Loss Of Vacuum Accident Research Articles

Analyses of loss of vacuum accident (LOVA) in ITER

A loss of vacuum accident (LOVA) is a unique event in fusion reactors. A reference event for ITER was postulated which was a failure of a penetration line (0.02 m 2 cross-section) into a room with stack ventilation. Behavior of ingress air and environmental release of the inventory were calculated by an accident analysis code (MELCOR). No in-vessel component cooling was assumed because of a non-safety system and 1350 g-T as tritium (HTO) and 30 kg as tokamak dust (tungsten) were set in the vacuum vessel (VV) as the initial mobile inventory. Operation of the maintenance detritiation system (MDS) after 1 h was credited to limit the release. The analytical results showed that the environmental release of tritium (19 g-T) from the stack was a factor of 5 below the accidental release limit for ITER, and the release of dust (21 g) from the stack is a factor of 25 below the release limit. To investigate the ultimate safety margin, a hypothetical event was also analyzed which was a failure of a penetration line (0.2 m 2 cross-section) into a room with ground level ventilation. Since the best estimate analysis assumed in-vessel cooling, which resulted in cooling down the walls, the mobile tritium inventory was reduced to 390 g-T. The large break size caused fast pressurization of the VV and thus the whole dust inventory (110 kg) was expected to be mobile. The ground level release was less than half of the no-evacuation limit under conservative weather conditions.

Quantitative Measurement of Dust Mobilization in a Vacuum Vessel of a Fusion Reactor during the Loss-of-Vacuum-Accident Event

The dust mobilization in the vacuum vessel under the Loss-of-Vacuum-Accident (LOVA) event was measured quantitatively using the preliminary LOVA apparatus. The particle size distributions in the mobilized dust were characterized and analyzed using a high performance optical-microscope and image analysis software. It was found that around 10% of the initial dust weight was transported upward inside the VV when the breach located at the roof of the VV and an incoming flow from the outside through the breach directly hit the dust. On the other hand, the transported dust weight was less than 1% of the initial dust weight when the breach position was at the side wall of the VV and an incoming flow from the outside indirectly hit the dust. The relationship between the dust mobilization and breach position was clarified from the present experimental results.

B135 真空破断事故時に浮力流に同伴されるダストの移行特性

Dust mobilization in a vacuum vessel (VV) of a fusion reactor during a loss-of-vacuum accident (LOVA) was studied numerically and experimentally from a viewpoint of the fusion safety research. After the LOVA occurred, air flows from the outside of the VV through a breach into the inside and the activated dust is blown up from the walls inside the VV, and then it can be considered that the dust is conveyed due to a buoyancy flow to the outside of the VV through the breach. The velocity, pressure and temperature distributions were obtained by the present numerical analyses and the dust mobilization behavior was predicted quantitatively. In addition, the dust transport characteristics from the inside of the VV through the breach to the outside due to the buoyancy effect was clarified by the visualization experiments.

236 相関法及びレーザーホログラフィー法による置換流及び移動微粒子の定量評価

Exchange flows through breaches of a vacuum vessel (VV) in a fusion reactor under the Loss-of-Vacuum-Accident (LOVA) conditions were observed to obtain velocity distributions and clarify those flow patterns. The exchange flows were visualized by smoke and those images taken by a CCD camera were processed by the correlation method, and velocity distributions in the exchange flows were calculated.In addition, moving small particles conveyed from the inside of the VV to the outside by the exchange flows under the LOVA conditions were visualized by the laser holography method and each particle size were recognized from those image data, and particle size distributions were calculated.It was found that the present flow visualization techniques would be an useful procedure for quantitative evaluation of the exchange flow patterns and particle size distributions.

Experiments on Exchange Flow Through Two Breaches During Loss of Vacuum Accident in Nuclear Fusion Experimental Reactor

When a loss of vacuum accident (LOVA) would occur in a nuclear fusion experimental reactor, a buoyancy-driven exchange flow takes place through breaches of a vacuum vessel. To concern the safety analysis, experiments were carried out to examine the effects of the enclosure configuration and the breach arrangement on the exchange flow rate at the vertical and/or horizontal breaches in an annular enclosure. Comparing the experimental results with the existing correlations of the exchange flow rate, the correlation based on the Bernoulli equation was found to provide the conservative results for small breaches in aspect of the release of radioactive materials from the vacuum vessel.

LOCA, LOFA and LOVA analyses pertaining to NET/ITER safety design guidance

The analyses presented pertain to loss of coolant accidents (LOCA), loss of coolant flow accidents (LOFA) and loss of vacuum accidents (LOVA). These types of accidents may jeopardise components and plasma vessel integrity and cause radioactivity mobilisation. The analyses reviewed have been performed under the assumption that the plasma facing components are protected by a carbon based armour. Accidental temperatures and pressure transients are quantified, the possibility of reaction products combustion is investigated and worst case accidental public doses are assessed. On this basis, design recommendations are given and design features such as low plasma facing components armour temperatures (on almost the entire surface) and inert gas adjacent to the vacuum vessel have been implemented.