# Numerical and Experimental Study of a Large-Scale Natural Circulation Helium Loop
## 4 Results
The interval started at 500 s at a pressure of \(p_{He1}=4.12\) MPa and at temperatures of \(t_{He1}=110\)\({}^{\circ}\)C and \(t_{He8}=28\)\({}^{\circ}\)C. This part of scenario 13 was characterized by the power \(P\) increasing in two steps from 49 to 132 kW and then decreasing in three steps to 70 kW. At the simulation time of 4 s, the flow rate \(Q_{H2O}\) of the cooling water started to rise gradually from 0.9 to 1.5 L/s. The calculated temperatures \(t_{He1}\) and \(t_{He8}\) were overestimated, resulting in a maximum overestimation of the calculated pressure \(p_{He1}\) of 0.6 MPa at 6000 s (peak of the curve). The calculated temperature values responded to the increasing and decreasing electrical input of the model generator in accordance with the measured values--that is, at the end of interval 2 at 8600 s, the temperature values dropped to \(t_{He1}=300\)\({}^{\circ}\)C and \(t_{He8}=145\)\({}^{\circ}\)C.  
Figure 15 shows interval 2 of scenario 13. The interval started at 10,000 s with measured values of \(t_{He1}=290\)\({}^{\circ}\)C, \(t_{He8}=115\)\({}^{\circ}\)C, and \(p_{He1}=5.5\) MPa. The electric power \(P\) was constant throughout interval 2 (within 22,600 per second) with \(P=70\) kW. In the monitored interval, only the flow rate \(Q_{H2O}\) of the cooling water was reduced from a value of 1.5 L/s to values of 0.9, 0.7, and 0.6 L/s with varying durations of the reduced flow rate. However, the cooling changes had only a negligible effect on the measured values, with the temperature of \(t_{He1}\) remaining in the range of 290 \({}^{\circ}\)C to 310 \({}^{\circ}\)C and the temperature of \(t_{He8}\) remaining in the range of 115 \({}^{\circ}\)C to 120 \({}^{\circ}\)C. Consistent with this was the pressure history of \(p_{He1}\), which did not exceed 5.5 MPa. The calculated quantities confirmed this trend. All of the investigated intervals of each scenario were statistically processed. Data were sampled according to the specified time step, and relative errors were calculated at each timestamp. The average values for each interval are provided in Table 1.  
The results show that the mathematical model's response was slightly different from the physical model's response, which was more significant in the reactor shutdown simulation (decrease in the power input \(P\), interval 2). This phenomenon resulted in higher relative errors in interval 2 when compared to that in interval 1. The accuracy of pressure prediction was highest among the monitored variables and varied from 1.0 to 6.8%. The hot branch temperature errors were between 2.2 and 21.2%, and the cold branch temperature errors ranged between 2.0 and 18.0%. The higher errors in the second intervals may have stemmed from one-dimensional model limitations. The sudden change in the value of the boundary condition (power input \(P\)) probably did not only lead to a higher rate of numerical error, but the modeled helium flow also became unstable, as it switched from a relatively stable turbulent regime to an unstable transient regime, which possibly magnified the three-dimensional effects, such as backflows, swirls, or velocity profile distribution non-uniformity. The heat energy exchange between the solid parts of components and the flowing medium probably became more complex and exceeded the model's simplifications. All of these effects also cast a shadow of uncertainty on the accuracy of the measurement devices. Figure 15: Scenario 13; interval 2. The effects of the sudden changes in boundary conditions should not have been so significant during interval 1. Nevertheless, the errors could also stem from the flow regime change and non-uniform heat accumulation in all circuit components.