J. Nucl. Eng., Volume 3, Issue 4 (December 2022) – 20 articles

Wendelstein 7-X (W7-X) is currently the largest optimized stellarator in operation in the world. Its main objective is to demonstrate long pulse operation and to investigate the suitability of this type of fusion device for a power plant. Maintaining the safety of the first wall is critical to achieving the desired discharge times of approximately 30 min while keeping a steady-state condition. We present a deep learning-based solution to detect the unexpected plasma-wall and plasma-object interactions, so-called hot-spots, in the images of the Event Detection Intelligent Camera (EDICAM) system. These events can pose a serious threat to the safety of the first wall, therefore, to the operation of the device. We show that sufficiently training a neural network with relatively small amounts of data is possible using our approach of mixing the experimental dataset with new images containing so-called synthetic hot-spots generated by us. Diversifying the dataset with synthetic hot-spots increases performance and can make up for the lack of data. The best performing YOLOv5 Small model processes images in 168 ms on average during inference, making it a good candidate for real-time operation. To our knowledge, we are the first ones to be able to detect events in the visible spectrum in stellarators with high accuracy, using neural networks trained on small amounts of data while achieving near-real-time inference times. Full article

For the EU-DEMO Helium-Cooled Pebble Bed (HCPB) concept, an indirect coupled design (ICD) with a molten salt (MS) loop as an intermediate heat transport and storage system (IHTS) is considered for the conceptual design phase. The IHTS with an energy storage decouples the primary heat transport system (PHTS) that undergoes pulse and dwell power cycles from the power conversion system (PCS), and thus can provide stable power to the turbine and grid. However, the maintenance of stable He and MS parameters during transitions from dwell to pulse and vice versa is challenging for the design of the MS loop, and the real performance of the helium–MS heat exchanger (He/MS HX) shall be verified. To investigate such components and conditions, a new R&D infrastructure HELOKA-US (Helium Loop Karlsruhe—Upgrade Storage) is under construction for the validation of prototypical components and the MS loop operation under stationary and transitional conditions. This paper provides the design features of Phase 1a of the project and the simulation results with EBSILON on the power generation phase. Full article

For the DEMO reactor, tungsten is considered as an armor material. Eurofer97 is planned to be used as a structural material for the first wall and in the divertor region, especially for the shielding liner component. To date, several joining solutions between W and Eurofer97 have been developed (copper brazing, W and Eurofer97 functional gradient material (FGM), etc.). Each existing joining solution has its own advantages (joining material, improved manufacturing process). In the present study, the choice of the joining material is driven, among other constraints, by a desire to minimize the thermal stresses at the materials’ interface. In this regard, FGM represents a promising solution. Another constraint that is taken into account in this study concerns the manufacturing process involved, which should be an improved industrial process. The present study proposes a joining solution, based on FGM, which, additionally to the advantages of the existing solutions, could reduce the long-term activation of the joining material. The development of a joining solution via Ti and Ta as materials constituting the FGM (Ti/Ta FGM) is presented in this paper. Due to the achieved density and the composition’s accuracy, the cold spray process is shown to be adapted for the Ti/Ta FGM’s manufacturing. Based on the feedback on the experience of joining between W, W/Cu FGM and CuCrZr, the final joining between W, Ti/Ta FGM and Eurofer97 is achieved using hot isostatic pressing, followed by a thermal treatment to recover Eurofer97’s mechanical properties, resulting in good joining quality. Full article

For the future fusion reactor, tungsten is the main candidate material as the plasma-facing material. However, considering the high thermal stress during operation, the intrinsic brittleness of tungsten is one of the issues. To overcome the brittleness, tungsten fiber reinforces tungsten composites (W_f/W) developed using extrinsic toughening mechanisms. The powder metallurgy process and chemical vapor deposition process are the two production routes for preparing W_f/W. For the powder metallurgy route, due to technical limitations, previous studies focused on short random distributed fiber-reinforced composites. However, for short random fiber composites, the strength and reinforcement effect are considerably limited compared to aligned continuous fiber composites. In this work, aligned long tungsten fiber reinforced tungsten composites have been first time realized based on powder metallurgy processes, by alternately placing tungsten weaves and tungsten powder layers. The produced W_f/W shows significantly improved mechanical properties compared to pure W and conventional short fiber W_f/W. Full article

During the Pre-Conceptual Design Phase of the EU-DEMO, two BOP solutions for WCLL and HCPB were elaborated, as close as possible to industrial standards. Nevertheless, each solution has open issues to be investigated, analytically and experimentally, in the Conceptual Design Phase (CDP). For the HCPB, the functionality and operability of the Helium-Molten Salt Heat Exchanger, and the coupling to a helium loop with a prototypic helium blower, is of primary interest. In addition, the operation of the pulse, dwell and transitions will be investigated within the new build infrastructure, HELOKA-US (Upgrade Storage), to be erected at KIT. The design requires a certain flexibility, since the final parameters of the Primary Heat Transfer System of DEMO may vary, due to plasma optimizations during CDP. HELOKA-US benefits from the high-pressure helium loop HELOKA-HP, erected to test HCPB-Breeding Blanket and First Wall modules, as well as from the competencies of preparing, handling and testing of various molten salts used for heat transfer optimization and natural convection. Full article

Despite the high performance exhibited by tungsten (W), no material would be able to withstand the huge loads expected with extreme plasma transients in EU-DEMO and future reactors, where the installation of sacrificial first wall limiters is essential to prevent excessive wall degradation. The integration of W lattices in the architecture of such components can allow for meeting their conflictual requirements: indeed, they must ensure the effective exhaust of the nominal thermal load during stationary operation; when transients occur, they must thermally insulate and decouple the surface from the heat sink, promoting prompt vapour shielding formation. Starting from the optimised layouts highlighted in a previous study, in this work, a detailed 3D finite element model was developed to analyse in depth the influence of the actual features of the latticed metamaterial on the overall performance of the EU-DEMO limiter PFC on the basis of a flat tile configuration. Its main goal is to help in identifying the most promising layout as a preconceptual design for the fabrication of a small-scale mock-up. For this purpose, the complex geometry of a W-based lattice armour was faithfully reproduced in the model and analysed. This allowed for a detailed assessment of the thermally induced stresses that develop in the component because of the temperature field in response to a number of plasma scenarios—above all, normal operation and ramp down. Structural integrity was verified through the acceptance criteria established for ITER. The two optimised layouts proposed for the PFC were able to effectively meet the requirements under normal reactor operating conditions, while they missed some requirements in the ramp-down case. However, the first HHF tests will be performed in order to benchmark the analyses. Full article

In the design of magnetic confinement nuclear fusion power plants, the breeding blanket (BB) plays a crucial role since it must fulfil key functions such as tritium breeding, radiation-shielding, and removal of the heat power generated by the plasma. The latter task is achieved by the first wall (FW) and breeding zone (BZ) cooling systems, which in the water-cooled lithium–lead (WCLL) BB employ pressurized water. Different arrangements of BZ coolant conduits have been investigated in the recent past to identify an efficient layout, which could meet the structural materials’ operational temperature constraint and which could provide the optimal coolant outlet temperature. However, most of the computational fluid dynamic (CFD) analyses that have been carried out until now have been focused on the equatorial WCLL elementary cell of the central outboard segment (COB). The aim of this work is to broaden the analysis to other relevant locations in the blanket. An assessment of the design of the cooling system of the COB bottom-cap elementary BZ cell has been identified as a top design priority due to its different geometry and thermal loads. The cooling efficiency of the BZ and FW systems is investigated to assess if the coolant-appropriate design conditions are matched and the temperature distribution in the cell is analyzed to identify the onset of hot spots. Different layouts of the FW systems are proposed and compared in terms of thermal–hydraulic reliability. Full article

The swelling of beryllium and titanium beryllide after irradiation at 70–750 °C to neutron fluences of (0.25–8) · 10²² cm⁻² (E > 1 MeV) was measured using methods of immersion, dimension, and helium pycnometry. Dependences of the swelling on the irradiation temperature and neutron dose were plotted and analyzed. The dose dependences show linear dependences of the swelling for all irradiation temperatures except 70 °C, where the swelling rate varies, depending on increasing neutron dose. Be-7Ti shows much less swelling than pure Be. Irradiation at 430–750 °C to neutron fluence of 1.82 · 10²² cm⁻² (E > 1 MeV) leads to swelling of Be at about 50%; for Be-7Ti, it is 2.7%. The microstructure study shows that the formation of bubbles and pores in beryllium occurs much more intense than in titanium beryllide. Full article

In the IFMIF-DONES facility of the future, the back-plate behind the Li target will receive strong irradiation from high-energy neutrons. The potential use of the back-plate for material specimens is attractive with respect to providing complementary irradiation data for Eurofer. In this work, DPA (displacement per atom) and gas production rates as well as DPA gradients and temperature distributions have been studied for the center segment of the back-plate, using both a nominal beam and a reduced beam footprint. It is shown that specimens can be produced with high DPA in similar conditions to the DEMO first-wall. Based on the size of the SSTT (small specimen test technology) specimens, the limited number of samples obtainable from the adopted arrangement scheme is driven by a major constraint: the thickness of the back-plate. A parametric study of the back-plate’s thickness provides an alternative arrangement scheme; thus, the DPA and gradient of the specimens are remarkably improved. Full article

Integration of safety aspects in IFMIF-DONES design is a main objective of EUROfusion and European Commission projects. IFMIF-DONES will be a radioactive facility of the first category, and stringent safety objectives must be achieved and demonstrated. A very low acceptable risk for the worker, the public and the environment is the main principle in the design phase. The progress of safety activities is performed iteratively as detailed engineering develops, taking into account the uniqueness of the facility: a high-power deuterons accelerator (125 mA, 40 MeV), a target of flowing liquid lithium, traps for activation products, a dedicated-design module for irradiated samples, a massive shielding cooled room with confinement function, and a number of conventional systems with safety functions. Several phases are developed: (i) identification of sources and materials at risk, radioactive and nonradioactive, subject to potential mobilization, (ii) failure mode analysis and effects of systems, starting at the functional level, and support with probabilistic analysis, (iii) identification of scenarios leading to unacceptable risk if unmitigated, (iv) proposal of layers of defense by means of safety-credited components and design features, (v) deterministic analysis of scenarios in support of requirements, and (vi) definition and demonstration of safety requirements charged to components. Full article

For the demonstration of fusion power plant technology, DEMO dedicated materials are necessary to cope with the harsh environment of high energy neutrons. For this purpose, the international neutron irradiation facility for fusion materials IFMIF/DEMO Oriented Neutron Source (DONES) is planned to be built in Granda, Spain. In the DONES facility, a deuteron beam hitting the lithium target produces a high energy neutron flux. Due to the high-power density, the windowless target is a free surface liquid lithium flow in a duct with a concave backplate. In order to keep the heat released by the beam within the liquid lithium and to avoid its intrusion in the backplate, a stable configuration of the free surface flow with a setpoint layer thickness of 25 ± 1 mm is crucial. In particular, stable wave structures, so called wakes, which occur from accumulated impurities at the nozzle edge, can cause a critical local decrease in the layer thickness of more than 1 mm. Therefore, it is necessary to better understand the nature of these wakes and to be able to monitor the surface profile to shut down the beam in case of a critical thickness loss, but to avoid unintended shutdowns. In the context of this work, currently available optical sensors were tested on their capability of detecting a specular liquid metal surface at measurement distances of several meters. After an initial selection, two optical sensors were further considered. Experiments with the liquid metal alloy GaInSn and simulations with the software Blender of the selected optical sensors for their capability of measuring distances of liquid metal were conducted. The results showed a significant dependency of the measurement results on the waviness of the liquid metal surface. Nevertheless, it was possible to resolve the wavy liquid metal surface with a sufficient resolution to detect critical wake structures. Full article

Data from the literature were analyzed and experimental data were obtained on the sublimation of uranium-zirconium carbonitrides with different contents of carbon, nitrogen and oxygen impurities in a wide temperature range (1773–2323 K). The composition of the gas phase was determined and the analytical dependences of the partial pressures of its components on temperature were calculated. It was shown that the sublimation of uranium-zirconium carbonitrides occurs incongruently with the predominant loss of nitrogen, which led to a shift in their compositions to the side richer in carbon. The chemical mechanism of sublimation was considered, according to which oxygen impurities in these materials contribute to the additional release of nitrogen and the appearance of oxide components UO, UO₂ and CO in the gas phase. The introduction of zirconium carbonitrides and an increase in the carbon content led to a decrease in the partial pressures of uranium monoxide and nitrogen, thereby increasing the thermal stability of materials. Full article

The second variant of the electron cyclotron heating and current drive system in DEMO considers the deployment of 2 MW power Gaussian microwave beams to the plasma by frequency steering. Broadband optical grade chemical vapor deposition diamond windows are thus required. The Brewster-angle window represents the primary choice. However, in the case of showstoppers, the double-disk window is the backup solution. This window concept was used at ASDEX Upgrade for injection of up to 1 MW at four frequencies between 105 and 140 GHz. This paper shows computational fluid dynamics conjugated heat transfer and structural analyses of such a circumferentially water-cooled window design aiming to check whether it might be used for DEMO microwave beam scenarios. This design was then characterized with respect to different parameters. Temperature and thermal stress results showed that it is a feasible window solution for DEMO, but safety margins against limits shall be increased by introducing design features able to make the fluid more turbulent. A first design change is proposed, showing that, in combination with a higher inlet flow rate, the maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower thermal gradients and stresses in the window. Full article

During the service life of plasma-facing components, they are exposed to cyclic stationary and transient thermal loads. The former causes thermal fatigue and potentially detachment between the plasma-facing material tungsten and the structural Cu-based materials (divertor) and steel (first wall). The latter causes surface roughening, cracking, or even melting, which could drastically increase the erosion rate. Employing thin flexible W wires (W_w) with a diameter of a few hundred µm can reduce mechanical stresses, and we demonstrated their crack resilience against transient loads within first proof of principle studies. Here, status and future paths towards the large-scale production of such W_w assemblies, including techniques for realizing feasible joints with Cu, steel, or W, are presented. Using wire-based laser metal deposition, we were able to create a homogeneous and shallow infiltration of about 200 µm of the W_w assembly with steel. A high-heat-flux test on such a µ-brush (10 × 10 × 5 mm³ W_w on a ~0.5 mm thick steel layer) using 5 MW/m² for 2000 cycles was performed without loss of any wire. Microstructural examination after and infrared analysis during the test showed no significant signs of degradation of the joint. Full article

The development of coatings for accident-tolerant fuels (ATFs) for light water reactor (LWR) applications promises improved corrosion resistance under accident conditions and better performances during operation. CrN and TiN coatings are characterized by high wear resistance coupled with good corrosion resistance properties. They are generally used to protect materials in applications where extreme conditions are involved and represent promising candidates for ATF. Zr cladding tubes coated with 5 µm-thick CrN or TiN, exposed in an autoclave to simulated PWR chemistry and BWR chemistry, were characterized with SEM, EDS, and STEM. The investigation focused on the performance and oxidation mechanisms of the coated claddings under simulated reactor chemistry. Both coatings provided improved oxidation resistance in a simulated PWR environment, where passivating films of Cr₂O₃ and TiO₂, less than 1 µm-thick, formed on the CrN and TiN outer surfaces, respectively. Under the more challenging BWR conditions, any formed Cr₂O₃ dissolved into the oxidizing water, resulting in the complete dissolution of the CrN coating. For the TiN coating, the formation of a stable TiO₂ film was observed under BWR conditions, but the developed oxide film was unable to stop the flux of oxygen to the substrate, causing the oxidation of the substrate. Full article

Tungsten-fibre-reinforced tungsten composites ($W_f/W$) have been in development to overcome the inherent brittleness of tungsten as one of the most promising candidates for the first wall and divertor armour material in a future fusion power plant. As the development of $W_f/W$ continues, the fracture toughness of the composite is one of the main design drivers. In this contribution, the efforts on size upscaling of $W_f/W$ based on Chemical Vapour Deposition (CVD) are shown together with fracture mechanical tests of two different size samples of $W_f/W$ produced by CVD. Three-point bending tests according to American Society for Testing and Materials (ASTM) Norm E399 for brittle materials were used to obtain a first estimation of the toughness. A provisional fracture toughness value of up to $346\mathrm{MPa}{\mathrm{m}}^{1/2}$ was calculated for the as-fabricated material. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied. A maximum value of the J-integral of $41{\mathrm{kJ}/\mathrm{m}}^2$ ($134.8\mathrm{MPa}{\mathrm{m}}^{1/2}$) was determined for the largest samples. Post mortem investigations were employed to detail the active mechanisms and crack propagation. Full article

A new target material combination was modelled to replace the existing uranium-aluminium design used for ⁹⁹Mo manufacture to increase the sustainability of the production process. Previous efforts to develop a more sustainable uranium target for ⁹⁹Mo production, resulted in the levels of ²³⁹Pu in the target after irradiation being elevated due to the increase in ²³⁸U present. MCNP6.2 was used to model 4 different cylindrical targets based on 4–7 days irradiation to further understand this effect. To reduce the resultant ²³⁹Pu levels, ratios of 0–99% of Ce were used as a replacement for ²³⁸U. The results show that the addition of ¹⁴⁰Ce and the removal of ²³⁸U reduced the ²³⁹Pu levels in the target significantly thus increasing the sustainability of the target and giving a slight increase to the ⁹⁹Mo output of the targets. Full article

The numerical analysis for the controllability assessment of a new design nuclear reactor is typically carried out by means of complex multiphysics codes, solving high fidelity partial differential equations governing the system neutronics as well as the fluid dynamics. Multiphysics codes deliver very accurate solutions at the expense of high computational times, which could be of several hours depending on the specific case study. In this work, to efficiently reduce runtimes, a sensitivity analysis (SA) is carried out to identify the most important input parameters affecting the solution of a multiphysics model developed for the controllability assessment of molten salt reactors (MSRs). The numerical modeling of these innovative systems is fundamental to allow for a safer and more sustainable power production (e.g., due to the lower radiotoxicity of the actinide inventory in MSRs and to the possibility of operation at atmospheric pressure). In this paper, four global sensitivity measures are calculated first, including the Pearson correlation coefficient, δ, Kolmogorov–Smirnov and Kuiper indices, whose results are aggregated by an ensemble strategy and confirmed by the CUmulative SUm of NOrmalized Reordered Output (CUSUNORO) plot. The results of the SA point out that the fuel density is the most important parameter yielding the largest variations in the system reactivity, fundamental for guaranteeing the MSR controllability. In light of this result, a simplified, surrogate model is then developed, which uses density as the only input parameter to determine reactivity, guaranteeing runtime reductions from several hours to a few seconds and, at the same time, a comparable level of accuracy of the multiphysics model. This result demonstrates the capability of global sensitivity analysis approaches to effectively identify the most relevant parameters in MSR systems, supporting the development of simplified, control-oriented models for these innovative reactors. Full article

This paper considers the possibility for large-scale production of plutonium isotope ²³⁸Pu in the light-water nuclear power reactor VVER-1000. ²³⁸Pu is a unique source of long-term autonomous energy supply in various devices for remote regions of the Earth and in outer space. The design of the irradiation device with ²³⁷NpO₂ as a starting material is proposed, which is placed in the central zone of the VVER-1000 reactor core and makes it possible to achieve 8% of the specific Pu production (Pu/²³⁷Np) by optimizing the pitch of NpO₂-rod lattice. The computations showed that the scale of ²³⁸Pu production in the irradiation device was remarkably larger (2 ÷ 7 times more) than similar values in research reactors. At the same time, the use of heavy neutron moderators with low neutron absorption (natural lead or lead isotope ²⁰⁸Pb) around the NpO₂ fuel assembly (FA) made it possible to obtain high-purity ²³⁸Pu with the content of ²³⁶Pu below 2 ppm. The paper also shows that if the irradiation device is placed in central zone of the VVER-1000 reactor core, then the displacement damage dose in the reactor vessel remains low enough to conserve its strength properties throughout the entire period of the reactor operation (60 years). Full article

Distributed multisensor networks record multiple data streams that can be used as inputs to machine learning models designed to classify operations relevant to proliferation at nuclear reactors. The goal of this work is to demonstrate methods to assess the importance of each node (a single multisensor) and region (a group of proximate multisensors) to machine learning model performance in a reactor monitoring scenario. This, in turn, provides insight into model behavior, a critical requirement of data-driven applications in nuclear security. Using data collected at the High Flux Isotope Reactor at Oak Ridge National Laboratory via a network of Merlyn multisensors, two different models were trained to classify the reactor’s operational state: a hidden Markov model (HMM), which is simpler and more transparent, and a feed-forward neural network, which is less inherently interpretable. Traditional wrapper methods for feature importance were extended to identify nodes and regions in the multisensor network with strong positive and negative impacts on the classification problem. These spatial-importance algorithms were evaluated on the two different classifiers. The classification accuracy was then improved relative to baseline models via feature selection from 0.583 to 0.839 and from 0.811 ± 0.005 to 0.884 ± 0.004 for the HMM and feed-forward neural network, respectively. While some differences in node and region importance were observed when using different classifiers and wrapper methods, the nodes near the facility’s cooling tower were consistently identified as important—a conclusion further supported by studies on feature importance in decision trees. Node and region importance methods are model-agnostic, inform feature selection for improved model performance, and can provide insight into opaque classification models in the nuclear security domain. Full article