Recent developments in nuclear heating measurement methods inside the OSIRIS reactor

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Computational support on the development of nuclear heating calorimeter detector design

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

Computational support on the development of nuclear heating calorimeter detector design

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

Assessment of Irradiation Performance in the Jules Horowitz Reactor (JHR) using the CARMEN Measuring Device

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

Fluorescence correction for activity measurement of 93^{93}mNb in niobium dosimeters: calculation and experimental validation

Conference ISRD 17: The Seventeenth International Symposium on Reactor Dosimetry, 21-26 May 2023 , Lausanne, SwitzerlandInternational audienceNiobium is a reference dosimeter for the survey of neutron damage to nuclear reactor vessels. Its characterization is based on the activity of $^{93}$mNb resulting from the $^{93}$Nb(n,n') activation reaction. The decay of $^{93}$mNb results mainly in the emission of niobium K X-rays which are used to determine the activity of $^{93}$mNb. 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 $^{182}$Ta, and other niobium isotopes remain present during the measurements and disturb the X-ray spectra. The fluorescence effect induces an additional X-ray emission from niobium to that due only to the decay of $^{93}$mNb, leading to an overestimation of the dosimeter activity. It is therefore necessary to assess the contribution of fluorescence effects to provide accurate values of $^{93}$mNb 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 concentration

Fluorescence corrections for activity measurements of 93m^{93m}Nb in niobium dosimeters by X-ray spectrometry

International audienceNiobium is a reference dosimeter for neutron damage survey of nuclear reactor vessels. This characterization is based on the activity of $^{93m}$Nb resulting from the $^{93}$Nb(n,n') activation reaction. $^{93m}$Nb is a metastable state of $^{93}$Nb that de-excites towards the ground state by a highly converted isomeric transition. This decay mainly results in the emission of niobium K X-rays which are used to determine the activity of $^{93m}$Nb, either by direct measurement using X-ray spectrometry or by liquid scintillation after the sample dissolution. The first method does not require any sample preparation, but fluorescence effects may perturb the measurement. Indeed, niobium foils may contain impurities that are activated during irradiation. Some of these radioactive impurities, in particular $^{182}$Ta, and other niobium isotopes ($^{94}$Nb, $^{95}$Nb and $^{92m}$Nb) are still present during the measurements and disturb the X-ray spectra. The fluorescence effect generates additional niobium X-ray emissions to those due only to the decay of $^{93m}$Nb, resulting in an overestimation of the dosimeter activity. Most activated impurities have a much shorter half-life than $^{93m}$Nb and the current method is to allow the dosimeters to decay for several months before measuring them to obtain their activity. However, in some circumstances, results are expected much earlier, not long after the end of irradiation. It is therefore necessary to assess the influence of the fluorescence effects to provide accurate $^{93m}$Nb activity values. Corrective factors for fluorescence were established by Monte Carlo simulation using the GEANT4 code. This required simulating the complete decay scheme of each radionuclide, taking into account interactions of the emitted particles with matter and atomic relaxation processes. The radioactive impurities were distributed isotropically within the dosimeter and the simulation provided the number of exiting niobium K X-rays as well as the information on the process responsible for the niobium ionization. With this information, it was shown that electrons are mainly responsible for the fluorescence effects in niobium dosimeters. These calculated fluorescence corrective factors were validated by an experimental approach: nine niobium dosimeters with different Ta levels were activated in MARIA reactor (National Centre for Nuclear Research, Poland) in the framework of a round robin initiated by the European Working Group on Reactor Dosimetry (EWGRD). These dosimeters were measured six times in the year following the end of irradiation using gamma- and X-ray spectrometry to determine the impurities activity and the Nb X-rays emission rates respectively. These rates were corrected for the impurities contribution using the calculated corrections before deriving the $^{93m}$Nb activity at the reference date. With the applied corrections, the standard deviation between the six measurements is reduced to about 1 %