During International Thermonuclear Experimental Reactor operation, due to plasma–wall interaction, particles/dust will be created in sizes ranging from nanometers to tens of microns. The dust properties, especially their ability to be covered by a thin oxide electrostatic insulating layer, and surface topology deeply affect their tritium inventory. Consequently, physico-chemical properties specific to tritiated tungsten particles and consequence on particle behavior in the facility and environment must be carefully assessed. For size-relevant tungsten particles, the measured tritium inventory is ~10 GBq g−1. However, it varies with the particle specific surface area. Due to tritium beta decay and the oxide-insulating layer, dust exhibits a positive electrostatic self-charging. For a 5 µm particle in diameter with a 10 GBq g−1 tritium inventory, self-charging rate could lead to 5.5 104 elementary electric charges per day. These electrostatic properties could change the adhesion of dust on walls. In the case of a single particle, the adhesion will be reinforced due to image and dielectric forces. However, if the tritiated particle is part of an aggregate, the adhesion remains unknown. Due to the limited free path of the β emission in material, the tritium inventory carried by airborne particles cannot be measured in real time by conventional continuous radioactive aerosols monitors, and a new measurement strategy is needed for atmospheric surveillance in the workplace and of facility exhaust. Toxicity studies dealing with exposure to untritiated/tritiated tungsten particles of 100 nm have been undertaken. It was shown that these particles are rapidly dissolved in biologic media. Finally, after collection, dust must be confined to avoid its spreading into the environment. Different technical solutions are presented in this paper.
This paper addresses the problem of false positive alarm when using a continuous air monitor (CAM) in decommissioning sites of nuclear facilities. CAMs are used to measure airborne activity and play an important role in the radiation protection of workers likely to be exposed to radioactive aerosols. Monitors usually sample aerosols on a membrane filter. Radioactive particles sampled are detected through the alpha and beta decays that they emit. These latter ionizing particles are measured online by spectrometry thanks to a Passivated Implanted Planar Silicon detector (PIPS). Alpha and beta decays, in this context, come mainly from the natural radon progeny ( 218 Po, 214 Pb, and so on) and, in the case of radioactive contamination, also from artificial radionuclides such as 239 Pu or 137 Cs. The aim of the CAM is to alert the workers when the artificial airborne activity occurs, always considering the presence of a variable background due to the natural particulate airborne activity. The CAM-specific algorithm considers this background dynamically and continuously, often by using a constant parameter. However, non-radioactive aerosols are also sampled on the membrane filter. These latter make the discrimination more difficult as they lead to the deterioration of the alpha-energy spectrum. In this paper, the effect of coarse non-radioactive aerosols on the CAM response is highlighted with four aerosol size-distributions. The evolution of the background is characterized as a function of the aerosol mass sampled, with the example of a simple algorithm. Thus, in this paper, results show a positive correlation of the background with the aerosol mass sampled by the CAM. In addition, results highlight at least two different evolutionary trends according to the aerosol size distribution. An explanation of these evolutions is given by considering the penetration profile of the natural radioactive aerosols in the granular deposit above the CAM filter. The main consequence of these results is that the background could not be considered as proportional to radon progeny as it is currently used.
The electrostatic self-charging rate of tokamak dust is investigated using Geant4, a toolkit for the simulation of the passage of particles through matter. To do so, the particles geometrical characteristics, the β disintegration energy spectrum and the deepness of tritium infusion are taken into account. The investigated materials are tungsten and beryllium, the plasma facing components (PFC) of ITER, considered as spherical particles from 20 nm to 200 μm in diameter, both tritiated. Two cases of tritium distribution in the particles are examined. On the one hand, tritium is homogeneously distributed over the whole sphere; on the other hand, tritium is homogeneously distributed within the external 100 nm layer of the sphere. The self-charging rate is assessed through the calculation of the particle exiting electron rate. Based on a tritium inventory of 10 GBq/g, relevant for ITER tokamak environment, our results show that, for a single tungsten or beryllium particle of 10 μm in diameter, the self-charging rate when the tritium is homogeneously distributed within the whole sphere is respectively 2.4 and 1.9 positive elementary charges per second. In the configuration where the tritium absorption is confined in the external 100 nm layer, the charge magnitude raises up to 37.1 and 8.4 respectively.
In nuclear facilities, the mandatory atmosphere surveillance is operated by Continuous Air Monitors. This standalone instrument is designed to measure the airborne aerosol activity concentration and to trig an alarm signal when a predetermined activity concentration is exceeded. However, a rapid resuspension event of coarse aerosol leads to a measurement error: the airborne aerosol activity concentration is over-evaluated. Prior results have shown that the coarse aerosol deposit disturbs the background evaluation for the radioactivity measurement. The interactions between radioactive aerosols (with radon daughters) and coarse non-radioactive aerosols have to be investigated by running together aerosol models and nuclear simulations. Therefore, this paper investigates different ways to represent an aerosol deposit in numerical simulations. We developed two numerical aerosol deposit models that we integrated into Geant4, a tool for the simulation of the passage of radiations through matter, and then compared these to experimental results. The simplest model was discarded, and by using the second model, we managed to correctly frame our simulation results as an experimental measurement: an aerosol has been correctly considered in a nuclear simulation. By combining theory, simulations, and experimentations on both aerosol science and nuclear physics, this research will be able to improve the comprehension of monitors’ behaviour in delicate situations and, more broadly, the filtration of aerosols using radioactivity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.