Development of a thermal-hydraulic model for the EU-DEMO Tokamak building and LOCA simulation

The EU-DEMO must demonstrate the possibility of generating electricity through nuclear fusion reactions. Moreover, it must denote the necessary technologies to control a powerful plasma with adequate availability and to meet the safety requirements for plant licensing. However, the extensive radioactive materials inventory, the complexity of the plant, and the presence of massive energy sources require a rigorous safety approach to fully realize fusion power’s environmental advantages. The Tokamak building barrier design must address two main issues: radioactive mass transport hazards and energy-related or pressure/vacuum hazards. Safety studies are performed in the frame of the EUROfusion Safety And Environment (SAE) work package to support design improvement and evaluate the thermal-hydraulic behavior of confinement building environments during accident conditions in addition to source term mobilization. This paper focuses on developing a thermal-hydraulic model of the EU-DEMO Tokamak building. A preliminary model of the heat ventilation and air conditioning system and vent detritiation system is developed. A loss-of-coolant accident is studied by investigating the Tokamak building pressurization, source term mobilization, and release. Different nodalizations were compared, highlighting their effects on source term estimation. Results suggest that the building design should be improved to maintain the pressure below safety limits; some mitigative systems are preliminarily investigated for this purpose

Identification of accident sequences for the DEMO plant

Safety studies are performed in the frame of the conceptual design studies for the European Demonstration Fusion Power Plant (DEMO) to assess the safety and environmental impact of design options. An exhaustive set of reference accident sequences are defined in order to evaluate plant response in the most challenging events and compliance with safety requirements.The Functional Failure Mode and Effect Analysis (FFMEA) has been chosen as analytical tool, as it is a suitable methodology to define possible accident initiators when insufficient design detail is available to allow for more specific evaluation at component level. The main process, safety and protection functions related to the DEMO plant are defined through a functional breakdown structure (FBS). Then, an exhaustive set of high level accident initiators is defined referring to loss of functions, rather than to specific failures of systems and components, overcoming the lack of detailed design information. Nonetheless reference to systems or main components is always highlighted, as much as possible, in order to point out causes and safety consequences. Through the FFMEA a complete list of postulated initiating events (PIEs) is selected as the most representative events in terms of challenging conditions for the plant safety. All the four blanket concepts of the European DEMO reactor have been analysed

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 hydraulics 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 hours. 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

ECART analysis of the STARDUST dust resuspension tests with an obstacle presence

The activated/toxic dust resuspension inside the vacuum vessel of future fusion devices as ITER or DEMO is a safety issue of main concern. In case of a LOVA or a LOCA, dusts produced during the normal and off-normal conditions can be released inside the tokamak building or towards the external environment. These accidents are not expected during the whole lifetime of the ITER machine, though in the past they were considered in the ITER Generic Site Safety Report with a conservative assumption for the lack of reliable resuspension models in the employed safety codes. To relax this strong assumption and validate resuspension models in fusion like conditions, different experimental campaigns in the STARDUST facility were performed at the ENEA laboratories in Frascati (Rome). In the first experimental campaign (2004), the resuspension of Tungsten (W), Carbon (C) and Stainless Steel (SS) dusts was investigated in an “empty tank” configuration, while the resuspension of the same dust types in presence of an internal obstacle was studied in the second campaign (2005). The obtained experimental results stressed that only a minor fraction of dusts is effectively resuspended. In the present work, focalized on the ECART code validation for the safety analysis of future fusion installations, a further step in the assessment of the semi-empirical “force balance” resuspension model implemented in the ECART code against the data obtained during the second experimental campaign (tank with an inner obstacle) is performed. The code predictions are quite in agreement with these STARDUST experimental data, and three charts (one for each dust type) were elaborated to predict the resuspension magnitude basing on the flow velocity on the structure where the dusts are initially collected

Parametric explorative study of DEMO galleries pressurization in case of ex-vessel LOCA

Abstract Radioactive toxins confinement is a main safety function for fusion power plants, hence the importance of confinement design parameters optimization. Moving from this perspective, two Loss of Coolant Accidents (LOCAs) were analyzed: loss of water from Vacuum Vessel (VV) cooling loop and loss of cryogenic helium from Toroidal Field (TF) cooling loop. In fact these two LOCAs may result into galleries pressurization possibly jeopardizing this second confinement boundary for radioactive toxins. Publicly available ITER data (2000 baseline design) have been scaled to DEMO. A set of sensitivity simulation analyses are performed on main variables (coolant inventories, enthalpy, rooms volume, etc.) in order to derive resulting galleries pressure and temperature conditions. As first design feedback to keep gallery pressure below an assumed design pressure of 120 kPa, the VV H2O LOCA requires pressure reduction (e.g. increase expansion volume, inventory partitioning, sprinkler), while TF He LOCA requires releasable inventory to be limited at about 4.4 tons and Cryo-systems designed against Common Mode Failure (e.g. seismic and fire, quench valve failure)

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

ECART analysis of the STARDUST dust resuspension tests with an obstacle presence

To validate dust resuspension models in fusion like conditions, different experimental campaigns in the STARDUST facility were performed at ENEA laboratories Frascati (I). In the first campaign, resuspension of Tungsten (W), Carbon (C) and Stainless Steel (SS) dusts was investigated in an “empty tank” configuration, while resuspension of the same dust types in presence of an internal obstacle was studied in the second campaign. In this work, focalized on the ECART code validation, the assessment of the code “force balance” resuspension model against the data obtained during the second campaign is performed. ECART was able to provide a good qualitative and quantitative description of the resuspension phenomena so a further step in its validation has been achieved. Further improvements for the ECART resuspension model have been also highlighted, as the necessity to treat “particle clusters” and their aggregate movements

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