Heat pipes can effectively transport heat from a heat source to a heat sink by means of phase transitions of the working fluid inside and capillary forces. Because of their high effective conductivity, they are under consideration for the DEMO in-vessel plasma-facing components. With proper condenser length, heat pipes can enlarge the heat transfer area to the cooling circuit, thus relaxing the requirements for the cooling circuit. The reduced fluid inventory of the heat pipe would also limit the amount of liquid released in case of damage or accidents compared to an actively cooled plasma-facing component, thus increasing the reactor’s safety. Recent engineering studies indicate that it is possible to design a water-based heat pipe with mixed capillary structures (axial grooves at the condenser and adiabatic zones and sintered porous material at the evaporator) that would have a capillary driving force large enough to transport an amount of heat corresponding to an applied heat flux of 20 MW/m $^{2}$ . However, to validate the design for such high heat fluxes, the capability of the evaporator to withstand such loads should be investigated first. Hence, a dedicated experiment focusing on the performance of the proposed heat pipe evaporator was designed. The experimental results show the operating characteristics of two different evaporator designs: one with a porous structure and one with channels on the porous surface. The influence of liquid inventory and heat sink flow rates on the heat pipe performance are also discussed here.

Fusion power plants are not yet considered specifically in European long-term energy system studies. In order to include them in such studies a corresponding and valid parametrization of their operating performance has to be established despite the fact that fusion reactor design is still an ongoing effort. The goal of the present paper is to specify and energetically represent the prospect of feasible operation and dynamics of tokamak and stellarator type fusion power plants from an energy system perspective. Special focus is given on time and operation mode dependent self-consumption. The basis of the parametrization is a one GW(el) power output plant. As a result, we propose the representation of fusion power plants as a system with three main components (fusion reactor, thermal energy storage (TES) and power conversion system), followed by a set of parameters for both tokamak and stellarator type devices. Five different operating states are defined for a fusion plant, depending on the required and active auxiliary subsystems. The comparison between operational dynamics of conventional and fusion power plants showed no tremendous differences due to the TES utilization. However, fusion plants had a lower full-load operation efficiency due to higher self-consumption as well as extensive pre-production losses.

1000K sodium loops SOLTEC (SOdium Loop for TEst materials and Corrosion): Material investigation and qualification for high temperature applications in CSP field SOLTEC-1: Steady-state/transient investigation of creep fatigue in circulating Na SOLTEC-2: Experimental investigation of steel erosion and corrosion Main technical data: SOLTEC -1, -2: cold side 450°C, hot side 720°C @ 3.5 bar, ~14L Na, ~300 kg/h SOLTEC facilities

The use of heat pipes (HP) for the DEMO in-vessel plasma-facing components (PFCs) has been considered because of their high capacity to transport the heat from a heat source to a heat sink by means of the vaporization and condensation of the working fluid inside and their ability to enlarge the heat transfer area of the cooling circuit substantially. Recent engineering studies conducted in the framework of the EUROfusion work package Divertor (Wen et al, 2021) indicate that it is possible to design a heat pipe with a capillary limit above 6 kW using a composite capillary structure (wherein axial grooves cover the adiabatic zone and the condenser, and sintered porous material covers the evaporator). This power level would correspond to an applied heat flux of 20 MW/m2, rendering such a design interesting with respect to a divertor target concept. To validate the results of the initial engineering analysis, several experiments have been conducted to evaluate the actual performance of the proposed heat pipe concept. The present contribution presents the experiment’s results regarding the examination of the operating limits of two different designs for an evaporator: one featuring a plain porous structure, and one featuring ribs and channels.

Liquid metals, such as sodium, have been already successfully used as heat transfer fluids (HTF) in concentrating solar power (CSP) plants up to ~550 °C. Even higher temperatures can be achieved and are envisioned for future CSP plants. The lack of measuring flow rate devices at high temperatures for liquid metals motivated this study. The present paper presents the experimental mock-up and the experimental results obtained with the SOLTEC-2 facility for two test flow sensors, one innovative eddy current flow sensor (ECFM) developed at HZDR, Germany and a built-in permanent magnet fly-wheel sensor for runs up to a sodium temperature of 700 °C. The signals of the sensors are compared also against the power level of the sodium pump.

. Liquid metal heat transfer systems are a technically attractive option to increase the efficiency of CSP plants. Sodium as heat transfer medium is promising especially due to high heat transfer rates. In this paper, basic considerations are made to match the requirements of a small-scale loop with sodium as heat transfer medium. The setup of the test facility KArlsruhe ReceIver test FAcility (KARIFA) to heat up this loop with a 20 kW IR laser is described. The loop is planned as an integrated design using an additive manufacturing process. Different technologies like selective laser melting (SLM) are established processes to realize an integrated design and to bring together components in one part. Some variations must be implemented to adapt these processes to liquid metal loops. The high heat fluxes also demand a flow simulation to ensure an efficient and safe operation. A Reynolds-averaged Navier-Stokes (RANS) steady state simulation was performed to show the surface and duct temperatures as well as dimensionless values to analyse the characteristics of the sodium flow.

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.

The Pre-Concept Design (PCD) of the Balance of Plant (BoP) systems of the EU-DEMO power plant is described in this paper for both breeding blanket (BB) concepts under assessment, namely the Water Cooled Lithium Lead (WCLL) BB and the Helium Cooled Pebble Bed (HCPB) BB. Moreover, the results of a preliminary evaluation of a number of BoP variants are discussed. This paper outlines the steps of the BoP design development, highlighting the project objectives and the strategy for their achievement under the very challenging requirements which include, among others, the intermittent nature of the DEMO plasma heat source. The main achievements during the PCD Phase will be reported together with the development plan for the Concept Design (CD) Phase to reach a mature (feasible) BoP concept for DEMO.

DEMO will be a fusion power plant demonstrating the integration into the grid architecture of an electric utility grid. The design of the power conversion chain is of particular importance, as it must adequately account for the specifics of nuclear fusion on the generation side and ensure compatibility with the electric utility grid at all times. One of the special challenges the foreseen pulsed operation, which affects the operation of the entire heat transport chain. This requires a time-dependant analysis of different concept design approaches to ensure proof of reliable operation and efficiency to obtain nuclear licensing. Several architectures of Balance of Plant were conceived and developed during the DEMO Pre-Concept Design Phase in order to suit needs and constraints of the in-vessel systems, with particular regard to the different blanket concepts. At this early design stage, emphasis was given to the achievement of robust solutions for all essential Balance of Plant systems, which have chiefly to ensure feasible and flexible operation modes during the main DEMO operating phases – Pulse, Dwell and ramp-up/down – and to adsorb and compensate for potential fusion power fluctuations during plasma flat-top. Although some criticalities, requiring further design improvements were identified, these preliminary assessments showed that the investigated cooling system architectures have the capability to restore nominal conditions after any of the abovementioned cases and that the overall availability could meet the DEMO top-level requirements. This paper describes the results of the studies on the tokamak coolant and Power Conversion System (PCS) options and critically highlights the aspects that require further work.

The present paper describes the 1000K sodium loop developed at the Karlsruhe Institute of Technology for creep fatigue and corrosion/erosion investigations of new materials for applications in the concentrating solar power plants. The construction of the high temperature loops is motivated also by the lack of experimental data for steels in flowing sodium at temperatures above -650'C, as revealed by the literature review. The study discusses the loop operation procedures and the safety measures considered. Some of the experimental results obtained so far are presented, namely the loop operations at maximal specified temperature, at maximal specified flow rate, the calibration of the sodium flowmeter and the dynamic flow conditions in the loop. The new materials proposed at KIT for the receiver are briefly presented.

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.

The experimental liquid metal loops hosted within the Karlsruhe Sodium laboratory (KASOLA) comprise a set of facilities to study liquid metal flows for various types of energy applications ranging from room temperature conditions used for education and training and fundamental research up to challenges posed by multiphysics problems such as material-fluid interactions at high temperatures. Extreme conditions, such as sodium boiling, relevant to thermo-electric conversion or fast reactor safety are covered in a dedicated small-scale facility (Karlsruhe receiver test facility (KARIFA)). The complete experimental range is complemented by system code support and computational fluid dynamics (CFD) simulation. The outcome of the experiments is used for code validation and development allowing application not only on component but also on system level. The paper presents main experimental facilities and supporting computational codes used at KASOLA.

When progressing from the International Thermonuclear Experimental Reactor (ITER) to the Demonstration Fusion Reactor (DEMO), a system for transferring plasma heat exhaust to a power conversion system is necessary for the so-called Balance of Plant (BOP). During the preconceptual phase of the EU-DEMO project, different BOP concepts were investigated in order to identify the main requirements and feasible architectures to achieve that goal in the most efficient way. This paper comprises the investigations performed during the DEMO preconceptual design phase (p-CDP) and compares the different variants. The main aspect was focused on the helium-cooled pebble bed (HCPB) breeding blanket (BB) concept. After all assessments were performed, the indirect coupled design (ICD) was chosen as the reference configuration for the DEMO HCPB BOP for further development and optimization. The ICD provides decoupling using a molten salt storage loop, which accumulates thermal power during plasma pulses that are released during dwell periods. The work is supported by simulations using design codes EBSILON and MATLAB/SIMULINK, providing the basis for the next design phase.

Wakes appearing downstream of disturbances on the surface of a water flow in a concave open channel were examined experimentally. The investigated channel geometry was similar to the liquid lithium target in DONES (Demonstration fusion power plant Oriented NEutron Source). The objective of the measurements was to analyze the effect of a disturbance on the downstream layer thickness. For measuring the height profiles in the channel, an optical measurement system based on laser triangulation was developed. It was shown that the wake of the undisturbed flow emerged from the nozzle corner, which was in accordance with analytical solutions. For sufficiently large disturbances at the nozzle edge, the height profiles located downstream showed symmetrical minima and maxima on both sides of the disturbance. The wake depth strongly depended on the diameter and penetration depth of the disturbance, as well as the circumferential position in the channel, which yields to a critical wake depth of one millimeter for the lithium target in DONES.

The European Research Roadmap to the Realisation of Fusion Energy foresees that the DEMO reactor is going to succeed ITER in the pathway towards the exploitation of nuclear fusion, achieving long plasma operation time, demonstrating tritium self-sufficiency and producing net electric output on an industrial scale. Therefore, its design must be more oriented towards the Balance of Plant (BoP) than it is in ITER. Since the early pre-conceptual phase of the DEMO project, emphasis has been laid on identifying the main requirements affecting the overall architecture of the BoP. For instance, specific efforts and proper solutions have been envisaged to cope with the pulsed nature of the heat source. Furthermore, the current development of two blanket concepts calls for two separate BoP options to be conceived. This paper summarizes the main alternatives outlined at the end of DEMO pre-conceptual design phase for the BoP concepts based on both the Helium-Cooled Pebble Bed (HCPB) and Water-Cooled Lithium Lead (WCLL) Breeding Blanket (BB) technologies. Then, the assumed reference configurations of both the BoP concepts are described in detail, highlighting the main features and the most relevant engineering aspects. Attention will be focussed on technological challenges, integration constraints and other open issues, highlighting pros and cons of the chosen BoP options to be further investigated in the next design phase.

Heat pipes (HP) are considered being used in the divertor target because of their high thermal conductivity and their capability to substantially enlarge the heat transfer area to the cooling circuit. Here, a divertor target concept based on a heat pipe design is introduced being able to dissipate heat fluxes of up to 20 MW/m2. It consists of a 230 mm long water-based heat pipe with a capillary structure that combines axial grooves with sintered porous material. The analysis of the integration of the HP in the divertor target indicates that design changes of the current divertor cassette design are required to accommodate such a solution.

This study is carried out within the DEMOnstration Balance of Plant (DEMO BoP) development activities. Despite the pulsed operation of the Tokamak reactor, a steady energy output of the DEMO BoP need to be ensured. To fulfil this requirement, a two-tank Intermediate Heat Transport and Storage System (IHTS) with HITEC molten salt as a heat transfer fluid is applied. The focus of the study is the development of a dynamic simulation model for the DEMO IHTS using MATLAB?/Simulink. The customized programme block is developed for MATLAB?, which is used for the simulation of the temperature dependences of HITEC parameters. The influence of tank geometry and molten salt temperature on the temperature decrease in the IHTS tanks in due course of service maintenance is studied. The simulation of fluid filling level as well as mass and energy flows through the IHTS during the pulse and dwell operation is carried out.

In this paper, we present experimental results for a non-isothermal vertical confined backward facing step conducted with a low-Prandtl number fluid. The eutectic alloy gallium–indium–tin is used as the working fluid. We conducted experiments for different Reynolds and Richardson numbers covering both forced and mixed convection regimes. Time-averaged velocity profiles were measured at six streamwise positions along the test section center-plane with so-called permanent magnet probes. The local Nusselt number was measured in streamwise and spanwise directions along the heating plate mounted right after the step. We further ran RANS simulations of the experiment to study the qualitative influence of assuming a constant specific heat flux thermal boundary condition for the experiment heating plate. The measured velocity profiles show the expected behavior for both studied convection regimes, while the measured streamwise local Nusselt number profiles do not. This is explained by how the heating plate thermal boundary condition is defined. We performed an order of magnitude estimate to estimate the forced- to mixed convection transition onset. The estimate shows good agreement with the experimental data, although further measurements are needed to further validate the estimated transition threshold. The measurement of fluctuating quantities remains an open task to be addressed in future experiments, since the permanent magnet probe measurement equation needs further adjustments.

The backward facing step geometry (BFS) is a representative geometry for sudden expansions in pipe, duct and channel flows. While this type of geometry by itself is not a part of engineering components, the flow separation and the accompanying flow features present in a BFS are of great importance when designing manifolds, heat exchangers or fuel bundles. In the frame of EU Horizon 2020 project SESAME, an extensive effort has been put forward to gain more insights into the flow and thermal features in a BFS geometry for low-Prandtl number fluids. The main motivation behind this effort is two-fold: to generate a reference database by means of experiments and high fidelity simulations, and accordingly utilize the reference database to validate and/or improve the turbulence models in engineering applications. In this paper, we present a broad description of the experimental facility and its expected capabilities, as well as the results of numerical efforts. The experimental results will be obtained in the DITEFA 2 facility of KIT with a GaInSn eutectic alloy. The expansion ratio of the BFS in the experiment is set to 2 and the geometry has one heated wall. Unlike the vast majority of the BFS experiments found in the literature, the present BFS experiment has an outflow in a shape of a square, that is, the width and height of the outflow are approximately the same. Second, a direct numerical simulation (DNS) is performed with a passive scalar and for an expansion ratio of 2.25. Similar to the experiment, the shape of the outflow is a square and the average flow is three-dimensional. Conjugate heat transfer DNS is performed for the heated solid walls, while the unheated walls were neglected. Finally, this reference DNS data is used to validate a LES and an advanced RANS modelling approach.