Conference Agenda

TRIGA research reactors are known for their pulse mode operation capability. Instrumented fuel elements are one key element ensuring safe pulse operation since they provide real-time fuel temperature.

In 2022, all three thermocouples failed in one of the two instrumented TRIGA fuel elements at the Jožef Stefan Institute (JSI). According to operational limits and conditions, fuel temperature shall be measured at least at one location in the fuel. The following corrective actions were considered to ensure redundancy in fuel temperature measurement.

The procurement of a new instrumented fuel element was not possible at the time. The second option was to equip one of the fresh fuel elements with thermocouples and convert it into an instrumented fuel. The third option was to get an instrumented fuel element from another TRIGA reactor. Lastly, operation without instrumented fuel was also considered, although this would likely preclude pulse mode in the future.

In April 2024, an in-house investigation was initiated to determine the root cause of the thermocouple failure. After dismantling the manipulation rod, corrosion of the thermocouple wires, caused by water ingress, was identified. After replacing the damaged wires and verifying thermocouple functionality, the element was successfully restored to full operational condition.

This case study highlights research reactors' challenges in securing specialised fuel components, especially as reactor infrastructure ages and suppliers become scarce. The successful refurbishment underscores the value of local expertise and technical agility in maintaining reactor instrumentation and operational continuity.

Nuclear research reactors are dedicated facilities used for scientific research, testing, and educational purposes related to nuclear energy. They are designed and used primarily to conduct experiments and tests on various materials under specific monitored operating conditions. However, the ageing research reactor park, and in particular the Material Testing Reactors, will be shrunk year by year, and cannot face up to the new challenges of growing safety criteria, new demands in terms of materials and fuels and innovative nuclear energy systems. Therefore, the Jules Horowitz Reactor (JHR), made possible through the collaboration of an international consortium and various partners, is under construction in Europe (CEA Cadarache, France). At its nominal power of 100 MWth, it will provide a substantial fast neutron flux (5.5 10¹⁴ n.cm^-2.s^-1 for energy from 1 MeV), high accelerated aging (up to 16 displacements per atom per year), and consequently, significant absorbed dose rate consequently called nuclear heating rate (20 W.g^-1 in Aluminum). In order to conduct new advanced in-pile studies on materials under such extreme conditions, the development of new instrumentation and sensors is necessary to ensure specific thermal conditions. A critical parameter for thermal and mechanical design, as well as result interpretation, is the nuclear heating rate, which represents the energy deposition rate per mass unit due to radiation-matter interactions. Consequently, the implementation of new sensors is essential to monitor and measure this parameter accurately. As a result, Aix-Marseille University (AMU) and the French Alternative Energies and Atomic Energy Commission (CEA) have been collaborating since 2009 through various research programs realized within the framework of their joint LIMMEX laboratory and aimed at enhancing calorimetric measurements and developing innovative sensors. Within this area of study, dedicated research work is being carried out on differential calorimeters and single-cell calorimeters. The research approach involves a comprehensive methodology that combines laboratory and real-condition experiments with numerical thermal work, including both 1-D calculations and 3-D simulations. One main objective of the work is to design and qualify new calorimeter prototypes with specific characteristics such as sensitivity, selectivity, and measurement range [1].

The paper will present experimental studies which will be conducted in the near future on a recently developed double-cell CALORRE calorimeter, specifically designed for an irradiation campaign in the Central Channel of the JSI TRIGA reactor within the framework of a research program of the LIMMEX laboratory (MICRO-CALOR) and a mobility program linking education and research for AMU students (Excellence Nucléaire Sud) [2]. The specificity and originality of this new calorimeter is that it contains two samples made of stainless steel and tungsten respectively for simultaneous fission and fusion purposes. The main objective of these studies is to perform, for the first time, a nuclear heating rate axial profile of this channel, using this double-cell CALORRE-type calorimeter with a low expected value (< 0.3 W.g^-1). The first part of the paper will deal with the design and key features of the sensor, including aspects such as size, material composition, instrumentation, and assembly to meet constraints in terms of sensor sensitivity and performance. The second part will describe the experimental set-up and the operating protocol used for calibration in the Central Channel when the reactor is shutdown. The main metrological characteristics obtained (linearity, response time, sensitivity, etc.) will be given and compared with those obtained with other CALORRE calorimeter prototypes. The third part will present the experimental and operating protocol to move the calorimeter to measure the axial profile of the nuclear heating rate in the Central Channel and the associated measurement methods. The last part will outline the in-pile results: the axial profile obtained thanks to each calorimetric cell for the two sample natures and the measurement of the nuclear heating rate peak as a function of the reactor power. These results will be compared and discussed in particular with results obtained in the TIC channel in two previous campaigns realized in 2023 and 2024.

By design, many research reactors are birthplaces of new and exciting ideas and melting pots for interdisciplinary collaboration, which in turn allows for groundbreaking research on complex and often underexplored topics. Yet the socioeconomic impact of this type of reactors is frequently underestimated since it does not only affect the scope of pure scientific research but extends further beyond, ultimately spreading its influence to many fields. These reactors, consequently, do not only serve as critical platforms for technical training but also as foundational tools in early-stage medical studies and as valuable assets in industrial processes and quality control systems.

As underlined in the 17 objectives of the 2030 Agenda for Sustainable Development, many of these topics are regarded as highly relevant, particularly in the context of ensuring quality education (SDG 4), promoting good health and well-being (SDG 3), and fostering industry and advancement (SDG9). Research reactors, especially those of low and medium power that are limited by their capability of producing radioisotopes for medical use, can still play a key role in enhancing national capabilities, supporting regional cooperation, and driving socioeconomic growth.

This paper aims to highlight the experience gathered over the years by Pavia’s TRIGA MKII research reactor as a case study for the impact on Italian soil.