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As part of the activities performed by the DEMO Central Team (DCT) to study alternative configurations of the Water-Cooled Lead Lithium (WCLL) Breeding Blanket (BB), to overcome the open issues that emerged at the end of the Pre-Conceptual Design phase, a new concept, the WCLL BB "double bundle" (db), was developed. This concept adopts an array of db tubes poloidally distributed inside the breeding zone, mimicking the arrangement inside heat exchangers. These are coaxial pipes the gap between which is filled with PbBi (or alternatively with gas and fins), which avoids direct contact between PbLi and water in case of in-box LOCA events. Similarly, the First Wall channel cooling water is separated from the PbLi. The use of PbBi as an inert fluid makes the Segment Box (SB) sizing less stringent: in the case of an in-box LOCA due to a break of the water cooling circuit inside the BB SB, the pressurized volume is only the PbBi one, instead of the whole SB volume as in the reference WCLL design configuration. Consequently, the total amount of steel inside the BB can be reduced, with benefits in terms of increased tritium breeding ratio. Within this framework, University of Palermo, in support of the DCT and under the EUROfusion action, preliminarily analysed the steady-state thermal, thermo-hydraulic and normal and off-normal thermo-mechanical performances of this concept, following a theoretical-numerical approach based on finite volume and finite element methods, and adopting the ANSYS CFX and ANSYS Mechanical codes. The analyses carried out allowed a preliminary optimization of the design, improving the thermal and thermo-mechanical performances of the WCLL-db BB, in compliance with the design requirements, while reducing the total steel amount. Models, assumptions and main results are herewith reported and critically discussed. [ABSTRACT FROM AUTHOR]

• The progresses of the Primary Heat Transfer System of the HCPB Breeding Blanket for the DEMO reactor are outlined. • The main components of Ex-Vessel PHTS has been sized to be compliant with the new In-Vessel requirements. • Results highlights that the 2017 HCPB BB PHTS has reached improved thermal-hydraulic and safety characteristics. • Further efforts are still needed to reduce PHTS pressure drops and volumes as well as to improve IHXs performance. In the frame of the activities promoted and encouraged by the EURO-fusion Power Plant Physics and Technology (PPPT) department aimed at developing the EU-DEMO fusion reactor, strong emphasis has been recently posed to the whole Balance of Plant (BoP) which represents the set of systems devoted to convert the plasma generated thermal power into electricity and to deliver it to the grid. Among these systems, a very important role is played by the Breeding Blanket (BB) Primary Heat Transfer System (PHTS) as it is responsible to extract more than 80% of the fusion plasma power. In this framework, University of Palermo, Ansaldo Nucleare and CREATE have focused their work to improve thermal-hydraulic, safety and integration features of the Ex-Vessel PHTS for the Helium-Cooled Pebble Bed (HCPB) BB concept of DEMO. Starting from the outcomes obtained in 2016 that have allowed to highlight some criticalities which needed design changes, the paper describes progress and developments of the 2017 HCPB PHTS as well as the goals which have been achieved. The results of the research activity carried out show, in fact, i) an increase of the thermal-hydraulic performances, since a reduction in both total coolant inventory and total pressure drop has been respectively reached, ii) a potential improvement of the overall safety characteristics of the system. Nevertheless a critical assessment of these key parameters reveals that some issues are still open in terms of design integration and feasibility of the whole DEMO BoP for the helium-cooled blanket option, indicating that additional efforts are required to make this technology more attractive. [ABSTRACT FROM AUTHOR]

• Neutronic analyses on the Water Cooled Lithium Lead blanket (WCLL) concept envisaged for DEMO reactor. • A theoretical-computational approach based on the Monte Carlo Method has been followed, adopting a qualified code MCNP5-1.6. • Parametric analyses on the influence of cooling water within fires wall channels both in terms of lay-out and inventory on the nuclear response of WCLL blanket under irradiation in DEMO reactor. In the framework of EUROfusion Work Package International Cooperation R&D activities, a close collaboration has been started among University of Palermo, ENEA Brasimone and ENEA Frascati for the development of the Water Cooled Lithium Lead (WCLL) Breeding Blanket (BB) concept. In this context, a research campaign has been carried out at the University of Palermo in order to investigate the influence of First Wall (FW) cooling water configuration on the nuclear response of the WCLL BB under irradiation in EU-DEMO, in order to gain useful indications for the WCLL BB pre-conceptual designs. To this end, three-dimensional nuclear analyses have been performed by MCNP5 v. 1.6 Monte Carlo code. A semi-heterogeneous MCNP model of the WCLL BB "single module segment" concept has been used and its nuclear response has been assessed for different FW configurations, centring the attention, mainly, on global quantities as nuclear power deposition and tritium breeding ratio. The results obtained provided some information on the nuclear features of the WCLL BB concept, useful for its design optimisation. The outcomes of this parametric study are herein presented and critically discussed. [ABSTRACT FROM AUTHOR]

The development of a sound conceptual design for the Water-Cooled Lead Lithium Breeding Blanket (WCLL BB) is pivotal to make a breakthrough towards the selection of the driver blanket concept for the EU-DEMO. To this goal, a research campaign has been launched over the last years at the University of Palermo, in close cooperation with ENEA Brasimone, under the umbrella of EUROfusion. In this frame, the analysis of the thermo-mechanical behaviour of the WCLL Left Outboard Blanket (LOB) segment is being performed. In a first phase, the assessment of the segment's overall structural performances was addressed, allowing the investigation of its global response under the selected loading scenarios. On this basis, the local structural analysis of the central region and of the upper and lower regions presenting geometric discontinuities (namely those regions where the stiffeners numbers changes) is presented in this paper, with the aim of assessing in detail their structural behaviour under the nominal BB operating conditions as well as steady-state accidental loading scenarios. Adopting the sub-modelling technique, the displacement field calculated in previous LOB global structural analysis can be mapped and applied at the boundaries of each local model. Moreover, it is possible to include there some structural details missing in the global analysis, like the Segment Box cooling channels. In this way, it is possible to study the thermo-mechanical behaviour of these regions in detail, assuming at the borders the mechanical action of the rest of the structure. The assessment has been performed in compliance with the RCC-MRx code, adopting the set of criteria on the basis of the nature of the considered loading scenario. The obtained results showed a promising structural behaviour of the segment and highlighted the necessity to revise the attachment system layout, which originates excessive deformation leading to the prediction of high stress. [ABSTRACT FROM AUTHOR]

• The structural performances of different poloidal regions of the WCLL BB central outboard blanket (COB) segment have been investigated. • Once investigated the overall structural behaviour of the COB segment, the sub-modelling technique has been adopted to study in detail some regions of interest. • Steady state thermo-mechanical analyses under normal and off-normal loading conditions have been performed. • The comparison between the global model and sub-models results has been carried out. • Results of the sub-models analyses confirmed the overall structural response of the COB segment, suggesting a revision of the attachment system. The development of a sound conceptual design of the Water-Cooled Lead Lithium Breeding Blanket (WCLL BB) is pivotal to make a breakthrough towards the selection of the driver blanket concept for the EU-DEMO. To achieve this goal, an intense research campaign has been performed at the University of Palermo, in cooperation with ENEA Brasimone, under the umbrella of EUROfusion. In this paper, structural analyses of different poloidal regions of the WCLL BB Central Outboard Blanket (COB) segment are reported. In particular, starting from the results of the thermo-mechanical analysis of the whole WCLL BB COB segment, the sub-modelling technique has been applied to the most significant poloidal regions, located at the top, middle and bottom of the segment. The aim is to focus on the stress field locally arising under purposely selected steady-state nominal and accidental loading scenarios. The nominal BB operating conditions, as well as steady-state scenarios derived from both the in-box LOCA and Vertical Plasma Disruption accidents have been considered. Thanks to the sub-modelling approach, the deformative action of the entire segment can be imposed at the boundaries of each local model to realistically assess its structural performances. Moreover, each local model reproduces structural details not included in the global one, such as the Segment Box (SB) cooling channels. Then, the structural behaviour of the selected regions has been assessed in compliance with the RCC-MRx code. The obtained results highlighted that the structural behaviour predicted by the whole segment analysis is similar to that predicted by sub-modelling calculations within the Stiffening Plates, whereas the application of the sub-modelling is a must to investigate in detail the SB structural performances. In addition, results indicate that the BB attachments should be revised, as they contribute to produce the WCLL COB large deformation originating excessive stresses, mainly within the equatorial region. [ABSTRACT FROM AUTHOR]

• The preliminary thermo-mechanical investigation of the DEMO WCLL LOB segment has been performed for the first time. • The DWTs layout within the equatorial region has been preliminarily optimized from a thermal point of view. • A 3D thermal field for the whole LOB segment has been extrapolated from the equatorial region by means of a purposely developed analytical procedure. • Steady state thermo-mechanical analyses under normal and off-normal loading conditions have been performed. • Results have highlighted that the attachment system should be revised to improve the WCLL LOB segment structural performances. The conceptual design phase of the EU-DEMO reactor has been recently launched, with the aim of evolving the DEMO pre-conceptual layout towards a more robust and articulated geometric configuration able to cope with most of the design requirements and to show further margins for the passing of the current potential show-stoppers. Hence, the achievement of the conceptual design of the Water-Cooled Lead Lithium Breeding Blanket (WCLL BB) is one of the milestones the EUROfusion consortium aims to achieve in the close future. To this purpose, within the framework of the research activities promoted by EUROfusion, a research campaign has been launched at the University of Palermo, in close cooperation with ENEA-Brasimone, in order to investigate, from the thermal and thermomechanical standpoints, the WCLL BB Left Outboard Blanket (LOB) segment performances. The scope of the analysis carried out has been the advancement of the WCLL LOB segment design in view of the prescribed thermal and structural design requirements. To this end, the nominal WCLL BB operating conditions as well as accidental steady-state scenarios derived from the in-box loss of coolant accident and the upper vertical plasma disruption event have been considered. In the first part of the study, the internal Double-Walled Tubes (DWTs) layout of the WCLL LOB equatorial region have been preliminarily optimized so to find a geometric configuration able to cool the segment structure maintaining the maximum temperature below the suggested limit of 550 °C. Then, a purposely set-up analytical procedure has allowed obtaining a set of interpolating functions able to accurately reproduce the calculated thermal field and to extrapolate it to the whole WCLL LOB segment. Afterwards, the thermo-mechanical analysis of the entire segment has been performed under the selected loading scenarios. The structural behaviour of the segment has been assessed in compliance with the RCC-MRx code, adopting the set of criteria on the basis of the nature of the considered loading scenario. The obtained results, herewith presented and critically discussed, have allowed introducing significant design improvements with respect to the WCLL LOB segment reference geometric layout, coming from the pre-conceptual design phase, paving the way for future, and more detailed, structural investigations. [ABSTRACT FROM AUTHOR]

Extensive modeling and analytical work has been carried out considering the Helium-Cooled Pebble Bed Breeding Blanket (HCPB BB) Balance Of Plant (BOP) configuration of the Demonstration Power Plant (DEMO) using the Apros system code, developed by VTT Technical Research Centre of Finland Ltd. and Fortum. The integral plant model of the HCPB BB plant has been improved with respect to the blanket and steam generator models. Based on HCPB-BL2017 v1 data, reported in 2019, the blanket has been remodeled by separate Apros process components, dedicated to average inboard and outboard segments, where the power deposition scheme of the breeding units took into account the output of high-fidelity neutronic analyses. A new helical coil steam generator model has been developed for primary–secondary system coupling using CAD data provided by EUROfusion partner University of Palermo. Transient analyses have been performed with Apros on the plant configuration that utilizes a molten salt technology-based small Energy Storage System (ESS). [ABSTRACT FROM AUTHOR]

Within the framework of the EUROfusion design activities concerning the EU-DEMO Breeding Blanket (BB) system, a research campaign has been carried out at the University of Palermo with the aim of investigating the structural behaviour of the DEMO Water-Cooled Lithium Lead (WCLL) Central Outboard Blanket (COB) segment. The assessment has been performed considering three different loading scenarios: the Normal Operation (NO), the Over-Pressurization (OP) and the Upward Vertical Displacement Event (VDE-up). In particular, NO scenario represents the loading case referring to the nominal operating conditions, whereas the OP scenario refers to the loading conditions due to an in-box LOCA accident, listed as one of the BB design basis accidental events. Lastly, the VDE-up scenario is an off-normal event reproducing the plasma disruption caused by an uncontrolled vertical motion of the plasma volume. This event brings the plasma in contact with the upper part of the plasma chamber, generating a sudden energy discharge accompanied by Electro Magnetic (EM) forces acting on the structure. The study has been carried out following a theoretical-numerical approach based on the Finite Element Method (FEM) and adopting the quoted ABAQUS v. 6.14 commercial FEM code. In particular, a detailed 3D FEM model of the whole COB segment, including the back-supporting structure and its attachment system to the vacuum vessel, has been set up. Several simulations have been run to assess the thermo-mechanical performances of the segment under the afore-mentioned loading scenarios, also taking into account the impact of the tungsten (W)-armour on the overall structural response. EM loads have been considered in all the assessed scenarios. In the first two, only magnetization forces have been taken into account, while in the VDE-up scenario Lorentz's forces have been also taken into account. The structural response has been evaluated according to the RCC-MRx structural design rules. The obtained results are herewith presented and critically discussed. [ABSTRACT FROM AUTHOR]

• Assessment of the spatial distribution of the absorbed dose in the DEMO Upper Pipe Chase due to nitrogen isotopes in BB PHTSs. • Investigation of the absorbed dose in a typical isolation valve of the BB PHTS considering a period of 7 full power years. • The calculation of the absorbed dose has been carried out adopting MCNP5 Monte Carlo code. Within the framework of the activities foreseen by the EUROfusion action on the cooling water activation assessment for a DEMO reactor equipped with a Water Cooled Lithium Lead Breeding Blanket (WCLL BB), the University of Palermo is involved in the investigation of the absorbed dose induced by the decay of nitrogen radioisotopes produced by water activation, in the main components (e.g. isolation valves) of both First Wall (FW) and Breeder Zone (BZ) cooling circuits. The aim of this work is to assess the spatial distribution of the absorbed dose in the DEMO Upper Pipe Chase (UPC), focusing the attention on the space neighbouring a typical isolation valve of the Primary Heat Transfer System (PHTS), taking into account a period of 7 full power years. To this end, a computational approach has been followed adopting MCNP5 Monte Carlo code. In particular, a totally heterogeneous neutronic model of a portion of the UPC has been set up, including the valve and the main FW and BZ PHTS piping, and the spatial distribution of nitrogen isotopes concentrations, previously assessed, have been used to model the photonic and neutronic sources. The results obtained, herewith presented and critically discussed, provided some information on the nuclear issues of the WCLL BB PHTS, to be considered as hints for the blanket design optimization. [ABSTRACT FROM AUTHOR]

• To assess the spatial distribution of nitrogen isotope concentrations a coupled neutronic/fluid-dynamic problem is solved. • The assessment of nitrogen production rate distributions within FW an BZ BB cooling channels by means of MCNP6.2 code. • The calculation of the nitrogen isotope concentrations within the In-Vessel domain has been carried out by ANSYS-CFX code. • The calculation the nitrogen isotope concentrations in the PHTS has been carried out by a lumped parameters 1-D approach. Within the framework of the activities foreseen by the EUROfusion action on the cooling water activation assessment for a DEMO reactor equipped with a Water Cooled Lithium Lead Breeding Blanket (WCLL BB), the University of Palermo is involved in the assessment of dose rates induced by the decay of nitrogen radioisotopes produced by water activation, nearby the main components (e.g. isolation valves) of both First Wall (FW) and Breeder Zone (BZ) cooling circuits. In particular, the aim of this work is to evaluate the spatial distribution of nitrogen isotopes (16N and 17N) in the WCLL BB cooling circuits. To this purpose, a coupled neutronic/fluid-dynamic problem is solved following a theoretical-numerical approach and adopting an integrated computational tool mainly relying on the use of MCNP6 and ANSYS CFX codes. The operative procedure adopted foresees the assessment of the production rate distributions of nitrogen isotopes within FW and BZ cooling channels and tubes by means of totally heterogeneous neutronic analyses. A fully 3-D fluid-dynamic approach is, then, used to compute the nitrogen isotope concentrations within the In-Vessel complex flow domain, while a 1-D lumped parameters approach is adopted to calculate their distribution along the Ex-Vessel BB Primary Heat Transfer System. The results obtained, herewith presented and critically discussed, provided the necessary data to perform dedicated neutronic and photonic transport analyses and, hence, to assess the dose rates in the aforementioned target locations. [ABSTRACT FROM AUTHOR]