## 4 Conclusions and Perspective  
In this paper, the Polynomial Chaos Expansion has been adopted to assess the uncertainty propagation in a transient relevant for the safe operation of a nuclear fusion plant. The uncertainty in the characteristic parameters of the varistors of the fast discharge units of the DTT TF coils has been propagated to the computed results during a fast discharge transient. The uncertainty has an impact on the actual current evolution during the discharge, which influences both the eddy currents induced in the coil casing and the TH behavior of the coil during quench propagation. The current evolution during the discharge has been computed as a function of the varistor parameters, with an object-oriented electrical model developed using the Modelica language. The uncertainty propagation from the varistor parameters to the effective current evolution has been assessed using a surrogate model based on the PCE, which has been successfully benchmarked against the Monte Carlo approach. The obtained statistical distribution of the current evolution has been used as input in the electromagnetic model developed using the 3D-FOX tool to compute, again with a PCE surrogate model, the statistical distribution of the power (and energy) deposition in the coil casing due to eddy currents. Finally, both these results (current and power evolution) have been used to identify the two possible worst-case scenarios to be simulated with the thermal-hydraulic model, developed with the 4C code. The two cases are the slowest current dump and the highest power deposition in the casing. The comparison of the results of the thermal-hydraulic model shows that the considered uncertainty on the varistor parameters leads to a wide range of different results. As concerns the quench protection, the reduction of the current discharge time, reducing the hot spot temperature, is preferable to the reduction of the power deposited in the casing due to eddy currents. Figure 11: Maximum hot spot temperature reached in each pancake during the transient for both WCS1 and WCS2. The effect of the latter is, however, important, especially with respect to hydraulic circuit pressurization and He venting. From this perspective, more detailed models, including the hydraulic circuits, will be used to assess this effect. The application of the statistical methods to the TH model will, in future, allow us to identify other worst-case scenarios between the two extreme examples analyzed in this work. Moreover, a reverse analysis will be performed with the aim of suggesting the maximum limits of uncertainty which are acceptable on the varistor parameters. The presented analysis is one example of the possible applications of the newly developed methodology for the uncertainty propagation in fault transients in superconducting magnets. Indeed, the presented approach is totally general and can be applied to any case and input data of interest and, with suitable parameter tuning, to any kind of numerical model. Conceptualization, A.A. and M.D.B. ; methodology, A.A. and M.D.B. ;original draft preparation, A.A. and M.D.B. ; supervision, R.B. and S.D. ; review and editing, R.B. and S.D. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Restrictions apply to the availability of these data. Data was obtained from DTT s.c.a.r.l. and are available with the permission of DTT s.c.a.r.l. OpenModelica Online Documentation. 2023. Available online: [[https://openmodelica.org/useresresources/userdocumentation/](https://openmodelica.org/useresresources/userdocumentation/)]([https://openmodelica.org/useresresources/userdocumentation/](https://openmodelica.org/useresresources/userdocumentation/)) (accessed on 27 June 2023). * [PERSON] et al. (2022) [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] 3D-FOX--A 3D Transient Electromagnetic Code for Eddy Currents Computation in Superconducting Magnet Structures: DTT TF Fast Current Discharge Analysis.