atw 2017-12

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atw Vol. 62 (2017) | Issue 12 ı December ENVIRONMENT AND SAFETY 750 TCLMAX is the temperature used to calculate for the cladding ruptures in MAAP5. The default value of this variable is 2,500 K, the minimum value is 2,200 K, and the maximum value is 2,700 K [15]. A sensitivity study is therefore conducted to estimate the impact of these two variables on the analysis results. 3 Results 3.1 Effect of Depressurization Time on Core Relocation Behavior Table 2 summarizes the key findings for different depressurization timing. The first column shows the onset of the severe accident. The second column is the timing of depressurization using POSRVs with an interval of 0.5 hour. The third column shows the injection time of the safety injection tank (SIT). The fourth column shows the first core relocation time slumping to the lower plenum of the RPV. Comparing the SIT injection time to the time of first relocation, it can be seen that the SIT flow alone cannot prevent the first core relocation, which is related to the loss of core integrity. For case A01, depressurization is carried out at 0.5 hour after SAM entry. The model predicts the first relocation time to the lower plenum of RPV to be 8.2 hours and the vessel failure to be 11.1 hours. This case marks the slowest accident progression in terms of the first relocation and vessel failure. Additional cooling water injection is required since the recovery of core water level due to SIT injection is incomplete as shown in Figure 7. A02 to A05 cases are depressurized at 1 hour to 2.5 hours, with 0.5 hour interval after SAM entry. For these cases, SIT injection time is delayed and the time to cool the core is insufficient as the depressurization is performed later. Accordingly, the progress of the accident is faster for these cases. The sixth column in Table 2 is the maximum mass of the melt produced in the core. With delayed depressurization, the maximum mass of melt produced in the core increases from ~ 40 tons for case A01 to ~ 66 tons for case A03 and then decreases to ~ 43 tons for case A05. Figure 8 shows the molten mass inside the core over time but there is no consistent trend. The seventh column is the total mass of melt accumulated in the lower plenum of RPV due to the failure of the core bottom plate or the sideward relocation until the vessel failure. Again, the smallest value is predicted for case A01, however no specific trend can be inferred from the simulated cases. The eighth and ninth columns indicate the time of the first relocation and the vessel failure measured from SAM entry. The late opening of POSRV shortens the time available for operators to take action and also increases the amount of melt in the RPV lower plenum, which is disadvantageous from a coolability perspective. It could be inferred from these results, that the early depressurization is desirable because it can delay the first relocation. Therefore, | | Fig. 7. Water Level in Core for A01 Case. | | Fig. 8. Molten mass in core for cases A01 to A05. Case no. In-vessel injection flow rate Onset of in-vessel injection Vessel Failure Maximum molten mass in core Molten mass in core at 72 hours Mass of total debris bed in RPV lower plenum B01 10 kg/sec 3.5 hr (12604 sec) Not occurred 6 ton 0.1 ton B02 10 kg/sec 4.5 hr (16204 sec) Not occurred 6 ton 0.1 ton B03 10 kg/sec 5.5 hr (19804 sec) Not occurred 6 ton 0.1 ton B04 10 kg/sec 6.5 hr (23404 sec) Not occurred 6 ton 0.1 ton B05 10 kg/sec 7.5 hr (27004 sec) Not occurred 6 ton 0.4 ton B06 10 kg/sec 8.0 hr (28804 sec) Not occurred 8 ton 1.4 ton B07 10 kg/sec 8.5 hr (30604 sec) Not occurred 50.5 ton 49.3 ton B08 10 kg/sec 9.0 hr (32404 sec) Not occurred 67.9 ton 61.9 ton | | Tab. 3. Analysis Results according to In-Vessel Injection Timing with Flow Rate of 10 kg/sec. The first relocation to LP has not occurred. B09 10 kg/sec 9.5 hr (34125 sec) Not occurred 40.7 ton 0 ton 53.9 ton B10 10 kg/sec 10.0 hr (35988 sec) Not occurred 40.7 ton 0 ton 64.9 ton B11 10 kg/sec 10.5 hr (37789 sec) Not occurred 40.7 ton 0 ton 69.5 ton B12 10 kg/sec 11.0 hr (39590 sec) Not occurred 40.7 ton 5.2 ton 66.5 ton Environment and Safety Analysis of the In-Vessel Phase of SAM Strategy for a Korean 1000 MWe PWR ı Sung-Min Cho, Seung-Jong Oh and Aya Diab
atw Vol. 62 (2017) | Issue 12 ı December | | Fig. 9. Molten Mass in Core for Cases B01 to B08 (with in-vessel injection before the first relocation to lower plenum). it is recommended to open the POSRVs within 30 minutes after SAM entry. 3.2 Effect of in-vessel injection on core relocation The effect of in-vessel injection on core relocation analysis is conducted based on A01. Three injection flow rates of 10, 30 and 50 kg/sec are examined. The effect of injection timing is investigated for each of the aforementioned flow rates. First, 10 kg/sec injection cases are examined (B series). For case B01, we assume that the injection can begin in 1 hour into SAMG entry, 30 minutes after the depressurization condition, then we increase the injection timing by 1 hour for cases B01 to B05 and by 30 minutes for case B06 to B12. Table 3 shows the results of the analysis for the injection flow rate of 10 kg/sec. When the flow rate of 10 kg/sec is injected before the first relocation in A01, the first relocation slumping to the lower plenum of RPV does not occur for 72 hours. As the core materials begin to melt inside the core, the molten mass gradually increases. In the cases of B01 to B06, the maximum amount of melt accumulated in the core is less than 8 tons. Most of the molten cores are then solidified again and 1.4 tons or less of the melt is present in the core at 72 hours. Figure 9 shows the amount of the melt generated in the core with 10 kg/s injection rate initiated at different timings, before the first relocation to the lower plenum. B07 and B08 cases show that larger amount of the melt are generated in the core (50.5 tons and 67.9 tons, respectively) compared to other cases (B01 to B06) for which only 6 to 8 tons are predicted. This is attributed to the prediction (or the lack of) type 5 nodes in the core. According to MAAP5, existence of type 5 core node indicates a fully molten mass. The sooner the in-vessel injection is implemented, the less likely it is that a type 5 node will occur (cases B01-B06). Figure 10 shows the configurations of the core node at 72 hours for cases B05 and B08. In contrast to case B05, for B08 with late in-vessel injection, the molten mass is relatively large with a number of completely melted (type 5) nodes. In contrast, B09 to B12 cases where the in-vessel injection is deployed after the occurrence of the first relocation, exhibit lesser amount of melt accumulated in the core (40.7 tons). Since some of the molten material | | Fig. 11. Total Debris Mass in RPV Lower Plenum for Case B08 to B11 (In-vessel injection after the first relocation to lower plenum). created in the core had already been relocated to the RPV lower plenum at different rates depending on the case. Figure 11 shows that the melt accumulated in the lower plenum and retained for 72 hours without further mass change. Table 4 and Table 5 show the results of 30 kg/sec (C series) and 50 kg/sec injection rates (D series), respectively. As the injection flow increases, the melt cooling becomes more effective, so the maximum mass of the melt accumulated in the core becomes less as depicted in C07, C08, D07, and D08. The total debris mass accumulated in the lower plenum also decreases as depicted in C09 to C12 and D09 to D12 cases. 3.3 Comparison for the Molten Mass in the RPV Lower Plenum The molten masses accumulated in the RPV lower plenum for all cases of the in-vessel injection analysis are summarized in Figure 12. No relocation is predicted for cases B01 to B08. Melt relocation is observed for cases B09 to B12 with much delayed injection. For these latter cases, the lowest injection flow rate, yields a maximum mass of about 70 tons of melt, while about 64 tons is observed for C and D cases with higher injection rates. ENVIRONMENT AND SAFETY 751 | | Fig. 10. Core Node Map at the End of Simulation for Case B05 (left) and Case B08 (right). 3.4 Input parameter sensitivity analysis A sensitivity analysis is performed to assess the impact of model input parameters on the amount of melt accumulated in the RPV lower plenum and the occurrence of reactor vessel failure. EPSCUT and TCLMAX, as representative variables affecting the core melting and relocation process, Environment and Safety Analysis of the In-Vessel Phase of SAM Strategy for a Korean 1000 MWe PWR ı Sung-Min Cho, Seung-Jong Oh and Aya Diab
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