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This report introduces a current status of the continuous tritium recovery test campaign of PbLi droplets in vacuum, using vacuum sieve tray (VST). The campaign aims to verify the viability of VST method at a prototype design level. The following verification are to be performed. 1) Verify the steady state extraction efficiency of tritium using a 0.6 mm nozzle. The target is greater than 80%. 2) Verify the mutual interference effects on efficiency degradation by multiple droplets by comparing 1, 4, 7, and 19 nozzles. 3) Verify the reliability of VST in continuous operation. At least 24 h of non-stop operation without efficiency degradation is the primary goal, which can be extended up to 48 h. A dedicated extraction setup is to be installed March 2019 into the Oroshhi-2 PbLi experimental loop at the National Institute for Fusion Science (NIFS) Toki-Japan, followed by a stand-alone test with a short flowing period at Kyoto University which is already finished. Deuterium (D2), instead of tritium, is used for the experiment. Extraction efficiency is calculated by comparing the D2 concentration before-drop and after-drop. The amount of released D2 is also measured by a quadrupole mass spectrometer (QMS) and verified by the concentration difference. The PbLi temperature is between 375 ℃ and 450 ℃, and the flow velocity at the nozzle is between 1.5 m s−1 and 4.5 m s−1. The final report is scheduled to be completed by the end of 2019.

The feasibility of liquid metal atomization in vacuum is studied by analysis and verified by experiment. Simultaneous tritium and heat recovery in vacuum from liquid lithium–lead (PbLi) has been previously proposed, where hot and tritium–rich PbLi is transformed into small droplets in vacuum, with the tritium then released by advection and the heat radiated away. Both mechanisms aimed to reduce the tritium permeation and membrane corrosion. The key hurdle is the atomization of liquid metal in vacuum by a large diameter nozzle. Conventional sprays function in air or other gases but not in vacuum. Therefore, an instability is applied that occurs at the velocity inflection in a parallel flow. The velocity inflection is generated by superimposing rotation on a liquid jet with a dedicated swirl nozzle. This can function in vacuum. An experiment using a low–temperature melting alloy (GaInSn) is performed in vacuum, using nozzle diameters of 0.7 mm, swirl angles 67°, and flow velocities of 3 and 5 ms−1. The instability that arises when using GaInSn at room temperature resembles that of PbLi at 400–500 °C, can be used as a proxy. The observed mean droplet Feret diameter was 0.31 mm at 3 m/s jet velocity and 0.23 mm at 5 m/s. These results support the feasibility of small droplet formation from a large diameter nozzle in vacuum. Further experiment must be performed to fully verify the droplet formation and proposed tritium and heat extraction method.

Any fusion power reactors such as “DEMO” that achieves tritium self-sufficiency with breeding blankets can produce tritium by DD reaction followed by exponential breeding in the blanket within reasonable total operation period. The present study further suggests that realistic Power Ascension Tests (PAT) of DEMO can produce its tritium to be needed in the series of tests by its own program until reaching steady state full power operation, with no external supply or additional operation costs. Closed tritium fuel plant was described by a system dynamics model, and analyzed considering realistic PATs of DEMO, that will be mainly pulsed DD and low concentration DT. Typical PATs require years of operation from zero power criticality to full power, with pulsed power output and long dwell time between them. Output power is gradually increased in PATs to check the functions of reactor systems and components. In the case of fusion DEMO, zero power criticality corresponds to DD operation. While plasma may be fired in pulses, tritium plant is continuously operated to recover all the tritium produced by the DD and low DT burn. Depending on the different time constant of tritium retention in components, tritium is transferred by deuterium purge, and high concentration tritium is finally collected in the storage, to be available for the next tests at virtually no additional cost. Realistic “initial loading tritium” will be 100 g for commissioning of tritium plant far prior to the plasma operation.

Quantitative feasibility analysis of the tritium recovery efficiency from multiple columns of liquid PbLi droplets was conducted.Then a case study based on the HCLL specification was performed. Main concern was whether the experimentally obtained recovery efficiency from a column of droplets is applicable for the efficiency estimation from the multiple columns of droplets without any mutual degrading effects. To maintaining a safe side assumption, the tritium once released and reabsorbed on another droplet was considered to be not re-emitted while falling. By the analogy with the thermal radiation theory, the view factor which expresses the intersection ratio of radiation on another surface was applied for the estimation.The dependences on nozzle design parameters, such as nozzle pitch, number of nozzles, chamber wall clearance, and exhaust port design, were investigated. Case study results suggest that, by choosing well-suited parameters approximately 40% to 60% of the single column recovery effi...

This paper describes the quantitative analysis for the design of a tritium extraction system that uses liquid PbLi droplets in vacuum (Vacuum Sieve Tray, VST), for application to the ITER helium-cooled lithium lead (HCLL) test blanket system (TBS). The parametric dependences of tritium extraction efficiency from the main geometrical features such as initial droplet velocity, nozzle head height, nozzle diameter, and flow rate are discussed. With nozzle diameters between 0.4 and 0.6 mm, extraction efficiency is estimated from 0.77 to 0.96 at the falling height of 0.5 m, with flow rate between 0.2 and 1.0 kg/s. The device has a height of 1.6 m, within the external dimensions of the HCLL Test Blanket Module (TBM), and no additional pumping power is required. The attained results are considered attractive not only for ITER, but also in view of the application of the VST concept as a candidate tritium extraction system for the European Union's demonstration fusion reactor (DEMO). The extraction efficiency of a single droplet column, which is the basis of the design analysis presented, has been validated experimentally with hydrogen. However, further experiments are required on an integrated system with size relevant to the proposed HCLL-TBS design to validate system-level effects, particularly regarding the desorption process in an array of multiple droplets.

The emissivity of a liquid lithium-lead (PbLi) droplet falling in a vacuum is theoretically analyzed and verified by infrared thermal imaging. The authors propose the concept of simultaneous recovery of heat and tritium from falling PbLi droplets in a vacuum. Permeation reduction in a heat exchanger is expected to occur by radiation heat recovery in a vacuum. The radiation power depends on the droplet emissivity, which remains unreported to date. An experimental measurement is performed using 0.8 mm diameter droplets at 400℃, falling with a velocity of 3 m s−1. The emissivity is calculated from the infrared normal temperature of a droplet, the true temperature measured using a thermocouple, and the wall temperature. Three different H2 gas pressures (instead of tritium) are applied (10-3, 10°, and 102 Pa) in a vacuum chamber to verify the effect of incident hydrogen molecules on the emissivity. The obtained emissivities are 0.35 ± 0.06, 0.33 ± 0.06, and 0.36 ± 0.06, respectively, which are higher than those of the precious metals. Deviations include random measurement errors and systematic errors. The incident H2 molecules have no effect on the emissivity. The droplet size effects and the dwell period elongation are to be considered to realize the concept of simultaneous heat and tritium recovery.

This study proposes a probability of the evaporated gas that agitates a growing instability wave in a thin liquid film first wall. The liquid first wall was considered to be in vacuum and the effect of the ambient gas was neglected but the evaporated gas by the high energy fluxes is a probable cause of unstable wave agitation. The criterion is approximately expressed by the density ratio ( Q 2 ) and the Weber number ( We ) as Q 2 × We 0.5 ≈ 5 × 10 −4 . Performed indirect experimental supported this criterion. For a case study of liquid Pb-17Li film with a velocity of 10 m/s, the evaporated gas pressure must be below 6.2 × 10 3 Pa to maintain stable conditions. By recent study, this pressure is generated at 1600 K temperature and it is believed to be attainable by the energy fluxes on the first wall. This result is so far not confirmed so the full verification by experimental is to be performed.

Release of deuterium from falling droplets of Pb-17Li in vacuum is experimentally studied. By comparing different diameter nozzle data each other, the effect of ambiguous solution is eliminated, and reliable result is attained. The amount of deuterium that is dissolved into Pb-17Li, followed by the release from the liquid droplets in vacuum, is measured with four different diameter nozzles ranging from 0.4 mm-1.0 mm under an initial velocity of 3.0 m/s and four temperatures between 375 °C and 450 °C. The resultant mass transport, represented by quasi-dispersion-coefficient is 3.4 × 10-7 [m2/s], which is approximately two orders of magnitude faster than previous studies under static condition. It also revealed different temperature dependency. Cyclic deformation of the sphere shape is observed with a high speed movie camera. These results show the falling droplets of liquid Pb-17Li in vacuum follow the mass transfer mechanism under convection prior domain by self- excited oscillation. This result suggests that the tritium recovery method from a breeding liquid Pb-17Li blanket is viable when using multiple nozzles in vacuum for the extraction.

In the design of the fusion blanket, it is important to gather generated tritium as quick as possible and supply them to a fuel supply system for keeping fuel cycle and reducing tritium inventories in the fusion reactor at the same time.In the advanced blanket concept which uses Lithium-Lead (LiPb) as the working fluid for heat removal, neutron shielding and tritium breeding, collection of generated tritium is thought not to be difficult as the solubility of hydrogen into the LiPb is small enough. But examination and design of these collecting systems was not fully studied.In this paper, we made the conceptual design of the tritium collecting device using vacuum sieve tray, and studied formation process of LiPb droplets by making a simple experimental device. It was found that droplets of about 0.9-mm radius were formed at 8~12-cm distance from nozzle when LiPb discharges from the nozzle with 1-mm diameter hole at pressure of 2.5×104 Pa. Using this value, it is estimated that the tritium collectin...

The feasibility of deuterium mass transport prediction from falling droplets of Pb-17Li was verified. This prediction is one of key techniques of the engineering design of tritium extraction device for the fusion reactor. The mass-transfer-coefficient, deduced on the surface-stretch-model was applied. As the experimental results, deuterium mass transport in the falling droplets from four different size nozzles, at four temperature conditions between 375 °C and 450 °C, performed by the authors, were compared. Resultant Sherwood number was between 494 and 598, and explained the experimental result of the two orders of magnitudes differences with the reported diffusion in static condition. Though, the ratio of theory and experiment still remained between 1.8 and 2.3. Simple boundary condition, not considering the number of oscillation, wide range of reported diffusivity value are considered to be main reasons of the deviation. The analysis model including these factors is to improve prediction accuracy. This result is expected to contribute to a preliminary design of a tritium extraction device.