# On the Dimensions Required for a Molten Salt Zero Power Reactor Operating on Chloride Salts
## 2 Codes and General Modelling  
The salt system is based on detailed data on the density versus temperature curve from Russian literature [17]; see Table 1. The data is used for the analysis based on a reference temperature of 980 K with changes of \(\pm 50\) K for the analysis of feedback effects with the aim to stay in the validity range of the available data given below for the reference case, as well as for the feedback analysis. Nevertheless, while being a good starting point for an initial analysis, the data is limited to a very narrow temperature window and for a temperature range which will not be the first choice for initial experiments in a zero-power environment. Especially, for a detailed study of a future zero-power experiment, the temperature range has to be extended down to room temperature to be able to judge the system behaviour under cold, solid conditions, which would be the perfect setting for first experiments. However, to be able to provide modelling and simulation results for room temperature, reliable thermo-physical data for the whole temperature range must be measured for the required salt system. This will require the creation of the salt mix in the molten state and then cooling it back down to room temperature. Using the coefficients given in Table 1, the temperature dependent density is calculated according to the formula:  
\[\rho\Big{[}\frac{\text{g}}{\text{cm}^{3}}\Big{]}=a-bT[\text{K}] \tag{1}\]  
Based on the given salt system investigation [15], the applied standard salt composition for the study for the nominal case is the eutectic 42.5% NaCl-40.5% UCl\({}_{4}\)-17% UCl\({}_{3}\). This composition is close enough to the third row of Table 1 to provide a reasonable density estimation. In addition, a heavy metal rich case is investigated with 20% NaCl-56.35% UCl\({}_{4}\)-23.65% UCl\({}_{3}\). The fuel temperature for both compositions is set with a uniform spatial distribution to 980 K in the core since no variation is expected in a zero-power system that typically has a thermal power of less than 1 kW. The simulations for this initial study of zero-power reactor configurations have been performed using the POLARIS module of the SCALE code system [18]. Polaris is a new module for SCALE 6.2 that provides a 2-D lattice physics analysis capability that uses a multigroup self-shielding method called the Embedded Self Shielding Method (ESSM) and a transport solver based on the Method of Characteristics (MoC). In general, POLARIS and its cross-section library has been developed and validated for light water reactors. Thus, validation we have recently published [16] was an initial verification/validation against the Monte-Carlo code SERPENT [19]. This will be extended here by comparisons to the multi-group deterministic S\({}_{n}\) transport calculation sequence TRITON-NEWT and multi-group as well as continuous energy Monte-Carlo code Keno VI, all parts of the SCALE package [20]. Polaris uses the v7-252 cross-section set of the SCALE package, based on ENDF/B 7.1, which is also the basis for the TRITON-NEWT as well as the multi-group (MG) Keno VI calculations, while the continuous energy version uses the ce_v7.1_endf library of the SCALE package. For the study, very specific 2-D and 3-D models of a fast molten salt reactor were built for the simulation. First, the POLARIS model, see Figure 3, consists of 2-D discretized ring core surrounded by a steel vessel and a reflector region with a large amount of absorbing material around the core. The outer region is required due to the reflecting boundary conditions at the outside of the square (a given setting in POLARIS). The arrangement models a leakage boundary condition due to the absorber.