Helium Leak Research Articles

Key technologies of manufacturing and testing for the ITER blanket shield block

The SB is one of the main in-vessel components manufactured with SS316L(N)-IG to play a major role in neutron shielding, to support the FW panels and to supply the FW panel with cooling water. Cooling channels are located inside of the SB to remove the neutron heat deposition while keeping the structure temperature to acceptable levels. The water coolant is routed first through the plasma-facing first wall panel which is attached to the SB and then through the SB. A number of deep slits are introduced into the SB to disrupt eddy current loops and reduce the Electro-Magnetic (EM) loads on the support system and VV. Water headers are machined on the side of the module with 7–10 mm welded cover plates. A Procurement Arrangements (PAs) for the manufacturing and testing of the SBs have been signed with Korea (50 %) and China (50 %).As part of the PA in Korean Domestic Agency (KODA), Process Qualification (PQ) to ensure the manufacturing and testing ability of the SB was completed successfully through manufacture of a Full Scale Prototype (FSP) as an essential step before starting series production. This paper reports key technologies, such as machining, welding, NDE and hot helium leak testing, of manufacturing and testing for the ITER SB series production in Korea toward successful completion of the missions.

Experimental investigation of rectangular nozzle aspect ratio effects on high-pressure helium leakage characteristics

High-pressure hydrogen leakage is one of the bottlenecks restricting the development of the hydrogen energy industry. In this study, a high-pressure helium leakage experimental system with stable pressure was built to research the influence of aspect ratio on concentration decay rate and mass flow rate. Similar to the circular nozzle, the centerline helium concentration of the rectangular jet is also inversely proportional to the distance from the nozzle. For the rectangular nozzles with aspect ratio of 1–4, the helium concentration decay rate is consistent with the circular nozzle when the leakage pressure is lower than 6.5 MPa. However, the decay rate is obviously larger for rectangular nozzles with aspect ratio of 10 when the leakage pressure is greater than 5.0 MPa. The aspect ratio has a greater influence on the flow coefficient, which increases with the aspect ratio under the same leakage pressure. To comprehensively consider the effects of nozzle size and shape, a weighted characteristic diameter of equivalent diameter and rectangular width, dc=αde+1−αdw, was proposed in this study. The results indicate that the equivalent diameter has greater impact on the centerline concentration decay rate, while the width of nozzles has substantial influence on the flow coefficient.

Evaluation of green laser source additive manufacturing technology for accelerator applications with ultra-high vacuum requirements

Additive Manufacturing (AM) offers different benefits such as efficient material usage, reduced production time and design freedom. Moreover, with continuous technological developments, AM expands in versatility and different material usage capabilities. Recently new energy sources have been developed for AM – green wavelength lasers, which provide better energy absorption for pure copper. Due to high thermal and electrical conductivity of copper, this novel AM technology is highly promising for various industries, particularly, there is a huge interest to use it for accelerator applications. In particular, these AM produced accelerator components should reach the associated Ultra High Vacuum (UHV) requirements. In this study, vacuum membranes of pure copper were produced by AM using a green laser source, in different thicknesses and built angles. Furthermore, a vacuum membrane helium leak tightness test was performed at room temperature by using a high-sensitivity mass spectrometer. Comparison of these test results was performed with previously established results. Through this study, novel knowledge and initial results are provided for green laser source AM technology usage for applications for UHV accelerator components.

Helium Leak Rate Measurements of Flight-like Mars 2020 Sample Tubes.

The sample tubes on board NASA's Perseverance rover are designed to contain rocks, regolith, and atmospheric gases and are hermetically sealed on the surface of Mars to minimize sample loss, alteration, and contamination. Following a robust testing program during mission development, it was determined that the helium (He) leak rates of flight-like sample tubes sealed under a range of conditions were typically no greater than ∼10-10 standard cubic centimeters per second (scc/s); leak rates below this value could not be measured since this is the detection limit of commercially available He leak detectors. This limit was adequate to meet mission requirements. However, some scientific objectives could be compromised by sample tube leak rates even below 10-10 scc/s, thus motivating a more sensitive technique for establishing leak rates. This study investigated He leak rates on six flight-like sample tubes using a static mode mass spectrometer. Room temperature He leak rates of the six sample tubes ranged from ∼8.8 × 10-17 to ∼4.6 × 10-14 scc/s. One sample tube was analyzed at eight different temperatures, ranging from -51°C to +42°C, and yielded He leak rates correlated with temperature that varied from ∼1.7 × 10-15 to ∼1.4 × 10-13 scc/s, respectively. Our results confirm and extend previous findings demonstrating that the Mars 2020 sample tube seals are likely to be very leak-tight, with leak rates <10-13 scc/s. These leak rates are sufficiently low that the impact of gas egress or ingress is expected to be negligible.

High -yield Fabrication of Thin Glass Interposers

Thin glass substrates with through vias (TGV) have excellent properties for a new generation of interposers. Glass has a large bulk resistance and low loss at high frequency (RF, mmWave). Physically, it is moisture insensitive, highly dimensionally stable, and has a thermal expansion coefficient that can be tailored to match silicon. The surface of as-received glass has roughness below 1 nm, like silicon wafers, and thus supports fine line/space patterning and advanced processes like damascene. A key focus of our current work involves metallizing thin glass interposers with TGVs, including both copper electroplating of TGVs and patterning of redistribution layers. We demonstrate void-free fill for vias ranging from in aspect ratio from 2:1 to 8:1, in a bottoms-up process like that used for through silicon vias. The vias are hermetic by helium leak detection, even after annealing in some cases to 450C. Since thin (≤ 200 micron) free-standing glass substrates are difficult to handle in current fabs, we discuss an advanced temporary bond approach using silicon or glass handles which enables high-yield processing. The bond is robust to temperatures exceeding 400 °C, and creates handle/glass stacks with total thickness variation (TTV) similar to that of a silicon wafer. The end result is a bonded pair that can be processed like a silicon wafer, with a simple mechanical debond to yield the processed thin glass. We discuss the bond and debond processes, showing the ability to control the bond strength. Data on the bonded glass stack resistance to various processing steps will also be shown.

Microstructure, mechanical properties and hermeticity of W-Cu/HTCC brazed joints with surface metallization

WCu alloy and high temperature co-fired ceramic (HTCC) were metallized through electroless nickel plating, then one series of W-Cu/Ni/AgCu/Ni/HTCC joints were successfully brazed with AgCu filler. The microstructure, mechanical properties, and hermeticity of W-Cu/HTCC joints were systematically investigated, and the fracture paths of the joints were summarized. The results revealed that the interface microstructure of the joints consisted of Ni(Cu)-W, Ni3P, CuNi, Cu(s,s), and Ag(s,s) phases. As the brazing temperature increased, Ni atoms gradually diffused from the NiP layer into the filler layer and substrate. At 900 °C, the NiP layer dissolved, and Cu atoms from the WCu alloy diffused into the solder layer, leading to a sharp growth in joint thickness. The shear tests indicated that the promotion of Cu content in the filler layer effectively alleviated the residual stress in the joints, resulting in higher joint strength. The maximum shear strength of 73.57 MPa was reached when the joints brazed at 840 °C. The hermeticity measurement results showed that the joints brazed at 900 °C for 10 min exhibited exceptional hermeticity, with a helium leakage rate of 2.51 × 10−11 Pa·m3/s. This work provided a reference for the connection of metal packaging materials with high temperature co-fired ceramics.

Optimization of Wall-Conditioning Techniques on the MT-II Tokamak

The MT-II is a small spherical tokamak that is currently under construction at the Pakistan Tokamak Plasma Research Institute. Wall conditioning of the MT-II vacuum vessel (VV) is an essential step to achieve a good quality vacuum for plasma experiments. This study presents an overview of the wall-conditioning techniques implemented on the MT-II VV, including baking and glow discharge cleaning (GDC). Prior to wall conditioning, the system is checked via a helium leak test machine and residual gas analyzer (RGA) to identify and remove leaks. The VV walls are baked at ~180 °C to get rapid desorption of water vapors and other impurities. After the baking process, the partial pressure of most of the carbon- and oxygen-containing impurities is reduced. In particular, the partial pressure of water vapors is reduced by 93%. Consequently, the total leak and outgassing rate is significantly reduced. To further improve the vacuum condition in the vessel, hydrogen GDC is carried out. The fill hydrogen pressure and anode voltage are optimized to get a stable glow discharge. The RGA scan shows that GDC reduces the partial pressure of H2O, O2, and CO2 by 57%, 63%, and 51%, respectively. The results signify that baking and GDC are effective techniques for wall conditioning of the MT-II VV.

Procedure qualification and integration of the continuous external Rogowski diagnostics to the superconducting magnets of ITER tokamak

Continuous External Rogowski (CER) are the primary plasma current diagnostics for the ITER tokamak, as well as the only diagnostics built into the ITER Toroidal Field Coils (TF coils). The integration of CER diagnostics to the TF coils faced numerous technical challenges, such as: the integrated components have to withstand the maximum pressure foreseen during ITER TF coil resin impregnation (350 kPa), comply with the high vacuum requirements that the helium leak rate shall be <2.7 × 10−9 Pa-m3/s, and survive at cryogenic temperature. The CER diagnostics integration campaign turned out to be a marked success, thanks to a comprehensive pre-qualification program and well-coordinated collaborations among the participating parties. All four sets of CER diagnostics have so far been integrated to the respective TF coils. As a pioneering attempt of building plasma diagnostics into reactor-scale superconducting magnets, the potential implications of this venture to the diagnostics development for DEMO and future fusion reactors are discussed. The gained experiences and lessons learned from this venture are summarized at last.

Handling of high-volume High Purity Germanium gamma ray detector and repair with minimum resources – A case study

A High Purity Germanium (HPGe) detector is a high-resolution detector, used for gamma-ray spectroscopy. One such HPGe detector of our laboratory was showing poor resolution either due to malfunctioning of components or vacuum. This detector being a sophisticated instrument need to be operated in an environment with adequate cooling and humidity control. The optimum conditions such as maintaining liquid Nitrogen filling in Dewar at proper time and holding capacity of Dewar and humidity control environment could not be maintained due to lockdown conditions during the pandemic. This disrupted the functioning of the detector resulting in poor performance and poor resolution. One reason for such a behaviour is attributed to the condensation of moisture at the feed through points of High Voltage and failures of components such as preamplifier and Field Effect Transistor (FET). Transportation of the detector for repairs to France involves lengthy procedures, additional expenditure, custom clearance and safe transit. The possibility of performing the detector repair with the available facility on premise was explored. The repair involved breaking of vacuum in Clean room Facility, availability of Variac, Vacuum gauge, Dry Nitrogen, Helium leak detector, Liquid nitrogen, Temperature sensor with controller, Turbomolecular pump, Vacuum plunger etc. The diagnosis of cascading failure of components in the germanium detector and their rectification to restore its functionality with desired performance has been discussed in the present paper.

Indigenous Development of Epoxy Resin System for Cryogenic Services and Fusion Application

An epoxy resin system is used in a superconducting tokamak to insulate the conducting components, such as superconducting windings, cooling pipes, metal electrodes, and bonding and sealing of dissimilar material joints at cryogenic temperature. The main aim is to develop and fabricate the dissimilar material joints of metal and glass fiber reinforced plastic (GFRP) polymer in the form of cryogenic components for the superconducting fusion magnet. To bond and fabricate the dissimilar joints, the epoxy resin needs to have low viscosity, good adhesion, resistance to moisture, long usable life, and high toughness at low temperatures. A two-component-modified diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was formulated with a modified polyamine-based hardener. To increase the toughness and minimize the induced thermal stress at low temperatures, a silane coupling agent, gamma-aminopropyltrithoxysilane, was used for its superior bonding and fast curing process. The tensile strength examination test results were found to 85 MPa, as per the International Organization for Standardization standard ISO 527-2, and an interlaminar shear strength of 12 MPa was found, as per the American Society for Testing and Materials standard ASTM D5868 at 77 K, respectively. The mechanical performance enhancements at 77 K overcome the issue of cracks and helium leaks that develop at cryogenic temperatures, as reported. The dissimilar material joints fabricated using the epoxy resin in the form of a cryo component have been validated in machine with an acceptable helium leak tightness of 1.0E-08 mbar-l/s. In this work, we report on the development, mechanical, thermal, and electrical performance tests, the testing and failures of various epoxy resins systems used, and the cryo components at 300 and 77 K.

Design and Fabrication of a Test Rig to Determine Helium Permeability through Fabrics

The fabric of a Lighter-Than-Air (LTA) system experiences permeation of Helium over a period of time. Due to the small size of the molecules of Helium, it permeates through the intermolecular spaces of the fabric. This leakage increases with time due to the degradation of the fabric. LTA systems need to be deployed for long durations. The loss of Helium due to permeation reduces the relative lift of the LTA system. The unknown permeation rates thus lead to uncertainties in the deployment time of the system. This paper describes the design and fabrication of a test rig to determine the permeability of Helium through different fabrics. This method involves a permeation cell divided into upper and lower chambers arranged to hold a specimen fabric between them, with the provision of a continuous supply of Helium in the lower chamber. A Phoenix L300i Helium Leak Detector (HLD) is connected to the upper chamber to detect Helium and measure its concentration. The further objective of this research is to study the variation of helium permeability through the fabrics at constant temperature and input pressure with time.

Preliminary Experimental Quantification of Helium Leakages from Flanged Connections at HCPB TBS Operative Conditions

The HCPB TBS (Helium-Cooled Pebble Bed Test Blanket System) is one of the two European TBSs that will be installed and tested in the ITER reactor. The use of flanged connections in the Helium Coolant System and the Tritium Extraction System of the HCPB TBS would make the remote maintenance operations easier and faster. Therefore, investigating the helium leakage from flanges becomes a fundamental step toward the control of the tritium activity in the Port Cell, as the helium flow will contain a variable but not negligible amount of tritium. The first set of experiments on helium leakages from flanged connections is described in this paper. The experiments were performed in a HeFUS3 facility, an eight-shaped helium loop designed to work at HCPB-TBS-relevant conditions. The facility can provide a helium mass flow rate in the range of 0.27–1.4 kg/s and can reach a pressure as high as 80 bar and a temperature up to 530 °C. Two types of gaskets were tested in this campaign: a spiral-wound gasket and an oval ring joint. The gasket/flange assemblies are described in detail in this paper, together with the test section that hosts them and the performed commissioning tests. The tests were carried out at 500 °C and 80 bar. In these conditions, the leak rate from the flange with the oval ring joint resulted in being, on average, 1.42·10−6 mbar∙L/s, while the leak rate from the flange with the spiral-wound gasket resulted in being, on average, 3.73·10−3 mbar∙L/s.

Construction of hot helium leak test facility for ITER blanket shield block

The ITER Blanket Shield Block (SB) is one of the main in-vessel components that provides nuclear shielding and removes nuclear heating for the Vacuum Vessel (VV) and external vessel components. The SB, which contains internal cooling channels, is manufactured from 316L(N)-IG stainless steel, and the internal channels are designed to provide the required thermal-hydraulic performance while aiming for a water/steel ratio that results in good nuclear shielding with headers and welded cover plates. Therefore, the soundness of all SB products' welded joints must be confirmed to eliminate the risk of leakage during manufacturing and the Factory Acceptance Test (FAT). However, a test facility capable of testing large components at high temperatures has never been constructed anywhere. The ITER Korea Domestic Agency (KODA) has therefore successfully completed the construction and commissioning of two Hot Helium Leak Test (HHLT) facilities, and all SB series products are being entirely tested under RT and an elevated temperature of 250 °C with a maximum operational pressure of 5.0 MPa. This paper provides information on the design features of the two HHLT facilities and lessons learned from the commissioning test.

A Threshold Helium Leakage Detection Switch with Ultra Low Power Operation.

Detecting helium leakage is important in many applications, such as in dry cask nuclear waste storage systems. This work develops a helium detection system based on the relative permittivity (dielectric constant) difference between air and helium. This difference changes the status of an electrostatic microelectromechanical system (MEMS) switch. The switch is a capacitive-based device and requires a very negligible amount of power. Exciting the switch's electrical resonance enhances the MEMS switch sensitivity to detect low helium concentration. This work simulates two different MEMS switch configurations: a cantilever-based MEMS modeled as a single-degree-freedom model and a clamped-clamped beam MEMS molded using the COMSOL Multiphysics finite-element software. While both configurations demonstrate the switch's simple operation concept, the clamped-clamped beam was selected for detailed parametric characterization due to its comprehensive modeling approach. The beam detects at least 5% helium concentration levels when excited at 3.8 MHz, near electrical resonance. The switch performance decreases at lower excitation frequencies or increases the circuit resistance. The MEMS sensor detection level was relatively immune to beam thickness and parasitic capacitance changes. However, higher parasitic capacitance increases the switch's susceptibility to errors, fluctuations, and uncertainties.

China's progress on hot helium leak test of ITER shield blocks

China Domestic Agency (DA) will provide ITER with 220 shield blocks (SB) which are important, large, and actively water-cooled components in ITER vacuum vessel, their leakage rate shall not exceed 2.69×10−10Pa·m3/s in hot helium leak tests (HHLT) at 250±20 °C. The HHLT work started with developing a HHLT facility and manufacturing a SB full-scale prototype (FSP). The result showed the FSP could satisfy ITER's requirement at 235 °C. H2 outgassing from stainless steel surfaces is the main factor that impacts both the background leakage rate and sensitivity of the testing system in hot testing cases. Hydrogen-removal measures were taken to improve the facility performance, and a new facility was built. A better facility performance came out at 250 °C rather than 235 °C, the background leakage rate decreased by one order of magnitude while the system sensitivity increased by two orders of magnitude. The background is at 10−10 Pa.m3/s and 10−9 Pa.m3/s level at room temperature (RT) and 250 °C, respectively, while the system sensitivity is at 10−12 Pa.m3/s and 10−11 Pa.m3/s level at the two temperatures. With the two facilities, series HHLTs have been carried out on 50 SBs covering two types, with all results complying with ITER requirements.

Experimental Research and Numerical Simulation of Laser Welding of 303Cu/440C-Nb Stainless-Steel Thin-Walled Natural-Gas Injector for Vehicles.

This paper presents the results of research on laser lap welding technology of heterogeneous materials and a laser post-heat treatment method to enhance welding performance. The purpose of this study is to reveal the welding principle of austenitic/martensitic dissimilar stainless-steel materials (3030Cu/440C-Nb) and to further obtain welded joints with good mechanical and sealing properties. A natural-gas injector valve is taken as the study case where its valve pipe (303Cu) and valve seat (440C-Nb) are welded. Experiments and numerical simulations were conducted where the welded joints' temperature and stress fields, microstructure, element distribution, and microhardness were studied. The results showed that the residual equivalent stresses and uneven fusion zone tend to concentrate at the joint of two materials within the welded joint. The hardness of the 303Cu side (181.8 HV) is less than the 440C-Nb side (266 HV) in the center of the welded joint. The laser post-heat treatment can reduce the residual equivalent stress in the welded joint and improve the mechanical and sealing properties. The results of the press-off force test and the helium leakage test showed that the press-off force increased from 9640 N to 10,046 N and the helium leakage rate decreased from 3.34 × 10-4 to 3.96 × 10-6.

Highly-sensitive investigation of a helium leak in a cryogenic valve

We report the results and techniques developed to find a small helium leak in a valve developed for use at cryogenic temperatures. The leak was found during testing of NASA's XRISM/Resolve Instrument after it was integrated into the Flight Dewar that would later be installed onto the spacecraft. This leak was determined to be on the order of ▪. Initial testing in the Dewar showed the leak disappeared when the valve was cooled below ▪. The rate and temperature behavior was verified by testing the valve in a controlled environment within a range of temperatures that span from room temperature to below the superfluid transition of helium-4. The techniques employed to minimize the detection of helium external to the measurement as well as the analysis necessary to tease the weak signal from the leak in the presence of a small helium background signal will be discussed.

Overview of recent ITER TBM Port Plug R&D activities

The objective of ITER project is the demonstration of the scientific and technological feasibility of fusion energy. Tritium fuel is required for the fusion reaction in DEMOnstration reactors and future commercial reactors but it is not available in nature because of its instability. To investigate tritium breeding self-sufficiency, two equatorial ports in ITER are dedicated to operating test blanket modules (TBMs) using the TBM Port Plug (TBM-PP). The TBM-PP design should be investigated with the physical mock-ups to validate the leak tightness in the primary vacuum boundary and to ensure TBM replacement and refurbishment compatible with Remote Handling (RH) operations in the ITER Hot Cell Facility during the ITER lifetime. This paper summarizes the achievements on the two most relevant R&D topics as described herewith. Several TBM-PP components (i.e., back parts of the TBM-Frame, the Dummy-TBMs, the TBM-Shields, and the TBM feeding pipes) are part of the vacuum boundary implying that these components are Vacuum Quality Classification (VQC) 1A. To test and develop high vacuum sealing for the TBM-Frame flange and the TBM-Set or Dummy-TBM flange, the ITER Large Seal Test Rig (LSTR), and metallic gasket seals for the TBM application have been manufactured and installed in the ITER site. The helium leak tests were performed at different operating temperatures to investigate the vacuum performance of metallic gasket seals with VQC 1A. The TBM-PP design shall guarantee the feasibility of a rapid replacement and refurbishment compatible with ITER Remote Handling (RH) operations by using RH tools in the Hot Cell Facility during the ITER's lifetime. Therefore, an experimental program has been performed to demonstrate the feasibility of the critical RH refurbishment tasks including the insertion and removal of TBM-Set/Dummy-TBM and the RH bolting.

Research on improving the system sensitivity for Hot Helium Leak Test

Hot Helium Leak Test (HHLT) can realize more reliable leak tightness assessment than the conventional cold helium leak testing for components that run at elevated temperatures. To ensure successful operation of the International Thermonuclear Experimental Reactor (ITER), the in-vessel components are required to ensure high vacuum sealing performance by HHLT. The biggest challenge for the HHLT is that the background leak rate and sensitivity of the test system can hardly meet the requirements of high accuracy at high temperature. Through years of research, it is found the main factor affecting the rate and the sensitivity of HHLT facility is the hydrogen released from the vacuum-facing materials at high temperature. By adopting a series of measures, the background leakage rate was successfully reduced by one and a half order of magnitude to 1.17×10−10 Pa·m3/s at 253℃, and the system sensitivity was improved by more than one order of magnitude to reach the order of 10−12 Pa·m3/s, even down to 9.85×10−13 Pa·m3/s at the temperature.

Mini-type low pressure helium detection from adsorption enhanced carbon nanotube field emission

The miniature low pressure helium detection technique is of great interest for some important industrial and scientific fields. A novel low pressure helium sensing method from carbon nanotube (CNT) field emission has been developed. The low field emission currents from multi-walled carbon nanotubes (MWNTs) with low crystallinities increase in helium pressure range of 10−7-10−3 Pa. The field emission analysis and first-principles simulation revealed that work functions of CNT emitters remain consistent with helium adsorptions. The simulation showed that the adsorption of He atom results in extra charge aggregation on carbon atoms at the defect, enhancing the field emission current. The helium sensing MWNT emitter normally doesn’t present good hydrogen, oxygen, and nitrogen sensing properties, benefiting the leak detection applications. The mini-type helium sensor in millimeter scale was demonstrated in various gas backgrounds with leak rates down to 10−8 Pa·l/s, very promising for helium detection and leak checking in wide types of in-situ measurement applications, including vacuum electronic devices, low power consumption instruments, and compact space facilities.