# Numerical and Experimental Study of a Large-Scale Natural Circulation Helium Loop
## 4 Results  
The helium loop is primarily intended to cool a nuclear reactor in critical conditions. The experiments carried out on the test loop were also subordinated to this objective. Measurements were performed by using predetermined time changes (scenarios) in input variables--or boundary conditions, to use mathematical terminology. These were the cooling-water flow rate at the heat exchanger and the electrical power input of the heater, which represented the nuclear reactor model. The temperature of the cooling water had a constant value during all measurements, as it was pumped from a deep borehole where conditions were very stable. The scenarios were chosen to cover a range of helium operating pressures up to a maximum value of 7 MPa. The helium charge in the loop system was also varied for each scenario--this is what determined what pressure was achieved at the given temperature conditions. Steady states and transients caused by a sudden change in one of the input variables (a change in the heater input power or a change in cooling) were monitored. In this method of experimentation, there were limits set by the loop design so that the temperature gradients did not cause the mechanical deformation of the device. For this reason, for example, operating conditions in which large temperature differences occurred in the cold and hot branches were excluded. The developed mathematical model was evaluated according to the agreement of the observed calculated quantities with the measured values. The observed quantities were chosen to be the pressure \(p_{He1}\) in the system, the temperature \(t_{He1}\) in the hot branch, and the temperature \(t_{He8}\) in the cold branch of the loop. Several results of measurements on the helium loop have already been presented in publications, e.g., [18]. In this paper, we retained the established numbering of the scenarios. We will document the comparison of the model results and the measured data in selected time intervals from these scenarios. Systemic responses to changes in input variables will be presented, with an emphasis on the comparison between calculations and experiments. Figure 4 represents interval 1 of scenario 3. In scenario 3, the pressure reached up to almost 7 MPa in the loop at the steady state (see Figure 5). Interval 1 of scenario 3 represents the loop's start from the initial cold state. At the beginning, the temperatures in the loops were around 100 \({}^{\circ}\)C (cold branch) and 200 \({}^{\circ}\)C (hot branch). The increase in the power input \(P\) was gradual in steps from 60 to 200 kW, taking about 2.5 h. The flow rate \(Q_{H2O}\) of the cooling water increased; the maximum value was 1.5 L/s, and at the end, it dropped to 1.0 L/s. In Figure 4, it can be seen that the temperature increase showed good agreement between the measurement and the calculation for both the hot and the cold branch. For the pressure \(p_{He1}\), it was similar, and the calculated values followed the measurement results very well. Figure 5 shows interval 2 of scenario 3, which started at 17,000 s. At the beginning of the interval, the system state was the steady state, and just before reaching 18,800 s, the generator input power \(P\) dropped to zero, but the flow rate \(Q_{H2O}\) of the cooling water remained unchanged. In the graph, we can observe the response of pressures and temperatures to this change. At the end of the scenario (after 24,200 s), the calculated temperatures \(t_{He1}\) and \(t_{He8}\) were at higher values than the measured temperatures. This was also related to the pressure \(p_{He1}\) (in addition, the calculated value was higher by about 0.4 MPa). Figure 6 shows interval 1 of scenario 4 starting at 1000 s and ending at 19,000 s. The system contained less helium than in scenario 3, and therefore, the maximum pressure reached about 5.8 MPa (see Figure 7). In Figure 6, the power \(P\) was gradually increased to 130 kW, and the flow rate \(Q_{H2O}\) was increased to 1.5 L/s.