Analysis of simulation results

As the simulation results indicate, a large LOCA can cause dramatic increase in reactor power and temperature. The positive reactivity brought by rapid voiding of PHT system destroys the thermal balance within the core. The shutdown system reacts to this situation and inserts sufficient amount of negative reactivity (-80 mk) to stop the chain reaction. That is why the power transient starts to drop quickly soon after the shutoff rods insertion. Thus, the sooner the SDS1 is activated, the smaller the power peaks will be.

By comparing the slopes of the transient curves, one can also note that the rate at which the transient rises for 10.5 ms shutdown time is much smaller than that of 100.0 ms

response time. Slower power increasing produces a lower peak in the transient. The difference between the two peaks, as described in Figure 5.11, is 1.778-1.5082 = 0.2698. In other words, it represents 27% reduction in the power surge peak, which means that 555.15 MJ less amount of heat has to be removed from the core eventually according to Table 5.2. Without getting into specifics of the safety limit in an operating reactor, it is evident that lower power peaks will be beneficial to safety margins.

Although the sheath temperature does not reach the peak value within this simulation time range, the temperature transient in 10.5 ms case does show improvement according to the simulation results. Moreover, within the 9-second simulation process the amount of

heat generated in 10.5 ms case is 6,649.04 MJ. It is 555.15 MJ less than the 7,204.19 MJ

of 100.0 ms case. Comparison of these two cases is summarized in Table 5.2. Table 5.2 Comparison of key parameters under two shutdown cases

Case Delay Time (ms) Power Peaks Temp. Peaks (℃℃℃℃) Heat (MJ) Time to Peak (s)

1 100.0 1.778 931.79 7, 204.19 0.839

2 10.5 1.5082 909.05 6, 649.04 0.757

∆ _{89.5 0.2698 22.74 555.15 0.082}

Although it has been shown that shorter response time produces lower peaks in power surges, how the variation of the response time affects the peak of surge is yet to be investigated. Further analysis on the relationship between the decision-making response time and the peak of the power surge under a large LOCA has been carried out. Simulations under different response time varying from 10.5 ms to 200.0 ms are performed. Based on the simulation results, the relationship between the response time and the power peak can be approximated by the following linear equation (5.1):

0.003 1.4765

peak d

P = t + (5.1)

where

peak

P is the peak value (in normalized power) of the thermal power transient as result of a LOCA; t_d is the decision-making time (in the unit of millisecond) needed by the trip channel, known as the response time.

Since the power surge peaking time is also of importance to the safety analysis, a formula to predict the surge peak time is also established based on the simulation results as follows (5.2): 0.0008 0.7547 peak d T = t + (5.2) where peak

T is the time (in the unit of second) that the thermal power transient reaches its peak after the LOCA is initiated; t_d is the decision-making time (in the unit of millisecond) needed by the trip channel, known as the response time.

The sampled data and the curve fitting results for these two relationships are illustrated in Figure 5.12. 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 S u rg e P ea k 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 2000.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 Response Time (ms) P ea k T im e (s ec )

data for surge peak data for peak time curve fitting for surge peak curve fitting for peak time

These two linear relationships clearly illustrate why shortening the response time of a shutdown system can be beneficial to mitigate the impact of an accident and to contribute to the NPP safety. In other words, by shutting down the chain reaction faster, less decay heat will be generated, which effectively increase the operational safety margin. Since, by definition, the safety margin of a particular system variable is defined as the difference between the operating values of this variable and its safety limit, the safety margin of reactor power is directly related to the operating power level. One can potentially increase the power level without scarifying the safety margins, i.e. maintain the same level of power surge, if the response time of the shutdown system can be shortened.

5.4 Summary

In this chapter, the test environment and test cases used in this work are described to show the evaluation methodologies.

The simulated results, for both the functionality and timing performance, are presented along with the analysis. The FPGA-based trip channel responds properly to the postulated “SG low level” scenario by issuing the expected trip signal. The reactor is then tripped at the moment when SG level goes below the predefined setpoint. The statistical analysis of the timing performance has highlighted the advantage of an FPGA-based trip channel as compared to an SDS1 controller utilized in an existing CANDU NPP.

On the basis of the timing evaluation, CATHENA simulation is carried out to quantify the safety margin improvement. Relationship between the response time and critical parameters, i.e. peak power surge, is established through data analysis.