C. Marinucci

Computer Simulation of Quench Propagation in QUELL

In the Quench Experiment on Long Length (QUELL), in the SULTAN facility, quench propagation was studied in a well instrumented sample wound using a Nb3Sn cable-in-conduit conductor (CICC) with central cooling channel, cooled by supercritical helium I. A few selected runs have been analyzed here by means of two different computer models — Mithrandir and Gandalf, using the same set of common input parameters, helium and material properties, heat transfer and fluid dynamic correlations. The predictions of the two codes are compared in detail with each other and against experimental data. Both codes are 1-D tools for the description of thermal-hydraulic transients in CICCs with cooling channel; Mithrandir differs from Gandalf mainly because it does not assume, as the latter does, the same thermodynamic state for the helium in the cable bundle region and the helium in the cooling channel. It turns out that although the 1-fluid model (Gandalf) gives a reasonable agreement with the experiment, 2-fluid (Mithrandir) modeling is more accurate both qualitatively and quantitatively.

The QUELL Experiment as a Validation Tool for the Numerical Code Gandalf

One goal of the QUELL experiment is the validation of numerical codes for the simulation of thermo-hydraulic transients in Cable-in-Conduit Conductors with Central Channel (C-5). The validation of Gandalf has focused on 17 quench propagation runs. In general, the good global agreement of the simulations with the experiments-the standard deviation is in the range between 5% and 25%-as well as the good computer performance qualifies Gandalf as a reliable engineering tool. One limitation of the simplified two-flow model implemented in Gandalf is that it tends to overestimate the quench propagation, and hence the helium pressure increase, in the initial phase of the quench evolution, and to underestimate them in the final phase. In standard quenches this limitation can be overcome by full two-flow modeling (Mithrandir code). Two further problem areas have been identified: the high sensitivity (in common with full two-flow modeling) to conductor parameters, if the coil is operated at low temperature margin, and the lack of accuracy when using simplified hydraulic boundary conditions

Pressure drop of the ITER PFCI cable-in-conduit conductor

Pressure drop in the ITER PFCI cable-in-conduit conductor (CICC) has been measured at CRPP using pressurized water at room temperature. The PFCI conductor is a dual channel CICC and the coolant flows in parallel in the central channel and in the annular bundle region. In our experiment the flow in the central channel is blocked and the longitudinal friction factor of the annular bundle region is deduced from measurements of pressure drop and mass flow rate. Two conductor samples are investigated, one with and one without subcable/outer cable wraps. The results show that the wraps have a negligible effect on the friction factor, and that the Katheder correlation overestimates the actual friction factor.

Heat slug propagation in QUELL. Part I: Experimental setup and 1-fluid GANDALF analysis

The experimental setup for heat slug propagation at zero current and field in the QUench Experiment on Long Length (QUELL) is described and re-evaluated. Eight inductively heated and four resistively heated runs are considered and analyzed with the 1-fluid GANDALF model, which assumes perfect coupling between bundle and hole helium in the two-channel cable-in-conduit conductor (CICC). Although adequate to predict the hydraulic response, the 1-fluid model exhibits an intrinsic limitation to accurately simulate the evolution of the conduit temperature at different sensor locations during heat slug propagation. In general it appears that the analysis of this type of slow transients requires a 2-fluid model, as considered in Part II of the present study.

A comparison between 1- and 2-fluid simulations of the QUELL conductor

In QUELL (QUench Experiment on Long Length) a cable-in-conduit-conductor with central cooling hole has been tested under fusion reactor relevant conditions. A first comparison is presented here between the results of a recently developed 2-fluid code-Mithrandir-and those of the reference 1-fluid code-Gandalf-for the case of the QUELL conductor. Mithrandir allows for different thermodynamic properties of the helium in the hole and that in the bundle, thereby providing a more accurate description of the physics involved when a central cooling hole is present.

Computations of AC Loss in the ITER Magnets During Fast Field Transients

The calculation of AC loss due to the control currents in ITER is a cumbersome task. The reason is that control transients require small field changes (0.1 T or less) at moderate frequency (up to 10 Hz), where effects of partial penetration of the filaments and shielding are important and need to be taken into account to produce sound AC loss estimates. In this paper we describe models developed for AC loss calculation, in particular hysteresis and coupling current loss, that are suitable for the above regime. Both hysteresis and coupling loss models are adapted to the conductor analysed through few parameters (the effective filament diameter and time constants) that can be derived from measurement of loss on short samples. We report an example of calculations of AC loss in the ITER TF and PF coils for two vertical control scenarios (VS1 and VS2) during high beta operation at flattop.

Stability analysis of the ITER TF and CS conductors using the code Gandalf

The stability of the toroidal field and central coil cable-in-conduit conductors for the International Thermonuclear Experimental Reactor (ITER) has been analyzed with the code Gandalf. The energy margins, computed for a number of disturbance scenarios, are in the order of some 100 mJ/ccst, well above the expected disturbances. A detailed convergence study is shown to be essential not only in principle but also in practice, e.g. dual stability was found in some cases, but disappeared when the integration time step was refined.

Quench Analysis of the European High Field Superconducting Dipole Magnet EDIPO

The European high-field superconducting dipole magnet, currently under development, will create magnetic fields of up to 12.5 T for performance tests of high-current superconducting cables. To study the behavior of the dipole during a quench (the energy stored is 16 MJ at 16.5 kA) a complex simulation model was developed and integrated in the 1-D thermal hydraulic code THEA. The detailed quench analysis has shown that the dipole can be adequately protected by proper selection of the discharge time constant and the quench detection voltage for all disturbances except if the initial normal zone is 50 m, an unlikely event. In this case the peak helium pressure exceeds the limit for plastic deformation.

Heat slug propagation in QUELL. Part II: 2-fluid MITHRANDIR analysis

In this Paper, heat slug propagation in the QUench Experiment on Long Length (QUELL) is studied with the 2-fluid MITHRANDIR model, which allows a finite coupling time between bundle and hole helium in a two-channel cable-in-conduit conductor. The same eight inductively heated and four resistively heated runs are considered in Part I. Good to very good agreement is shown between computed and experimental evolution of the conduit temperature T-jk at downstream sensors, as opposed to the poor accuracy of the 1-fluid simulations shown in Part I. The inlet mass flow is typically slightly overestimated, as with the 1-fluid model, resulting in a phase shift between computed and experimental T-jk. Some parametric effects are also analyzed

The hydraulic solver Flower and its validation against the QUELL experiment in SULTAN

Knowledge of the hydraulic boundary conditions is a prerequisite for accurate estimates of the quench characteristics of superconducting magnets. A set of routines (Flower) has been designed and interfaced to the code Gandalf to provide a simplified model of the hydraulic connections to a cryogenic plant of a coil using cable-in-conduit conductors with central cooling channel. The validation against experimental data provided by the Quench Experiment on Long Length (QUELL) in the CRPP facility SULTAN have shown that Flower is able to simulate the hydraulic boundary conditions within engineering limits of accuracy.