## 2 Codes, Models and Methods  
For the simulations, the HELIOS code system version: HELIOS 2.03 with the internal 173 group library [21] based on the ENDVB-VII.1 cross-section library. The code is a 2D spectral code with wide unstructured mesh capabilities and a transport solver based on the collision probability method [22] and a newer extension based on the Method of Characteristics [23]. The general model is based on the EVOL benchmark configuration [24], which is transferred to a volume-corrected 2D HELIOS model (see Figure 1). The model has been adopted for as close as possible reproduction of the 3D structure and the relations between the different materials. Furthermore, the benchmark model has been extended to represent the outer structures and the 16-heat exchanger pipe arrangement for a better representation of the real geometry. The leakage in the third dimension is introduced into the calculation through the insertion of a buckling correction available in HELIOS (BSQ: 0.00002). This value has been fixed by a comparison of 2D and 3D calculations within the EVOL benchmark exercises [25]. Using this setting, a k of ~1.005 is required for a pseudo-3D keff of 1.0. The leakage in radial direction is directly modelled through vacuum boundary conditions. The salt system chosen for iMAGINE is based on NaCl-UCI3-UCI4 with the eutectic composition 42.5%-17.0%-40.5%; more detailed discussion of the data of the salt system and the ratio behind the choice is given in [26], the blanket area is filled with sodium, while the protector is based on B4C. The core size of model has been adopted to a radius of 287.5 cm and a U-235 enrichment of 11.06%, which is slightly different to POLARIS due to the consideration of the axial leakage, see discussion above. This is required to take the different salt compositions into account while keeping the system critical. The used data are based on the results of two earlier studies [26; 27] based on SCALE/POLARIS [28]. The HELIOS code is an industrial standard software which is designed to perform the neutron transport calculation, the burnup calculation, and, if requested, the cross-section preparation in the whole system or in defined calculation areas under the consideration of the boundary conditions set for the unit cell. Originally, the HELIOS code was written for the simulation of solid structured fuel assemblies; thus, the possibility of online refuelling and online reprocessing is not foreseen. To deal with these special features required for the simulation of molten salt reactors, a PYTHON script has been developed [17], which is based on the special features of the HELIOS package. All input data, which does not change during the whole reactor operation, is stored in a so-called expert input. The changing material configuration is fed into the system through a user input which is re-written in every cycle using a PYTHON script. Within each of the cycles, 5 burnup steps are calculated through HELIOS. The expert input and the updated user input are merged in the pre-processor AURORA [29], which creates the updated input for the HELIOS run used for the determination of the neutron flux distribution and the burnup of the materials. The results are finally evaluated at the end of each cycle in the post-processor ZENITH [30]. On the one hand, here, it is decided which elements are reduced or increased to which extent. On the other hand, isotopes will be fed back into the next new user input, which is created with the help of the PYTHON script (see Figure 2). Theoretically, it would be possible to simulate a molten salt reactor precisely by using small time steps in this calculation loop. In a real MSR, two different time scales for the salt clean-up can be observed based on two  
Figure 1: Volume corrected 2D HELIOS model of the molten salt reactor. different processes, the helium bubbling for gaseous and volatile fission products with a comparably short acting time and the online salt clean-up for the dissolved fission products with a significantly longer-acting time.