# Numerical and Experimental Study of a Large-Scale Natural Circulation Helium Loop
## 5 Conclusions  
The goal of the presented paper was to introduce a one-dimensional transient mathematical model of flow in a natural circulation helium loop. The model was developed with STU in the context of ongoing studies in the field of emergency cooling for Generation IV nuclear reactors. The model was implemented in C++ and verified on an experimental setup. The experimental device was a large-scale STU-owned helium loop, which was sized to transfer power up to 250 kW at a maximum helium temperature of 520\({}^{\circ}\) and an overpressure of 7 MPa. Verification of the mathematical model was performed in six scenarios. The scenarios differed in either the steady-state helium pressures and temperatures or the time courses of the heating and cooling performance. On the basis of the presented results, it can be concluded that the simulated pressure and temperature waveforms were qualitatively in agreement with those from the experiments. A quantitative comparison of the results showed that the average deviations of the calculated temperatures of the experiment normally did not exceed 10% of the measured values of the variables. The exception was in scenario 3, where the relative error in the cooling phase (interval 2) was more than 20%. In general, it can be concluded that the simulated temperatures were overestimated during heating and underestimated during the cooling phases of the experiments. In steady states, the agreement between the experiments and simulations was in units of percent. In general, the results of the simulations and experiments differed the most in scenarios with rapid changes in input parameters during the start-up or after-cooling phase of the system. In Scenario 3, the heating was abruptly turned off while the cooling-water flow was maintained. As a result, the large differences in simulated temperatures were the most dramatic (21.22%--hot branch, 18.02%--cold branch). For the scenario with the smallest amount of helium in the system (Scenario 8), significant differences in the time courses of the temperature curves were also noticeable. In this scenario, the mean deviations in the helium temperatures reached up to 9.5%. The percentage agreement of the calculated pressures was less than 6.8%, regardless of the scenario. The above observations can be attributed to heat accumulation in the solid parts of the loop. With abrupt changes in the input parameters, the transition between steady states is slowed by the heat exchange between the helium and the solid material. Similarly, with less helium mass in the system, the relative influence of heat released or absorbed \begin{table}
\begin{tabular}{c c c c c} \hline
**Scenario—Interval** & **Time Interval (s)** & \(T_{\textit{HE,err}}\) **(\%)** & \(T_{\textit{HE8,err}}\) **(\%)** & \(p_{\textit{HE,err}}\) **(\%)** \\ \hline Scenario 3—interval 1 & 4200 \(\div\) 15,900 & 4.12 & 11.61 & 1.02 \\ Scenario 3—interval 2 & 17,000 \(\div\) 24,200 & 21.22 & 18.02 & 6.43 \\ \hline Scenario 4—interval 1 & 1000 \(\div\) 19,000 & 2.18 & 2.08 & 3.08 \\ Scenario 4—interval 2 & 20,000 \(\div\) 25,000 & 3.19 & 3.56 & 3.44 \\ \hline Scenario 5—interval 1 & 2000 \(\div\) 12,200 & 5.21 & 6.89 & 6.25 \\ Scenario 5—interval 2 & 15,000 \(\div\) 25,000 & 5.65 & 4.10 & 6.77 \\ \hline Scenario 6—interval 1 & 1000 \(\div\) 8200 & 4.35 & 4.64 & 2.48 \\ Scenario 6—interval 2 & 13,000 \(\div\) 23,800 & 7.35 & 6.45 & 4.56 \\ \hline Scenario 8—interval 1 & 800 \(\div\) 12500 & 9.47 & 9.67 & 3.71 \\ Scenario 8—interval 2 & 15,000 \(\div\) 25,000 & 7.71 & 7.66 & 2.64 \\ \hline Scenario 13—interval 1 & 500 \(\div\) 8600 & 4.28 & 7.28 & 6.26 \\ Scenario 13—interval 2 & 10,000 \(\div\) 22,000 & 2.94 & 2.44 & 3.96 \\ \hline \end{tabular}
\end{table} 
Table 1: Simulation error estimation. Average relative errors. by the solid parts is larger. Thus, our simulation results are burdened with errors that resulted from an incorrect estimation of the device's mass.