Nuclear heating plays an important aspect in design and deployment of both fission and fusion reactors and experimental devices in terms of cooling requirements. Two experimental campaigns in the framework of a collaboration project between the French Atomic and Alternative Energy Commission (CEA) and Jožef Stefan Institute (JSI), Slovenia, have been performed at the JSI TRIGA reactor for the experimental assessment of nuclear heating in fission and fusion-relevant materials by the differential calorimetry technique, based on the CALMOS and CARMEN differential calorimeters, previously developed at CEA. The results of the first campaign performed at reactor powers between 100 and 250 kW have already been reported, highlighting some measurement difficulties. Therefore, the second campaign was performed at a lower reactor power of 30 kW to overcome these issues. Moreover, a computational analysis of the experiments was performed using the JSIR2S code package to calculate the nuclear heating levels. Both experiments and their reproduction by simulations are described in detail. We present a comparison of the previously reported measured nuclear heating values of the first campaign with the computational results, with consistent underestimation by simulations by 8–35

Neutron activation dosimetry is the primary method for the determination of the neutron flux or fluence, and in general, it is sensitive to the thermal and resonance energy ranges (radiative capture reactions-(n,gamma)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(n,\gamma )$$\end{document} reactions) and the fast energy range (threshold reactions). However, there are very few nuclear reactions which are sensitive specifically to neutrons in the intermediate-epithermal-energy region. This energy region, along with the fast energy range, will become particularly important in the development and deployment of new reactor technologies (Generation IV reactors and Small Modular Reactors-SMRs), which are currently being championed as technologies enabling a meaningful contribution to decarbonization and the fight against climate change, as well as nuclear fusion. The epithermal neutron energy range is also of particular importance for Boron Neutron Capture Therapy (BNCT), a neutron-based cancer therapy, particularly effective for the treatment of head and neck cancer, malignant meningioma, melanoma and hepatocellular carcinoma. This work investigates and demonstrates the applicability of a particular set of (n,gamma)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(n,\gamma )$$\end{document} reactions in conjunction with boron-based neutron filters to achieve sensitivity in the epithermal energy region, and discusses avenues for future research in this context.

Niobium is a dosimeter used to characterize fast-neutron reaction rates to monitor nuclear reactor vessels. Its characterization is based on the activity of 93mNb resulting from the 93Nb(n,n’) activation reaction. The decay of 93mNb results mainly in the emission of niobium K X-rays which are used to determine the activity of 93mNb. Direct measurement using X-ray spectrometry does not require any sample preparation, but fluorescence effects, due to impurities which are activated during irradiation, must be taken into account. Indeed, some of these radioactive impurities, in particular 182Ta, and other niobium isotopes remain present during the measurements and disturb the X-ray spectra. The fluorescence effect induces additional niobium X-ray emission to that due to the 93mNb decay alone, which leads to an overestimation of the dosimeter activity, especially if the results are expected shortly after the end of the irradiation. It is therefore necessary to assess the contribution of fluorescence effects to provide accurate values of 93mNb activity. Fluorescence correction factors were established by Monte Carlo simulation. The calculated fluorescence correction factors were validated by an experimental approach, using activated niobium dosimeters with different tantalum concentrations.

Following an initial Round Robin inter-comparison of gamma spectrometry measurements reported in 2014, this paper presents results from the first part of a second Round Robin inter-comparison commissioned by the European Working Group on Reactor Dosimetry in 2018. Measurements were performed by thirteen European organisations on a set of irradiated neutron activation detectors representative of those commonly used by the Reactor Dosimetry community to measure neutron fluence using gamma spectrometry methods. The radionuclides measured were110mAg,58Co,60Co,54Mn, and46Sc. The purpose of the exercise was to demonstrate the level of consistency between participating organisations in blind tests of measurements. The samples used were disks of iron, nickel, titanium, and two standard alloys of aluminium, one of 1% cobalt and the other 1% silver. They were irradiated in the MARIA reactor operated by National Centre for Nuclear Research, Poland. Participants provided their results to an independent referee who collated and compared the data. The results are presented in an anonymised form together with discussion and conclusions which may be drawn from the exercise. Good agreement was obtained with standard deviations for individual measurements between 2.0% (54Mn) and 5.6% (60Co). Overall, the results obtained from the latter Round Robin show less consistency than those from the first Round Robin, which is attributed to the participation of a wider pool of organisations.

Following on from an initial Round Robin inter-comparison of gamma spectrometry measurements reported in 2014, this paper presents the results of the second part of a further Round Robin inter-comparison commissioned in 2018 by the European Working Group on Reactor Dosimetry. The purpose of the exercise was to demonstrate the level of consistency between different organisations’ measurements of the 93Nb(n,n’)93mNb reaction, which plays a key role in underwriting reactor dosimetry assessments. To achieve this, measurements of 93mNb activity were performed by twelve European organisations on six sets of near identical niobium samples, each having its own geometry and tantalum concentration. The samples were provided by CEA, France and irradiated in the MARIA reactor, operated by National Centre for Nuclear Research, Poland. Participants provided their results to an independent referee who collated and compared the data. The inter-comparison has demonstrated agreement to within standard deviations ranging from ±2.2% to ±7.9%, with a tendency for some organisations to measure elevated values. The results of the inter-comparison are presented in an anonymised form together with discussion and conclusions which may be drawn from the exercise.

The knowledge of nuclear heating inside nuclear (fission or fusion) reactor is essential for safe reactor operation, to know and meet limits expressed in terms of cooling requirements, and it represents critical input information for the design of any experimental irradiation device. In the framework of a research collaboration project between the Atomic and Alternative Energy Commission (CEA) in France, and the Jozef Stefan Institute (JSI) in Slovenia, a new project was launched in 2018 with the objective to measure the total heating rates in different materials of interest inside the JSI TRIGA Mark II reactor using the calorimetry technique. Three standard materials, graphite, aluminum and tungsten, and a fusion-relevant material, Eurofer-97, have been selected. The design of the calorimeters to host the four selected material samples is based on the CALMOS and CARMEN differential calorimeters, previously developed at the CEA, which were used to perform nuclear heating measurements in the OSIRIS material testing reactor (MTR) at the Saclay research center. The design of these specific calorimeters, and the dimensioning of the material samples located within, necessitated preliminary particle transport calculations using the MCNP V6.1 Monte Carlo code in order to estimate the expected heating levels in the experimental Central Channel of the core, and to avoid excessive perturbation of the measured radiation field. On the other hand, a parametric study was carried out using the COMSOL multiphysics code, to evaluate the expected temperature differences that could be measured using calorimeters designed specifically for low heating rates in the JSI TRIGA reactor core. A dedicated measurement campaign took place in July 2021 in which heating measurements were carried out at 100 kW reactor power and at nominal reactor power - 250 kW. This paper presents the design and preparation of the calorimeters, their preliminary calibration, the measurements carried out and the analysis of the obtained experimental results.

The development of the JHR experimental devices rely on the operational feedback from previous French material testing reactors (i.e. SILOE and OSIRIS). The experimental devices used for the irradiation of structural material were already facing technological limitations, in particular regarding the control of irradiation temperature and of the thermal gradients in the experimental samples, which is essential to ensure the quality of the experiments. Obtaining satisfactory thermal fields (in compliance with the setpoint and the homogeneity) is all the more difficult as the level of nuclear heating is higher in the JHR. This paper attempts to characterize the irradiation conditions in different experimental positions of the JHR and to compare them with the conditions and the empirical criteria of maximum acceptable temperature measured in OSIRIS. The study shows that the irradiation conditions obtained inside the experimental devices can sometimes be significantly different from the measured conditions using instrumentation devices. The interpretation of the experimental results and their transposition to other situations will always require a calculation versus measurement adjustment and the intensive use of computer simulation. However, despite all simulation and transposition efforts, the control of temperature conditions is not yet fully demonstrated and nothing will ultimately replace experimental validation.

Heating due to energy deposition of intense ionizing radiation in samples and structural materials of nuclear reactors poses severe limitations in terms of cooling requirements for safe reactor operation, especially in high neutron and gamma flux environments of material testing fission reactors (MTRs) and novel fusion devices. A bilateral CEA-JSI research project was launched in 2018 with the objective to measure the gamma heating rates in standard reactor-related materials (graphite, aluminium, stainless steel and tungsten) as well as fusionrelevant materials (low-activation steel Eurofer-97 and Nb3Sn superconductor) in the JSI TRIGA reactor my means of gamma calorimeters. The calorimeter design will be based on the the CALMOS-2 calorimeter developed at the CEA and used to perform gamma heating measurements in the OSIRIS MTR in Saclay. In order to optimize the detector response inside the JSI TRIGA reactor field and not to perturb the measurement field, a detailed computational analysis was performed in terms of energy deposition assessment and measurement field perturbation using the MCNP v6.1 code, and in terms of heat transfer using the COMSOL Multiphysics code. The abovementioned activities enabled us to finalize the detector design with the experimental campaign planned for the end of year 2019.

Nuclear heating inside a Material Testing Reactor (MTR) needs to be known in order to design and to run irradiation experiments that have to fulfill target temperature constraints. To improve the incore nuclear heating knowledge of the French OSIRIS reactor operated by CEA, an innovative calorimetric system CALMOS (French acronym for CALorimètre Mobile OSiris) has been studied, manufactured and tested. This device can be inserted in any in-core experimental location. It is based on a mobile probe moving axially along the core height. First tests of this new probe, offering several ways for the heating evaluation, were performed in the reactor periphery. Then, two complete prototypes dedicated to incore measurements (calorimeter and the associated displacement system) were designed and tested during several reactor cycles. This paper presents a comprehensive analysis of all the results collected during the measurement campaigns carried out between 2013 and 2015 with these new prototypes in various reactor conditions. A comparison is made with previous calorimeters and obtained advantages are emphasized. This new calorimeter has been designed as a real operational measurement system, well suited to characterize the radiation field inside an MTR reactor.

Activation measurements, the primary method of determination of the neutron fluence, are generally sensitive to the thermal and resonance energy ranges (neutron capture reactions) and the fast energy range (threshold reactions). However, there are very few nuclear reactions which are sensitive specifically to neutrons in the intermediate (epithermal) energy region. In recent work, conducted in the framework of a research collaboration project between the French Atomic and Alternative Energy Commission (CEA) and the Jozef Stefan Institute (JSI), Slovenia, indications were obtained that boron-based neutron filter materials, due to the very large magnitude and almost pure 1/v energy dependence of the B-10 absorption cross-section, are able to shift significantly the sensitivity of neutron capture reactions towards the epithermal energy range. A new collaboration project between the CEA and the JSI was launched to perform a broad investigation of neutron capture reactions in combination with boron-based neutron filters of different geometry, material and isotopic composition (i.e. B-10 enrichment level) could be suitable for measurements of the epithermal neutron flux, in terms of their energy sensitivity and in terms of the possibility of gamma spectrometry measurements of the reaction products. This paper presents the preparation process of a dedicated experimental campaign, to be performed November 2018 in the JSI TRIGA reactor.

Nuclear heating inside a materials testing reactor needs to be known in order to design irradiation experiments that have to fulfill temperature constraints. To improve nuclear heating knowledge, an innovative calorimeter, CALMOS, has been studied, manufactured, and tested for the 70-MWth OSIRIS reactor operated by the French Alternative Energies and Atomic Energy Commission. This device is based on a mobile calorimetric probe that can be inserted in any in-core location and moved axially along the core height and above. Obtained results and advantages brought by the first CALMOS-1 equipment already have been presented. However, a thermal limitation in the cell did not allow the monitoring of nuclear heating up to the nominal power, and some significant discrepancies were observed at high heating levels between results deduced from the calibration and those obtained by the "zero method." Feedback-based, the new CALMOS-2 calorimeter has been designed both for extending the heating range up to 13 W.g-1 and for improving the "zero method" procedure. In addition, the CALMOS-2 calorimeter is an operational measurement system, suited to characterize the radiation field evolution throughout the reactor cycle. To meet this requirement, a programmable system associated with specific software allows automatic cell mobility in the core, the acquisition of data and their processing. This paper presents an analysis of results collected during the 2015 measurement campaign carried out with the new prototype. The four-wire technique was tested up to about a 4 W.g-1 heating level and allowed to quantify discrepancies between "zero" and calibration methods. Thermal neutron flux and nuclear heating measurements from CALMOS-1 and CALMOS-2 are compared. Thermal flux distributions, obtained with a self-powered neutron detector suited to the calorimeter, are compared with those obtained with current devices. Finally, the analysis emphasizes advantages brought by the human machine interface, which deeply refined the profiles definition.

Nuclear heating rate inside an MTR has to be known in order to design and to run irradiation experiments which have to fulfill target temperature constraints. This measurement is usually carried out by calorimetry. An innovative calorimetric system, CALMOS, has been studied and built in 2011 for the 70MWth OSIRIS reactor operated by CEA. Thanks to a new calorimetric probe, associated to a specific displacement system, it provides measurements along the fissile height and above the core. Development of the calorimetric probe required manufacturing and irradiation of mock-ups in the ex-core area, where nuclear heating rate does not exceed 2 W.g -1 . The calorimeter working mode, the different measurement procedures, main modeling and ex-core experimental results have been already presented in previous papers. In this paper, we present in-core results obtained from 2011 to 2013 with the final device. For the first time, this new experimental measurement system was operated in several experimental locations, with nominal in-core thermal hydraulic conditions, nominal neutron flux and nuclear heating rate up to 6 W.g -1 (in graphite). After a brief presentation of the displacement system specificities, first nuclear heating distributions are presented and discussed. The Finite Element model of the calorimeter was upgraded in order to match calculated temperatures with measured ones. This “validated” model allowed to estimate a Kc factor which tends to correct small nonlinearities when heating rate is calculated from the “calibration method”. A comparison is made between nuclear heating rates determined from “calibration” and “zero methods”. In addition, an evaluation of the global uncertainty associated to the measurements is d- tailed. Finally, a comparison is made with available measurements obtained from previous calorimeters.

Nuclear heating inside an MTR reactor has to be known in order to be able to control samples temperature during irradiation experiments. An R&D program has been carried out at CEA to design a new type of in-core calorimetric system. This new development, started in 2002, has for main objective to manufacture a calorimeter suitable to monitoring nuclear heating inside the 70 MWth OSIRIS material testing reactor operated by CEA's Nuclear Energy Division at the Saclay research center. An innovative calorimetric probe, associated to a specific handling system, has been designed to provide access to measurements both along the fissile height and on the upper part of the core, where nuclear heating still remains high. Two mock-ups of the probe were manufactured and tested in 2005 and 2009 in ex-core area of OSIRIS reactor for process validation, while a displacement system has been especially studied to move the probe along a given axial measurement range. This paper deals with the development, tests on preliminary mock-ups and the finalization of the probe. Main modeling and experimental results are presented. Moreover, alternative methods to calibration for nuclear heating rate measurements which are now possible with this new calorimeter are presented and discussed.