Thermal-hydraulic design and analysis of the STEAM and water loop facilities to support the development of the EU-DEMO tokamak fusion reactor

The Department of Astronautical, Electrical, and Energy Engineering at Sapienza University of Rome and the Division of Experimental Engineering – Nuclear Department – of ENEA collaborate within the framework of the EUROfusion project. This project, co-funded by the EURATOM program, aims to develop the design and technologies for the realization of a demonstration nuclear fusion reactor capable of producing electrical energy, known as DEMO. Italy plays a fundamental role in the EUROfusion consortium, with ENEA as the main beneficiary, and universities, research institutions, and industrial partners as affiliates. One of the most significant technological challenges for the realization of DEMO concerns the design and development of the technologies for the Water-Cooled Lithium-Lead (WCLL) breeding blanket concept. This component has an essential role in generating fuel (tritium), absorbing heat for energy production, and protecting the internal parts of the reactor. The WCLL breeding blanket adopts a mixture of lithium and lead (PbLi) as fertile material and pressurized water in the primary cooling circuit. The conceptual design of the Balance of Plant (BoP), developed by ENEA and its partners, involves the generation of electrical energy through a Rankine cycle: a single-phase, high-pressure, high-temperature primary circuit cools the WCLL breeding blanket and transfers power to the secondary circuit via steam generators, where electricity is generated by a turbine. In this context, ITER represents an indispensable step toward the realization of DEMO. This experimental reactor, currently under construction in Cadarache, France, is designed to demonstrate the scientific and technological feasibility of nuclear fusion. ITER will explore critical aspects of plasma confinement and stability, producing fusion power of up to 500 MW for short durations. Unlike DEMO, which aims to demonstrate the economic feasibility of fusion reactors, ITER will not generate electricity but will focus on validating operational regimes and testing key systems under fusion-relevant conditions, demonstrating fusion reactors technology feasibility. Among the pivotal systems tested in ITER there are the Test Blanket Modules (TBM), which play a crucial role in advancing the technologies required for the DEMO breeding blanket. These modules are integrated into the ITER structure to evaluate the performance of various breeding blanket concepts. The TBM program will provide invaluable data on heat transfer, neutron flux resilience, and tritium recovery, directly informing the design and optimization of various concepts, including the DEMO WCLL breeding blanket. Thus, ITER acts as a bridge between experimental fusion research and the practical realization of fusion energy, laying the groundwork for the technological innovations required in DEMO. Given the current understanding, continuous operation at constant power is not feasible in a fusion reactor based on tokamak technology. Reference studies for DEMO propose pulsed operation, with full-power cycles lasting 2 hours (referred to as pulse) followed by 10 minutes at nearly zero power (dwell). This mode of operation introduces new and significant challenges for the design, reliability, and safety of the breeding blanket and the DEMO BoP. In particular, the components most impacted by this operation are the first wall, the steam generator, and the turbine. For the first two, the design, analysis, and experimental demonstration of feasibility and reliability are ENEA responsibility and are, in part, the subject of this thesis. To address the unique challenges posed by the pulsed operation of DEMO, the Once-Through Steam Generator (OTSG) was selected for the steam generator design due to its lower thermal inertia compared to conventional U-tube generators. This reduced thermal inertia, a result of the OTSG lower water mass, allows it to respond more effectively to the rapid power fluctuations. Furthermore, the OTSG benefits from extensive validation in Pressurized Water Reactor (PWR) applications, where thermodynamic conditions at full power are comparable, further enhancing its suitability for DEMO. The main objective is the conceptual design of the DEMO steam generator test section and the experimental campaign aimed at demonstrating the stability of the main thermal-hydraulic parameter during the component operation, as well as its performance and the effectiveness of the adopted control strategy. The achievement of this objective included fulfilling several other significant secondary milestones relevant to both research and professional growth, such as: 1) contributing to the design of the STEAM facility, acquiring expertise in steam system design and in using support codes; 2) studying measurement devices and their application; 3) providing support through numerical simulations in the design of control and acquisition systems for the test section; 4) contributing to the design of the Water Loop facility; 5) participating in the drafting of technical supply specifications for the Water Loop and STEAM facilities; 6) participating in a large international project, working at the Brasimone Research Centre with the ENEA Division of Experimental Engineering; 7) improving skills in thermal-hydraulics and in the use of the RELAP5 code, further enhanced through a three-month internship at the Idaho National Laboratory (INL). The activity involved contributing to the design and analysis of the W-HYDRA experimental platform (Section 3), which includes the Water Loop facility for experimental tests on DEMO breeding blanket components and ITER TBM, as well as the STEAM facility for steam generator experimentation. Water Loop and STEAM facilities address complementary objectives in the study of fusion reactor components, but, after sensitivity analyses, their primary loops were integrated into a single W-HYDRA primary circuit, with the aim of optimizing the design while maintaining the distinct functionalities of the facilities, reducing redundancy and minimizing the costs. Initially, the scaling and conceptual design of the steam generator test section (i.e., Once Through Steam Generator) were developed, followed by a study to assess its representativeness under the reference operating conditions (Section 4). The thermal-hydraulic analyses performed using the RELAP5 code validated the design and enabled comparisons with the DEMO reference configuration. The goal was achieved by proposing the experimental campaign to be conducted, the control strategy to be implemented, the conceptual design of the test section (including supply specification), and the necessary instrumentation for control and data acquisition. Subsequently, the focus shifted to the design and analysis of the STEAM experimental facility (Section 5). The objective was to ensure that the facility had a power variation dynamic capable of adequately simulating the power variations expected in DEMO during the test section experiments. A critical issue in choosing the plant architecture was limiting the absorbed power to 4 MW. Additionally, numerical analyses were performed to assess critical components such as air coolers and the performance of the low-pressure section of the secondary loop under various operating scenarios. Finally, incident scenarios focused on pump failures (e.g., Loss Of Flow Accident and Loss Of Heat Sink) were studied to assess the facility dynamics. Since the primary circuit of the STEAM facility is interconnected with that of the Water Loop, the performed studies and analyses covered both high-power operation (3.1 MW) and lower power operation (800 kW). In the former case, the power from the electric heaters is removed through the steam generator, while in the latter, it is removed through single-phase heat exchangers, whose design and regulation were also investigated. Sensitivity analyses were conducted to optimize the integration between Water Loop and STEAM to reduce facility costs by minimizing the number of components. The proposed configuration of the Water Loop can test various test sections, including plasma-facing components, over a wide range of power, flow rates, temperatures, and pressures. The RELAP5 analyses showed that it is also possible to perform pulse-dwell-pulse transitions and simulate incident transients. Even though STEAM and the Water Loop share part of the primary loop, they are characterized by extremely different purposes. STEAM aims to provide a structure for a component-level investigation, focusing on the characterization of the steam generator mock-up under various operational and accidental scenarios. Within this framework, the performed analyses supported the design of the facility to ensure that it replicates the thermal-hydraulic responses of the DEMO plant in terms of boundary conditions across all considered scenarios, even though it is not representative of the DEMO plant itself. For this reason, proportional-integral control systems were implemented to ensure the accuracy and stability of the boundary conditions. Consequently, safety analyses performed for STEAM—such as transient simulations—are not aimed at supporting the safety of the DEMO reactor. Instead, they serve as pre-test analyses to guarantee the proper functioning of the facility itself. In this phase, in fact, STEAM primary purpose is to demonstrate the performance and reliability of the steam generator mock-up under pulsed operational conditions, with other analyses focused exclusively on supporting the facility operation. In contrast, the Water Loop is designed to perform system-level analyses where the thermal-hydraulic response of the entire system is as critical as that of the test section. For this reason, the analyses conducted with the Water Loop aim to support the safety aspects of the ITER design by assessing the integrated behavior of the system under a variety of scenarios. Thanks to the work undertaken and the results obtained, an experimental activity was designed to demonstrate the feasibility of operating a steam generator in pulsed mode. An experimental platform capable of testing test sections for components such as steam generators, plasma-facing components, and simulating transient accidents has been developed. Additionally, technical specifications for the supply of systems and components currently under construction or subject to tendering have been produced. Therefore, this PhD activity represents a significant and decisive contribution to defining and realizing a unique experimental infrastructure in the international landscape for qualifying components and technologies of ITER TBM, DEMO breeding blanket, and BoP.