atw 2017-12

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atw Vol. 62 (2017) | Issue 12 ı December ENVIRONMENT AND SAFETY 752 Case no. In-vessel injection flow rate Onset of in-vessel injection Vessel Failure | | Tab. 4. Analysis Results according to In-Vessel Injection Timing with Flow Rate of 30 kg/sec. Maximum molten mass in core Molten mass in core at 72 hours C01 30 kg/sec 3.5 hr (12604 sec) Not occurred 6 ton 0.1 ton C02 30 kg/sec 4.5 hr (16204 sec) Not occurred 6 ton 0.1 ton C03 30 kg/sec 5.5 hr (19804 sec) Not occurred 6 ton 0.1 ton C04 30 kg/sec 6.5 hr (23404 sec) Not occurred 6 ton 0.1 ton C05 30 kg/sec 7.5 hr (27004 sec) Not occurred 6 ton 0.4 ton C06 30 kg/sec 8.0 hr (28804 sec) Not occurred 6 ton 0.2 ton C07 30 kg/sec 8.5 hr (30604 sec) Not occurred 15.8 ton 9.4 ton C08 30 kg/sec 9.0 hr (32404 sec) Not occurred 51.9 ton 50.3 ton Mass of total debris bed in RPV lower plenum The first relocation moved to LP has not occurred. C09 30 kg/sec 9.5 hr (34125 sec) Not occurred 40.7 ton 0 ton 30.3 ton C10 30 kg/sec 10.0 hr (35988 sec) Not occurred 40.7 ton 0 ton 50.4 ton C11 30 kg/sec 10.5 hr (37789 sec) Not occurred 40.7 ton 0 ton 62.5 ton C12 30 kg/sec 11.0 hr (39590 sec) Not occurred 40.7 ton 3.4 ton 64.1 ton 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 D01 50 kg/sec 3.5 hr (12604 sec) Not occurred 6 ton 0 ton D02 50 kg/sec 4.5 hr (16204 sec) Not occurred 6 ton 0.1 ton D03 50 kg/sec 5.5 hr (19804 sec) Not occurred 6 ton 0 ton D04 50 kg/sec 6.5 hr (23404 sec) Not occurred 6 ton 0 ton D05 50 kg/sec 7.5 hr (27004 sec) Not occurred 6 ton 0.1 ton D06 50 kg/sec 8.0 hr (28804 sec) Not occurred 6 ton 0.1 ton D07 50 kg/sec 8.5 hr (30604 sec) Not occurred 13.7 ton 7 ton D08 50 kg/sec 9.0 hr (32404 sec) Not occurred 55.6 ton 48.4 ton Mass of total debris bed in RPV lower plenum The first relocation moved to LP has not occurred. D09 50 kg/sec 9.5 hr (34125 sec) Not occurred 40.7 ton 0 ton 26.8 ton D10 50 kg/sec 10.0 hr (35988 sec) Not occurred 40.7 ton 0 ton 47.4 ton D11 50 kg/sec 10.5 hr (37789 sec) Not occurred 40.7 ton 0 ton 61.9 ton D12 50 kg/sec 11.0 hr (39590 sec) Not occurred 40.7 ton 3.1 ton 63.8 ton | | Tab. 5. Analysis Results according to In-Vessel Injection Timing with Flow Rate of 50 kg/sec. are examined. For B01 to B12 cases, ten sensitivity analyses are conducted by changing EPSCUT from 0.00 to 0.25 with a step of 0.05, keeping TLCMAX parameter at the default value of 2,500 K. Additionally, TCLMAX has been varied from 2,200 K to 2,700 K, keeping the | | Fig. 12. Total Debris Mass Accumulated in Lower Plenum as a Function of Timing for Different In -Vessel Injection Flow Rates. EPSCUT parameter at the default value of 0.1. The sensitivity of the debris mass accumulated in the RPV lower plenum to these parameters is shown in Figure 13. The accumulated mass generally increases as the invessel injection timing is delayed, however, the spread can be significant for different EPSCUT and TCLMAX values. Unlike the base cases, the sensitivity study predicts the first relocation to the lower plenum for B06, B07, and B08. In accordance with the base case, B01 to B05, exhibit no relocation to the lower plenum irrespective of the values of EPSCUT and TCLMAX. No specific tendency can be derived but the results show the phenomenological uncertainty. Therefore, considering those uncertainties, it is necessary to initiate the in-vessel external injection within 7.5 hours (5.0 hours after entry of a severe accident) to maintain the RPV integrity without the first relocation to the lower plenum. The success region for the external injection flow rate and timing is shown in Figure 14. It is given that the POSRVs are opened within 30 minutes after the entry to SAM. The first relocation to the RPV lower plenum is not occurred if injection is initiated within 5.0 hours after the entry to SAM at flow rates ranging from 10 to 50 kg/sec. The phenomenological uncertainties considered indicate that 1.5 hours of uncertainty regions of the core relocation. The mass fraction of metallic layer in the RPV lower plenum is shown in Figure 15. It ranges from 14 % to 22 % in general. Changing the sensitivity parameters changes the time of the first relocation to the RPV lower plenum. Figure 16 shows the results of Figure 13 as a rearrangement based on the time of first relocation. As a result, the in-vessel injections before the first relocation time show the total debris in lower plenum from 28 to 80 tons. The in-vessel injections after the first relocation time show 51 to 114 tons. 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. 13. Debris Mass Changes in Sensitivity Analysis. | | Fig. 14. Success Region for In-Vessel Injection without the first relocation to the RPV lower plenum. ENVIRONMENT AND SAFETY 753 | | Fig. 15. Metallic Layer Mass Fraction in RPV lower plenum. | | Fig. 16. Total Debris Mass Rearrangement based on the first relocation time. 4 Summary and conclusions In this paper, a sensitivity study is performed to the in-vessel phase of SAM for a Korean 1000 MWe NPP using ROAAM+ framework. The selected scenario is SBO with RCS depressurization followed by in-vessel external injection. The impacts of injection timing and flow rate as well as the uncertainties associated with the core melting and relocation process have been examined. The main conclusions that could be drawn from this work are summarized as follows: • In order to implement the in-vessel phase of SAM strategy, it is recommended to open the POSRVs within 30 minutes after the entry of a severe accident. • If the external injection with the flow rate of more than 10 kg/sec into the vessel performed within 5.0 hours after the entry of a severe accident, the molten mass is retained in the core. The first relocation to the RPV lower plenum is not occurred. • When the reactor vessel integrity is maintained through the derived SAM strategy, the maximum amount of melt accumulated in the RPV lower plenum is about 114 tons. The derived SAM strategy and the information on core melting and relocation may be used as the initial conditions for the next phase of SAM evaluation such as ERVC and the ex-vessel phase in the future. Acknowledgments This study was supported by the 2017 research fund of the KEPCO International Nuclear Graduate School (KINGS), Republic of Korea. References [1] Westinghouse, Status report 81 – Advanced Passive PWR (AP 1000), IAEA, 2011. [2] Gidropress, Status report 93 – VVER-1000, IAEA, 2011. [3] AREVA NP & Mitsubishi Heavy Industries, Status Report – ATMEA1TM, IAEA, 2015. [4] KEPCO & KHNP, CHAPTER 19 PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION, U.S. NRC, 2014. [5] KEPCO & KHNP, Status Report 103 – Advanced Power Reactor (APR1000), IAEA, 2011. [6] T. G. Theofanus, On Proper Formulation of Safety Goals and Assessment of Safety Margins for Rare and High- Consequence Hazards, in Reliability Engineering & System Safety, 1996. [7] W. Y. S. A. J. S. K. F. X. C. T. S. T.G. Theofanous, Lower head integrity under steam explosion loads, in Nuclear Engineering and Design, 1999. [8] C. L. S. A. S. A. O. K. T. S. T.G. Theofanous, In-vessel coolability and retention of a core melt, in Nuclear Engineering and Design, 1997. [9] P. K. Sergey Galushin, Analysis of core degradation and relocation phenomena and scenarios in a Nordic-type BWR, in Nuclear Engineering and Design, 2016. [10] W. V. P. K. C.-T. T. A. Goronovski, Effect of Corium Non-Homogeneity on Nordic BWR Vessel Failure Mode and Timing, in ICAPP-2015, 2015. [11] S. L. N. K. P. Yakush, Risk and uncertainty quantification in debris bed coolability, in NURETH 15, 2013. [12] D. B. S. K. P. Grishchenko, Development of TEXAS-V code surrogate model for assessment of steam explosion impact in Nordic BWR, in NURETH-16, 2015. [13] S. G. Pavel Kudinova, A FRAMEWORK FOR ASSESSMENT OF SEVERE ACCIDENT MANAGEMENT EFFECTIVENESS IN NORDIC BWR PLANTS, in Probabilistic Safety Assessment and Management PSAM 12, 2014. [14] Westinghouse Electric Company, APR 1400 RCP Seal Design and ELAP Capability, 2014. 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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