Thermofluid-dynamic assessment of the EU-DEMO divertor single-circuit cooling option

Until 2019, the thermo-hydraulic development of the EU-DEMO divertor was based on the “double-circuit” concept, in which two independent cooling circuits served by two different Primary Heat Transfer Systems were used to cool the Plasma-Facing Components (PFC) and the Cassette Body (CB). During the Divertor Final Design Review Meeting, held in May 2020, the possibility to adopt a single cooling circuit to serve both components was suggested. This new cooling circuit was originally conceived with the aim of simplifying remote maintenance, with potential benefits for some aspects of safety and balance of plant design and integration. During the years from 2020 to 2022, in the framework of the Work Package DIV 1 - “Divertor Cassette Design and Integration” of the EUROfusion action, University of Palermo and ENEA carried out a research campaign focussed on the preliminary thermofluid-dynamic assessment of this new concept, highlighting its strengths and weaknesses. The research campaign was carried out following a theoretical–computational approach based on the finite volume method and adopting the commercial computational fluid-dynamic code ANSYS-CFX. The steady-state thermal-hydraulic performances of the single-circuit DEMO divertor concept were assessed in terms of coolant pressure drop and flow velocity distribution, mainly in order to check coolant aptitude to provide a uniform and effective cooling to CB, shielding liner, reflector plates, PFCs and the newly introduced neutron shields to improve the shielding of the vacuum vessel. Moreover, the margin against critical heat flux distributions among the plasma-facing channels were assessed by adopting appropriate correlations, to check the compliance with the applicable constraints. Models, loads and boundary conditions assumed for the analyses are herewith reported and critically discussed, together with the main results obtained

Hydraulic characterization of the full scale mock-up of the demo divertor outer vertical target

In the frame of the pre-conceptual design activities of the DEMO work package DIV-1 “Divertor Cassette Design and Integration” of the EUROfusion program, a mock-up of the divertor outer vertical target (OVT) was built, mainly in order to: (i) demonstrate the technical feasibility of manufacturing procedures; (ii) verify the hydraulic design and its capability to ensure a uniform and proper cooling for the plasma facing units (PFUs) with an acceptable pressure drop; and (iii) experimentally validate the computational fluid-dynamic (CFD) model developed by the University of Palermo. In this context, a research campaign was jointly carried out by the University of Palermo and ENEA to experimentally and theoretically assess the hydraulic performances of the OVT mock-up, paying particular attention to the coolant distribution among the PFUs and the total pressure drop across the inlet and outlet sections of the mock-up. The paper presents the results of the steady-state hydraulic experimental test campaign performed at ENEA Brasimone Research Center as well as the relevant numerical analyses performed at the Department of Engineering at the University of Palermo. The test facility, the experimental apparatus, the test matrix and the experimental results, as well as the theoretical model, its assumptions, and the analyses outcomes are herewith reported and critically discussed

Hypothetical porous medium concept as a virtual swirl tape: A novel modelling technique towards efficient CFD simulation of swirl tape cooling pipe

The EU-DEMO divertor target cooling circuit is equipped with Swirl Tape (ST) inserts to improve its thermo-hydraulic performance in terms of heat transfer coefficient and critical heat flux. Due to the presence of the STs, accurate 3D CFD-based thermofluid-dynamic assessments of the divertor targets cooling circuit require a high computational cost and a laborious pre-processing modelling effort. To this end, a cost-efficient CFD simulation technique based on an equivalent porous medium concept, namely the Virtual Swirl Tape (VST) approach, has been developed. In this work, the mathematical formulation of different VSTs models is presented, and the porous media calibration procedure and validation are shown. This technique enables the reduction of computational costs by decreasing the number of volumes required for a single Plasma-Facing Unit (PFU) assembly cooling channel by a factor of 10, while lowering the calculation time by ≈86%. The results obtained show that it is possible to correctly reproduce the friction factor profile and pressure drop of a PFU assembly cooling channel, this latter with errors within 10% considering a wide range of coolant inlet velocities. Some limitations have been observed concerning the VST thermal performance, which is still unsatisfactory and requires further development. The VST approach has been studied using the commercial CFD code ANSYS CFX, coupled with a multi-objective optimization algorithm available in the ANSYS Direct Optimization tool

Divertor of the European DEMO: Engineering and technologies for power exhaust

In a power plant scale fusion reactor, a huge amount of thermal power produced by the fusion reaction and external heating must be exhausted through the narrow area of the divertor targets. The targets must withstand the intense bombardment of the diverted particles where high heat fluxes are generated and erosion takes place on the surface. A considerable amount of volumetric nuclear heating power must also be exhausted. To cope with such an unprecedented power exhaust challenge, a highly efficient cooling capacity is required. Furthermore, the divertor must fulfill other critical functions such as nuclear shielding and channeling (and compression) of exhaust gas for pumping. Assuring the structural integrity of the neutron-irradiated (thus embrittled) components is a crucial prerequisite for a reliable operation over the lifetime. Safety, maintainability, availability, waste and costs are another points of consideration. In late 2020, the Pre-Conceptual Design activities to develop the divertor of the European demonstration fusion reactor were officially concluded. On this occasion, the baseline design and the key technology options were identified and verified by the project team (EUROfusion Work Package Divertor) based on seven years of R&D efforts and endorsed by Gate Review Panel. In this paper, an overview of the load specifications, brief descriptions of the design and the highlights of the technology R&D work are presented together with the further work still needed

On the modelling of tritium transport phenomena at fluid-structure interfaces

One of the main functions of the DEMO Breeding Blanket (BB) system is to ensure the tritium breeding inside the reactor. Tritium is a beta emitter radioactive isotope, subjected to several processes that determine its permeation across materials and its leakage towards the environment, posing potential safety issues in terms of radiological hazard. Thus, the evaluation of tritium inventories inside components and tritium losses towards the environment plays a key role in the fulfilment of the pertinent BB safety requirements. In this regard, a research activity has been carried out, in close cooperation between the University of Palermo and the Karlsruhe Institute of Technology, focussing on the development of a multiphysical model that might realistically simulate 3D tritium transport phenomena across complex fluid-structure interfaces. Models, source terms and boundary conditions assumed for the analyses are herewith reported and critically discussed, together with the main results obtained

Mid-term results of bicuspid aortic valve repair guided by morphology and function assessment.

OBJECTIVES: Bicuspid aortic valve (BAV) is frequently associated with aortic insufficiency (AI) due to cusp disease and/or aortic root dilatation. Based on functional classification and morphology, a systematic surgical approach was used for aortic valve repair (AVr). METHODS: : From 2004 to 2014, 152 consecutive patients (mean age 55 ± 7 years) with BAV underwent AVr with or without concomitant aortic root surgery. Cusp pathology was treated with central plication in 60 (39.5%) patients, free edge reinforcement in 45 (29.6%), triangular resection in 28 (18.4%) and pericardial patch in 19 (12.5%). Aortic root dilatation was corrected with valve sparing reimplantation in 65 patients. Mean follow-up was 68 ± 36 months. RESULTS: : In-hospital death was 1.3%. Overall 5-year survival was 88.6 ± 3.6%. Freedom from recurrent ≥ grade 2 AI requiring reoperation at 5-year was 93 ± 3.1%. Five-year freedom from reintervention was 98.4 ± 1.6%, 93.3 ± 6.4% and 82.6 ± 9.6 in the group of patients treated with AVr and reimplantation technique, AVr and subcommissural plasty and AVr repair alone (log-rank P  = 0.005). Commissural orientation 29 mm were identified as predictors for recurrent ≥ grade 2 AI. CONCLUSIONS: : BAV repair could be achieved with low incidence of recurrent AI, especially when concomitant annulus stabilization was performed. Systematic preoperative morphological and functional characteristics of the aortic valve should be carried out in order to reduce the risk of repair failure

Aortic stenosis: insights on pathogenesis and clinical implications

Aortic stenosis (AS) is a common valvular heart disease in the Western populations, with an estimated overall prevalence of 3% in adults over 75 years. To understand its patho-biological processes represents a priority. In elderly patients, AS usually involves trileaflet valves and is referred to as degenerative calcific processes. Scientific evidence suggests the involvement of an active “atherosclerosis-like” pathogenesis in the initiation phase of degenerative AS. To the contrary, the progression could be driven by different forces (such as mechanical stress, genetic factors and interaction between inflammation and calcification). The improved understanding presents potentially new therapeutic targets for preventing and inhibiting the development and progression of the disease. Furthermore, in clinical practice the management of AS patients implies the evaluation of generalized atherosclerotic manifestations (i.e., in the coronary and carotid arteries) even for prognostic reasons. In counselling elderly patients, the risk stratification should address individual frailty beyond the generic risk scores. In these regard, the co-morbidities, and in particular those linked to the global atherosclerotic burden, should be carefully investigated in order to define the risk/benefit ratio for invasive treatment strategies. We present a detailed overview of insights in pathogenesis of AS with possible practical implications