Masana Nishikawa

Development of general methodology of safety analysis and evaluation for fusion energy systems (GEM-SAFE)☆

Abstract A synthesized methodology of safety analysis and evaluation for fusion systems has been developed to concretely assess the adaptability of fusion systems to the environment from the earliest stages of system development. The methodology objective was to summarize both the safety design requirements and achieve rational safety in fusion systems. The framework of the methodology was constructed to clarify its logical consistency. The safety characteristics of fusion systems were then investigated in detail paying attention primarily to potential hazards, so that a fusion system was identified as a distributed system in regard to energy sources and radioactive materials. Based on this recognition, a General Descriptive Model (GDM) of a fusion system has been constructed which is a highly generalized and integrated expression. The safety ensuring principle, on the other hand, set up items to be protected and categorized events for a fusion system. The development of the safety ensuring principle was a key to the practical performance of safety analysis and its evaluation in a general fusion system. Finally, by using the Function-Based Safety Analysis (FBSA) on the GDM, abnormal events were summarized into 16 typical events, according to the safety ensuring principle. Consequently, 20 design based events for the general fusion system were selected to envelope all credible abnormal events.

Thermomechanical behavior of graphite and coating materials subjected to a high heat flux

This study has been performed for the development of limiter and divertor plates. Their thermal and thermomechanical behavior were examined in heat load experiments with an electron beam facility, and were compared with analysis results. Graphite was proven to have a high thermal shock resistance. Its erosion thickness and thermal contact conductance were also studied. Copper alloy with coating and graphite brazed to metal were tested, and their feasibility was demonstrated for use as limiter and divertor plates of an advanced-type concept.

High density, single-null poloidal divertor results in doublet III☆

Abstract Experimental results of divertor characteristics on impurity suppression, helium ash exhaust and remote radiative cooling are reviewed. In a throatless (open) divertor such as the single-null poloidal divertor in Doublet III, a dense and thick scrape-off layer provides strong particle shielding in high density divertor discharges, which results in a high neutral pressure and radiative cooling of the divertor even in an impurity-free condition. The feasibility of employing an open divertor in a reactor-grade device like INTOR is discussed. A preliminary result of divertor experiments with a 1.6 MW neutral-beam injection is presented.

Bubble formation on silicon by helium ion bombardment

A silicon crystal sample was exposed to helium ions in an ECR ion irradiation apparatus for various substrate temperatures and ion fluences. The bubble formation and helium retention properties of the silicon were investigated. High density small bubbles and low density large bubbles were observed after the irradiations at RT and 573 K, respectively. The irradiated sample was subjected to the thermal desorption spectroscopy. After the helium ion irradiation and the heating up to 1273 K, the surface morphology of silicon was largely changed due to the rupture of bubbles. In the desorption spectrum of helium, the sharp peak corresponding to the rupture was observed at 800 K. In the case of irradiation at 573 K, the desorption peak became very broad, compared with the case at RT. The amount of retained helium at 573 K was about a half of that at RT.

Application of the general methodology of safety analysis and evaluation for fusion energy systems (GEMSAFE) to the FER design

Abstract A safety analysis for the FER has been carried out along the framework of the GEMSAFE. First, the safety characteristics of the FER were investigated so that the RI and energy sources were identified and the General Descriptive Model (GDM) was defined to be consistent with the conceptual design of the FER. Second, the radioactive materials and the boundaries of subsystems were classified based on the definition of the proposed event categorization. Finally the Function-Based Safety Analysis (FBSA) has been performed to select 19 Design Basis Events (DBEs), i.e., seven Category-1 events, eight Category-2 events and four Category-3 events. These DBEs not only elucidate the safety requirements to be satisfied with the event categorization, but also suggest crucial items for the safety evaluation of the FER. It is noted that, for establishing the rational safety assurance for the FER, future studies should be performed upon (1) spectra of event propagations which may occur following plasma disruption and coolant inleakage into the vacuum area, and (2) technological feasibility of safety features which can prevent or mitigate these event propagations.

Graphite limiter damage due to runaway electrons and abrupt Ip termination in doublet III

Abstract Two kinds of plasma-related damage, spotty damage and wide area damage, have been photographically recognized on the TiC-coated surface of the new graphite primary limiter in Doublet III. TV and infrared-camera observations revealed the relationship of these damages to the plasma operations. The spotty damage was generated by a single runaway discharge. The runaway electron energy was deposited not only on the surface but also penetrated into the bulk graphite. Thermal analysis showed that runaway electron of 5–6 MeV penetrates into the limiter to approximately 1.5–2.0 cm. Wide area damage was caused by one or more successive discharges which exhibited rapid termination of the plasma current. In this case, the electron energy was deposited only on the limiter surface.

Study on divertor materials and heat load experiments with electron beam facility

Typical heat load conditions on a divertor plate in a fusion reactor are 2 MW/m 2 at normal operation and 500 MW/m 2 × 5 ms at plasma disruption. Test Materials I and II were prepared for basic material tests, corresponding to the followings, (1) Design I: tube structure of copper alloy with or without protection layer, and (2) Design II: graphite tiles brazed to a metal tube. Test Material I is a plate of a OMC Cu-Cr-Zr alloy. The molybdenum coating (∼500 μm) was applied by plasma spray. Test Material II is graphite brazed to a molybdenum base plate. Heat load experiments were performed on the test materials for normal operation or disruption simulation by using a 120 kW electron beam facility. Thermomechanical and erosion analyses were also performed. This basic study showed that these two types of materials have good characteristics at the typical heat load conditions.

Preparation of silicon oxide with bubble layer by helium plasma irradiation followed by oxidation

Silicon oxide bubble layer on silicon wafer was produced by helium ion irradiation followed by oxygen plasma irradiation or thermal oxidation. By the helium ion irradiation, a bubble layer was formed. The thickness and porosity of bubble layer depended on the irradiation condition. By the oxygen plasma irradiation to the bubble layer, only the surface of the bubble layer was oxidized. By the thermal oxidation, an entire region of the bubble layer was oxidized. In the latter case, the thickness of the silicon oxide bubble layer was as high as approximately 200 nm in the present experiment. Estimated dielectric constant of this layer was approximately 3.