Processing of the quench detection signals in W7-X

The Wendelstein 7-X (W7-X) project uses superconductive coils for generation of the magnetic field to keep the plasma. One of the important safety systems is the protection against quench events. The quench detection system of W7-X protects the superconducting coils, the superconducting bus bar sections and the high temperature superconductor of the current leads against the damage because of a quench and against the high stress by a fast discharge of the magnet system.Therefore, the present design of the quench detection system (QDS) uses a two-stage safety concept for discharging the magnetic system. This paper describes the present design of the system assembly from the quench detection unit (QDU) for the detection of the quench to the quench detection interface (QDI) to implement the two-stage safety concept.     

Quench detection of fast plasma events for the JT-60SA central solenoid

The JT-60 is planned to be modified to a full-superconducting tokamak referred to as the JT-60 Super Advanced (JT-60SA). The maximum temperature of the magnet during its quench might reach the temperature of higher than several hundreds Kelvin that will damage the superconducting magnet itself. The high precision quench detection system, therefore, is one of the key technologies in the superconducting magnet protection system.The pick-up coil method, which is using voltage taps to detect the normal voltage, is used for the quench detection of the JT-60SA superconducting magnet system. The disk-shaped pick-up coils are inserted in the central solenoid (CS) module to compensate the inductive voltage. In the previous study, the quench detection system requires a large number of pick-up coils. The reliability of quench detection system would be higher by simplifying the detection system such as reducing the number of pick-up coils. Simplifying the quench detection system is also important to reduce the total cost of the protection system. Hence the design method is improved by increasing optimizing parameters. The improved design method can reduce the number of pick-up coils without reducing the sensitivity of detection; consequently the protection system can be designed with higher reliability and lower cost. The applicability of the disk-shaped pick-up coil for quench detection system is evaluated by the two dimensional analysis. In the previous study, however, the analysis model only took into account the CS, EF (equilibrium field) coils and plasma. Therefore, applicability of the disk-shaped pick-up coil for the quench detection system remains open question because the fast plasma events, such as disruption, mini collapse and ELM (edge localized mode), directly influences on the voltage of pick-up coil making the quench signal undetectable. Consequently, a new analysis model proposed in the present paper was designed to avoid this difficulty by introducing the passive coil series such as vacuum vessel and stabilizer. The influence of fast plasma events is absorbed by passive coil series like real system, and the evaluation of applicability can be examined in detail. The analysis results show that the disk-shaped pick-up coil is applicable whenever the standard operation, disruption, mini collapse and ELM.     

Study of electromagnetic interference on quench detecting system of HTS current leads for EAST

High temperature superconducting (HTS) material B-2223/Ag-Au has been used for EAST poloidal field (PF) coil current leads for reducing construction and operation cost of cryogenic system. The quench propagation velocity of HTS superconducting material is several orders of magnitude lower than that of normal low temperature current leads. It is difficult to detect weak signal of quench which is easily influenced by strong electromagnetic interference (EMI). In this paper, the sources of EMI from quench detecting system of high temperature current leads have been introduced. And we have chosen reasonable methods for good transformation and protection on the basis of electromagnetic compatibility simulation diagnosis experiments. Recent experimental results showed that the restraint of EMI has been achieved and has met the requirements of experiment.     

Quench propagation and quench detection in the TF system of JT-60SA

In the framework of the JT-60SA project, France and Italy will provide to JAEA 18 Toroidal Field (TF) coils including NbTi cable-in-conduit conductors. During the tokamak operation, these coils could experience a quench, an incidental event corresponding to the irreversible transition from superconducting state to normal resistive state. Starting from a localized disturbance, the normal zone propagates along the conductor and dissipates a large energy due to Joule heating, which can cause irreversible damages.The detection has to be fast enough (a few seconds) to trigger the current discharge, so as to dump the stored magnetic energy into an external resistor. The JT-60SA primary quench detection system will be based on voltage measurements, which are the most rapid technology. The features of the detection system must be adjusted so as to detect the most probable quenches, while avoiding inopportune fast safety discharges. This requires a reliable simulation of the early quench propagation, performed in this study with the Gandalf code.The conductor temperature reached during the current discharge must be kept under a maximal value, according to the hot spot criterion. In the present study, a hot spot criterion temperature of 150 K was taken into account and the role of each conductor component (strands, helium and conduit) was analyzed. The detection parameters were then investigated for different hypotheses regarding the quench initiation.     

Study of contact resistance in the design of a pyro-breaker applied in superconducting fusion facility

Pyro-breaker is a fast-responding and high-reliable explosive-driven circuit breaker, which has been applied in several quench protection systems, such as International Thermonuclear Experimental Reactor and Experimental Advanced Superconducting Tokamak. As an indispensable back-up switch, Pyro-breaker guarantees the reliability and safety of the system and avoids tremendous loss when quench happens. Electrical contact, a crucial part of an electrical device greatly determines the steady current capacity of a Pyro-breaker. However, due to the complexity of the model and the deficiency knowledge of the microstate of contact areas, an accurate calculation for contact resistance is difficult to acquire. A study of electrical contact in the design of a Pyro-breaker has been presented in this paper. An engineering calculation method is verified with experiments. Parameters are fitted for the presented model, which will be a significant theoretical basis for the future designing.     

Design and implementation of quench detection instrumentation for TF magnet system of SST-1

Steady State Superconducting Tokamak-1 (SST-1) at Institute for Plasma Research (IPR), India is now in engineering validation phase. The assembled Toroidal Field (TF) magnet system of SST-1 will be operated at 10 kA of nominal current at helium cooled condition of 4.5 K. A reliable and fail proof quench detection (QD) system is essential for the safety and the investment protection requirements of the magnets. This QD system needs to continuously monitor all the superconducting coils, which include 16 TF magnets, return-loop, bus bars and current leads. In case of any event initiating the normal resistive zone and reaching thermal run-away, the QD system needs to trigger the magnet protection circuits. Precision instrumentation and control system with 204 signal channels had been developed for detection of quench anywhere in the entire TF magnet system. In the present configuration of quench detection scheme, the voltage drop across each double pancake (DP) of each TF coil are compared with its two adjacent DPs for the detection of normal zone and cancelation of inductive couples. Two identical redundant systems with one out of two configurations are successfully commissioned and tested at IPR. This paper describes the design and implementation of the QD system, Installation experience, validation test and initial results from the recent SST-1 magnet system charging.     

A commutation analytical model for quench protection of the CFETR central solenoid model coil

A central solenoid model coil will be set up to develop and verify the technique for the full-size central solenoid coil of the China Fusion Engineering Test Reactor. In case of quench and failures of superconducting coils, the quench protection(QP) system, which employs fuse-based commutation technology, is designed. This paper presents an analytical model to investigate the commutation process in the QP circuit. The model consists of the QP circuit equations, the breaker arc model, the fuse pre-arc model, and the fuse arc model. The model is employed in the whole commutation process including current transfer from breaker branch to the fuse branch model, then from fuse branch to the discharge resistor branch, and current decrease to zero in the discharge resistor. The experiment result verified the effectiveness of the presented model. The model might be helpful for design of the fuse and optimization of the commutation circuit.     

Analysis of Internal and External Bypass Circuit Design for ITER Poloidal Field Converter System

This paper describes the bypass circuit design applied to International Thermonuclear Experimental Reactor (ITER) poloidal field converter system. In the baseline design, an internal bypass scheme was designed for purposes of quench protection and reactive power reduction by bypass operation. However, due to the potential risks that exist in case of fault protection, internal bypass scheme is abandoned finally and external bypass scheme is proposed to replace the internal bypass design. In this paper, the weakness of internal bypass design has been reviewed and the reliability of external bypass design at different sites is analyzed. Simulations based on SIMULINK are performed to confirm the reasonability and availability of external bypass proposals.     

不同淬灭水平混合~(90)Sr/~(90)Y液闪谱的简易拟合方法

针对不同淬灭水平混合~(90)Sr/~(90)Y液闪谱,利用Origin软件建立了基于非对称双S函数的简易拟合方法。由于常用外标准淬灭指数受外标准谱的统计涨落影响(测定高活度样品时可能会产生显著误差),因此根据~(90)Y样品液闪谱特征自定义了不受样品活度影响的淬灭指数。利用该淬灭指数和不同淬灭水平~(90)Y样品的液闪谱,将~(90)Y液闪谱位置和形状参数与自定义淬灭指数进行关联,并在此基础上完成了不同淬灭水平混合~(90)Sr/~(90)Y液闪谱的解析,解析结果与实验结果符合良好。该法有望用于同时测定不同淬灭水平样品中几种放射性核素的活度。     

A moving subgrid model for simulation of reflood heat transfer

In the quench front and froth region the thermal-hydraulic parameters experience a sharp axial variation. The heat transfer regime changes from single-phase liquid, to nucleate boiling, to transition boiling and finally to film boiling in a small axial distance.One of the major limitations of all the current best-estimate codes is that a relatively coarse mesh is used to solve the complex fluid flow and heat transfer problem in proximity of the quench front during reflood. The use of a fine axial mesh for the entire core becomes prohibitive because of the large computational costs involved. Moreover, as the mesh size decreases, the standard numerical methods based on a semi-implicit scheme, tend to become unstable.A subgrid model was developed to resolve the complex thermal-hydraulic problem at the quench front and froth region. This model is a Fine Hydraulic Moving Grid (FHMG) that overlies a coarse Eulerian mesh in the proximity of the quench front and froth region. The fine mesh moves in the core and follows the quench front as it advances in the core while the rods cool and quench. The FHMG software package was developed and implemented into the COBRA-TF computer code. This paper presents the model and discusses preliminary results obtained with the COBRA-TF/FHMG computer code.     

Scaling of quench front and entrainment-related phenomena

The scaling of thermal hydraulic systems is of great importance in the development of experiments in laboratory-scale test facilities that are used to replicate the response of full-size prototypical designs. One particular process that is of interest in experimental modeling is the quench front that develops during the reflood phase in a pressurized water reactor (PWR) following a large-break loss of coolant accident (LOCA). The purpose of this study is to develop a scaling methodology such that the prototypical quench front related phenomena such as the entrainment of liquid droplets can be preserved in a laboratory-scale test facility which may have material, geometrical, fluid, and flow differences as compared to the prototypical case. A mass and energy balance on a Lagrangian quench front control volume along with temporal scaling methods are utilized in developing the quench front scaling groups for a phenomena-specific second-tier scaling analysis. A sample calculation is presented comparing the quench front scaling groups calculated for a prototypical Westinghouse 17×17 PWR fuel design and that of the geometry and material configuration used in the FLECHT-SEASET series of experiments.     

环形通道内再淹没过程骤冷前沿推进速度实验研究

骤冷前沿推进速度是衡量失水事故中再淹没过程堆芯冷却效率的重要参数之一。本文通过实验研究了竖直环形通道内骤冷前沿的推进特性,获得初始壁温、入口温度、入口质量流速及加热功率对骤冷前沿推进速度的影响。实验结果表明,骤冷前沿推进速度随初始壁温、入口温度和加热功率的增加而减小,随入口质量流速的增加而增加。      

Coolability study on two-bundle scale flow blockage in the reflood process

A two-bundle scale channel flow blockage in the eight-bundle slab core test facility (SCTF) was examined for core cooling capability study in the reflood phase of a PWR-LOCA. The coplanar blockage with 60% blockage fraction resulted in promotion of quench for the elevation immediately above the blockage in relatively high reflood velocity tests. Conversely, this blockage led to delay of quench in low reflood velocity tests. Peak clad temperature, however, was not affected much by the existence of the blockage. These results are examined in comparison with the results of similar small scale test facilities. This examination revealed that the promotion of quench above the blockage was confined to a shorter length but the quench delay time was slightly longer for a large partial blockage than for a small blockage.     

Process Design of Cryogenic Distribution System for CFETR CS Model Coil

The superconducting magnet of Central Solenoid(CS) model coil of China Fusion Engineering Test Reactor(CFETR) is made of Nb_3Sn/Nb Ti cable-in-conduit conductor(CICC),and operated by forced-flow cooling with a large amount of supercritical helium.The cryogenic circulation pump is analyzed and considered to be effective in achieving the supercritical helium(SHe) circulation for the forced-flow cooled(FFC) CICC magnet.A distributed system will be constructed for cooling the CFETR CS model coil.This paper presents the design of FFC process for the CFETR CS model coil.The equipment configuration,quench protection in the magnet and the process control are presented.     

Characterization of plasma current quench during disruptions at HL-2A

The most essential assumptions of physics for the evaluation of electromagnetic forces on the plasma-facing components due to a disruption-induced eddy current are characteristics of plasma current quenches including the current quench rate or its waveforms.The characteristics of plasma current quenches at HL-2A have been analyzed during spontaneous disruptions.Both linear decay and exponential decay are found in the disruptions with the fastest current quenches.However,there are two stages of current quench in the slow current quench case.The first stage with an exponential decay and the second stage followed by a rapid linear decay.The faster current quench rate corresponds to the faster movement of plasma displacement.The parameter regimes on the current quench time and the current quench rates have been obtained from disruption statistics at HL-2A.There exists no remarkable difference for distributions obtained between the limiter and the divertor configuration.This data from HL-2A provides basic data of the derivation of design criteria for a large-sized machine during the current decay phase of the disruptions.     

Correlation of Quench Phenomena for Bottom Flooding during Loss-of-Coolant Accidents

The three quench modes based on the observation with an outside-heated quartz tube experiment, i.e. (1) liquid column type, (2) dryout type and (3) droplet-rewetting type are discussed quantitatively to provide correlations for a reflood analysis code.

For the quench velocity of liquid column type and dryout type, using the data of PWR-FLECHT experiments, a correlation is obtained. This correlation is compared with experimental data and found that it can predict the inverse quench velocity within ±20% accuracy under the following conditions: pressure 1.0–4.1 kg/cm²-a, quench temperature 370~510°C, local subcooling 0~30°C, flow rate 5~25 cm/sec.

For the quench temperature of droplet-rewetting type, a correlation is derived and the prediction from it gives the qualitative agreement with few available data.     

Response to G. Yadigaroglu''s letter

The rewetting or quench temperature is the temperature of a hot solid surface at which a liquid can reestablish contact with the dry surface. An estimation of this temperature is essential in predicting the rate at which the coolant quenches the core of a light-water reactor (LWR) after a loss-of-coolant accident. The present study reviews and evaluates previous work in this area and presents a model relating experiments to theory for the different possible types of reflood in LWRs. It is postulated that, with the exception of those cases of top reflood by a film in a single-rod geometry and bottom reflood with a very low mass flow rate, the quench temperature corresponds to either the minimum film boiling temperature or the Leidenfrost temperature. In cases where there are such exceptions, the quench temperature corresponds to the critical heat flux temperature. New correlations for the rewetting or quench temperature are presented.     

Quench front propagation during bottom reflooding of a heated annular channel

A time-dependent two-dimensional conduction model of quenching, via bottom reflood, of hot, dry cylindrical tube of finite length in an annulus is presented. The method of finite integral transform is used in the solution. Multiregion heat transfer is considered at the heater surface, and no a priori assumption is made in regard to the quench front velocity. A possible mode of variation of the quench temperature with coolant inlet velocity is proposed. Predictions of the quench front velocity are compared with a few experimental data for Zircaloy-2 tube available in the literature.     

液闪分析中闪烁体积对各种核素测定的影响

除了淬灭以外,闪烁体积是影响液闪计数效率的另一个重要因素。采用可区分闪烁体积影响和淬灭影响的实验方法,在两种液闪仪上测定了多种代表性核素。结果表明:在实验淬灭水平范围内,闪烁体积对α粒子和高能β粒子的计数效率影响很小,小闪烁体积测量时的壁效应仅仅影响液闪谱图的形状,对计数效率的影响可以忽略;闪烁体积对γ射线和低能β粒子的计数效率影响很大,相同水平的淬灭对闪烁体积较大样品的计数效率影响较大。在用淬灭校正曲线计算计数效率或用"最优PSA-淬灭指数"曲线设置α/β甄别参数时,为了减小测量误差,不管采用什么淬灭指数,待测样品都需要与淬灭标准样品保持相同的闪烁体积和闪烁瓶尺寸。     

Validation of the 3D code SARUMAN in a stability and quench experiment with the superconducting magnet SAFFO

Experiments on stability and quench propagation were performed on a NbTi hollow conductor test module allowing heat conduction only between adjacent layers, while the turns in a layer are thermally insulated (2-D winding). The measurements performed over a wide range of operating conditions were used as the experimental data-base for the comparison with the results of the simulations done using the quench analysis code SARUMAN. In the paper the experimental results are compared with the predictions of a 2-D version of SARUMAN, which show that the simulation is able to reproduce the general features of the quench propagation experiments.