial resolution and reduced noise. Within this context, the Cultural Heritage Network (CHNet) of the Italian National Institute for Nuclear Physics (INFN) installed an operational neutron imaging station at the thermal beam port of the 250 kW TRIGA Mark-II research reactor operated by the Laboratorio Energia Nucleare Applicata (LENA) in Pavia, Italy. This work will focus on the neutron-based diagnostic capabilities for multidisciplinary cultural heritage investigations of the newly installed facility.