Rapporto preliminare del progetto del target di IFMIF comprensivo di disegni, analisi numeriche e valutazione dei costi

The International Fusion Materials Irradiation Facility (IFMIF) is an international joint project aimed at developing an accelerator-based neutron irradiation facility to test and qualify candidate materials to be used in future fusion power reactors. Within the current IFMIF Engineering Validation and Engineering Design Activities (EVEDA) phase, ENEA is involved in the design of the Target Assembly (TA) with removable bayonet Backplate which is one of the two alternative concepts envisaged for the TA system of IFMIF (the other being the Integral Target Assembly to be developed by JAEA in Japan). In the present annual period covered by the current MSE-ENEA Agreement, the design of the Target Assembly with bayonet Backplate has advanced starting from a reference model based on the previous design which was described in the system Design Description Document of the Engineering Design Activities phase I (DDD-I). In particular, the integration of the Target Assembly with bayonet Backplate into the Li loop and Target Test Cell models has been performed. Moreover, some important features previously not present in the model, such as the TA supporting structures and positioning system and the thermal compensating system were developed and introduced in the design. As a further advancement, some work has been done on the numerical analyses in support to the TA design and the safety assessment of the IFMIF Lithium Target Facility that is also a task in charge of ENEA within the IMIF/EVEDA project. Regarding this latter activity, a contribution has been given to the preparation of the IFMIF main safety documents. Updated activation calculations for the Activated Corrosion Products (ACP) have been also performed in order to provide the radioactive inventories of Lithium containing ACP due to the irradiation of both neutron and deuteron fluxes. Concerning the numerical analyses, the work performed has been mainly devoted to setting up the software tools (in particular the McCad interface software) to perform the neutronic analysis of the integrated TA model, that will be carried out in the next phase of the project. Calculations of the power deposition of Deuteron beams in the Lithium jet, which is needed for thermo-hydraulics calculations, have been also performed. In the present document, a detailed description of all the above mentioned activities is reported. Moreover, a rough cost evaluation of the TA system with bayonet Backplate is also given

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

Modeling of a confinement bypass accident with CONSEN, a fast-running code for safety analyses in fusion reactors

The CONSEN (CONServation of ENergy) code is a fast running code to simulate thermal-hydraulic transients, specifically developed for fusion reactors. In order to demonstrate CONSEN capabilities, the paper deals with the accident analysis of the magnet induced confinement bypass for ITER design 1996. During a plasma pulse, a poloidal field magnet experiences an over-voltage condition or an electrical insulation fault that results in two intense electrical arcs. It is assumed that this event produces two one square meters ruptures, resulting in a pathway that connects the interior of the vacuum vessel to the cryostat air space room. The rupture results also in a break of a single cooling channel within the wall of the vacuum vessel and a breach of the magnet cooling line, causing the blow down of a steam/water mixture in the vacuum vessel and in the cryostat and the release of 4 K helium into the cryostat. In the meantime, all the magnet coils are discharged through the magnet protection system actuation. This postulated event creates the simultaneous failure of two radioactive confinement barrier and it envelopes all type of smaller LOCAs into the cryostat. Ice formation on the cryogenic walls is also involved. The accident has been simulated with the CONSEN code up to 32 h. The accident evolution and the phenomena involved are discussed in the paper and the results are compared with available results obtained using the MELCOR code. © 2013 Published by Elsevier B.V

Design basis accident analysis for the Ignitor experiment

A safety analysis study has been applied to the Ignitor machine. The main initiating events have been identified, and accident sequences have been studied. A deterministic assessment of the main accidental sequences has been performed. The consequences of the radioactive environmental releases have been assessed by means of a population dose code. This paper analyzes the deterministic consequences of two accidental sequences, serving as the "design basis accidents" because of the extent of radioactive release involved, either outside or inside the building. The two sequences with higher releases have been considered. The deterministic analysis has achieved the following results: the Ignitor machine, both during routine operation and accidental sequences, presents a negligible environmental impact and radiological risk. © 2015 Elsevier B.V. All rights reserved

Experimental and numerical analysis of the air inflow technique for dust removal from the vacuum vessel of a tokamak machine

In fusion facilities, the dust production inside the plasma chamber is a concern from the viewpoint of both machine performance and safety. To the purpose of a correct handling of the experimental devices the problem of its removal must be properly solved. This work deals with the experiments carried out in the STARDUST facility by using as dust removal technique an air inflow into the volume representing the vacuum vessel. The goal was to evaluate the effectiveness of such an approach, less invasive as compared to all the others so far. These experiments, performed by using characterized carbon, tungsten and stainless steel dusts, show that the mobilization capability of the air inflow is between few percent and 100%, mainly depending on dust type of and deposit shape. The capture efficiency in a filter reached a maximum of about 7.5% in the STARDUST geometrical configuration. In conclusion, this simple and clean (from the radioactive point of view) removing technique needs particular care to be more effective and is not the perfect solution due to its low efficiency in the collection of removed powder in proper surfaces (i.e., filters). Nevertheless improvements are possible and worthwhile. © 2007 Elsevier B.V. All rights reserved

Large eddy simulation of Loss of Vacuum Accident in STARDUST facility

The development of computational fluid dynamic (CFD) models of air ingress into the vacuum vessel (VV) represents an important issue concerning the safety analysis of nuclear fusion devices, in particular in the field of dust mobilization. The present work deals with the large eddy simulations (LES) of fluid dynamic fields during a vessel filling at near vacuum conditions to support the safety study of Loss of Vacuum Accidents (LOVA) events triggered by air income. The model's results are compared to the experimental data provided by STARDUST facility at different pressurization rates (100 Pa/s, 300 Pa/s and 500 Pa/s). Simulation's results compare favorably with experimental data, demonstrating the possibility of implementing LES in large vacuum systems as tokamaks. © 2013 Elsevier B.V

ICE layer growth on a cryogenic surface in a fusion reactor during a loss of water event

The design of fusion devices often includes water as primary coolant of the first wall/blanket system and a potential accident scenario is the steam/water injection from the primary circuit into the magnets' cryogenic chamber (cryostat). An important question to be answered for the above scenario is whether the pressure suppression created by the cryogenic surfaces is sufficient to prevent cryostat damage. The computer codes used for the assessment of ITER safety were validated in the past years against the EVITA (Experimental Vacuum Ingress Test Apparatus) experiment at CEA-Cadarache, which was designed for the simulation of the physical phenomena occurring during a coolant ingress into the cryostat, namely ice formation on a cryogenic structure, heat transfer coefficient between walls and fluid, flashing, two-phase critical flow. The paper presents the results obtained by the CONSEN computer program for seven post-test calculations of the EVITA facility relating to the cryogenic experiments carried out in 2004 and 2005, in which the kinetics of the ice layer formation was analysed. The comparison with the experimental data has been performed and the main agreements and differences are remarked. © 2014 Elsevier Ltd

Analysis of an ex-vessel break in the ITER divertor cooling loop

In the present work the integrated ECART code, developed for severe accident analysis in LWRs, is applied on the analysis of a large ex-vessel break in the divertor cooling loop of the international thermonuclear experimental reactor (ITER). A comparison of the ECART results with those obtained by Studsvik Nuclear AB (S), utilizing the MELCOR code, was also performed in the general framework of the quality assurance program for the ITER accident analyses. This comparison gives a good agreement in the results, both for thermal-hydraulics and the environmental radioactive releases. Mainly these analyses, from the point of view of the ITER safety, confirm that the accidental overpressure inside the vacuum vessel and the Tokamak cooling water system (TWCS) Vault is always well below the design limits and that the radioactive releases are adequately confined below the ITER guidelines. © 2006 Elsevier B.V. All rights reserved