# Numerical and Experimental Study of a Large-Scale Natural Circulation Helium Loop
## 4 Results
The calculated parameters \(t_{He1}\), \(t_{He8}\), and \(p_{He1}\) showed good agreement with the experimental data. Figure 7 presents interval 2 of scenario 4. The interval started at 20,000 s and represented the reactor model's power failure. Figure 8 shows interval 1 of scenario 5. The start of the interval was at 2000 s, and its duration was 10,200 s. Throughout the time interval, the electrical power \(P\) of the model generator increased to 149 kW, and the flow rate \(Q_{H2O}\) of the cooling water increased to 1.48 L/s. For this heating and cooling method (the power input increased in 20-30 kW steps), we observed at the end of the interval that the measured and calculated temperature data \(t_{He1}\) differed by about 30 \({}^{\circ}\)C. The measured and calculated pressure values \(p_{He1}\) diverged between 0.3 and 0.5 MPa throughout the interval (the calculation predicted lower values than those of the measurement). Figure 9 shows interval 2 of scenario 5. The interval started at 15,000 s and showed the effects of the power drop in two steps; first, it decreased to 100 kW, and then to zero after 6300 s. The flow rate \(Q_{H2O}\) of the cooling water was unchanged at 1.5 L/s. The figure shows the response of the temperatures \(t_{He1}\) and \(t_{He8}\) and the pressure \(p_{He1}\) to the power changes made. We can see that both the calculated and  
measured values were sensitive to these changes. In Figure 10, interval 1 of scenario 6 is at 1000 s and lasts 7200 s. The increase in power \(P\) of the reactor model to 120 kW occurred in four steps. The cooling-water flow rate \(Q_{H2O}\) increased from 0.5 to 0.8 L/s only at the end of the interval. The temperature and pressure curves--with small deviations between the calculations and measurements--tracked the changes in the power input. Figure 11 shows interval 2 of scenario 6. It started at 13,000 s and ended at 23,800 s. The power \(P\) first dropped from 105 to 80 kW and then to zero. At the same time, the flow rate \(Q_{H2O}\) of the cooling water changed from 1.05 to 1.55 L/s. The calculated temperature and pressure waveforms responded to these changes in good agreement. Figure 12 shows interval 1 of scenario 8 with its start at 800 s. At 810 s, there was a step in power \(P\) from zero to 60 kW. The flow rate \(Q_{H2O}\) of the cooling water was held at 0.7 L/s throughout the observed interval. In the next steps, the power input was gradually increased up to a maximum of 87 kW and gradually decreased to 78 kW at the end. The helium charge was small, with a maximum pressure of \(p_{He1}=2.48\) MPa (significantly less than in the other scenarios) at a maximum temperature of \(t_{He1}=530\)\({}^{\circ}\)C (hot branch). The differences between the calculation and the measurement in the case of temperature \(t_{He1}\) were significant, especially in the first part, where the effect of the increase in the power input \(P\) was evident. The cooling effect of the smaller amount of helium was small, and a large part of the energy was consumed to heat the solid parts of the loop (piping, structure, and containment). This phenomenon (energy accumulation in the solid parts of the loop) was captured with lower accuracy by the mathematical model. This led to the calculated temperature \(t_{He1}\) being underestimated in the first part and overestimated relative to the measurement at the end of the interval. Figure 13 shows interval 2 of scenario 8. The start was at 15,000 s. The power decreased from 78 to 60.5 kW in the first step and then dropped to zero at 20,280 s. We observe what we saw in interval 1 of this scenario: When the power input changed abruptly, there were much larger differences between the calculated and measured temperatures \(t_{He1}\) and \(t_{He8}\). This was due to the smaller helium charge, for which the energy accumulation in the solid parts of the system was more pronounced. The difference between the calculations and measurements was also reflected in the pressure \(p_{He1}\) through the temperatures. Figure 14 shows interval 1 of scenario 13.