atw 2017-12

More documents

Recommendations

Info

atw Vol. 62 (2017) | Issue 12 ı December ENVIRONMENT AND SAFETY 748 | | Fig. 2. Core nodalization. | | Fig. 3. Event Tree for the Representative Sequence. framework in addressing the uncertainties associated with both phenomena (epistemic) and scenarios (aleatory). In this paper, we use similar approach to the in-vessel phase of SAM evaluation. 2.3 MAAP model and nodalization The Modular Accident Analysis Program (MAAP) has been used to simulate the various severe accident phenomena including actions taken as part of the Severe Accident Management Guidelines (SAMGs). MAAP5, and its predecessor MAAP4, have been used widely by the nuclear industry throughout the world. In this study, MAAP5 version 5.04 released in 2016 is used [15]. The RCS for the target plant was simulated with the nodalization for CE type plants already provided in MAAP5. The core model is modeled using 7 rings (channels) and 13 rows as shown in Figure 2. 2.4 Initial Event and Representative Sequence The initial event and scenario is based on the PSA results of APR1400 [4] because it is assumed that the target plant has all of the latest safety systems in APR1400. SBO is selected as the initial event with the highest cumulative CDF as shown in Table 1 [4]. With a SBO as the initiating event, a conservative accident sequence has been set assuming that all safety related functions fail. Thus, the representative sequence considers the failure of alternative alternating current (AAC) power source, the failure of secondary heat removal with Turbine-Driven Auxiliary Feed Water Pumps (TDAF WPs), no RCP seal leakage, and the failure of offsite power recovery within 72 hours as shown in Figure 3. It should be noted that the Westinghouse's KSB type F RCP seal has been commercialized recently. It is designed to maintain the integrity for 72 hours even in a high-temperature and high-pressure environment of 573 K (572 F) and 16 MPa (2335 psia), respectively. This seal has been applied to the latest APR1400 design [14] and it is therefore assumed that this RCP seal is applied to the target NPP and as such no RCP seal leakage is assumed in this analysis. An analysis was performed to obtain the baseline information of the accident progress for the representative sequence. Figure 4 shows the RCS pressure and core exit temperature for the representative case without implementation of any SAM actions. As shown in the figure, the SAM phase starts at 2.5 hours. The SAM entry condition is activated when the core exit temperature is higher than 922 K (1200 F) in accordance with the SAMG of APR1400 [15]. According to the model the first relocation, a phenomenon in which the melt inside the core slumped to the lower plenum of RPV, occurs at 4.8 hours, and the failure of the reactor pressure vessel (RPV) is predicted to occur at 5.3 hours. 2.5 Model assumptions and case studies Following the SAM strategy, RCS depressurization is first implemented followed by in-vessel injection. RCS depressurization is achieved by the opening of POSRVs [16]. For the representative scenario, the battery can be used for 16 hours, and the power cannot be recovered until 72 hours. Therefore, it is assumed that the primary side injection is achieved by the external injection using the FLEX portable pumps [16]. Through the above SAM strategy and assumptions, the effect of depressurization timing is analyzed. The strategy for in-vessel external injection is examined with respect to the injection flow rate and timing. Furthermore, in terms of in-vessel phase of SAM, key parameters affecting the core relocation, EPSCUT and TCLMAX, are selected. The sensitivity analyses are performed to examine the effect for the variation of those parameters on the results. 2.5.1 Impact of depressurization timing The depressurization timing is varied on a 30 minutes time interval, starting from 3 hours. The impact of depressurization timing is assessed by analyzing a total of six cases (A01 to A06) as shown in Table 2. 2.5.2 Impact of injection timing and flow rate The injection timing may be started from the SAM entry point all the Initiator Frequency (/year) Percent Contribution (%) Station BlackOut (SBO) + Loss Of Offsite Power (LOOP ) Total Loss Of Component Cooling Water (TLOCCW) + TLOESW (Total Loss Of Essential Service Water) 5.11×10 -7 39.4 1.58×10 -7 12.2 Medium Loss Of Coolant Accident (MLOCA) 1.23×10 -7 9.5 Partial Loss Of Component Cooling Water (PLOCCW) 1.22×10 -7 9.4 | | Fig. 4. Pressure and Core exit temperature for the representative case without SAM actions. Partial Loss Of Essential Service Water (PLOESW) 7.04×10 -8 5.4 | | Tab. 1. CDF Contribution by Initiating Events (APR1400) [4]. 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 Case no. A01 A02 A03 A04 A05 1 Onset of severe accident 2.5 hr (9003 sec) 2.5 hr (9003 sec) 2.5 hr (9003 sec) 2.5 hr (9003 sec) 2.5 hr (9003 sec) 2 Depressurization timing 3 hr (10804 sec) 3.5 hr (12604 sec) 4 hr (14405 sec) 4.5 hr (16205 sec) 5 hr (17926 sec) way to the reactor vessel failure. The fastest injection is initiated one hour following the SAM entry, to account for the time required to open the POSRV, confirm the RCS depressurization, and align the FLEX portable pump for the external injection. To determine the ranges of injection flow, Equation 1 is used to calculate the flow rates in con sideration of the heat removal by the vaporization, the saturation, and the subcooling of the injected water, respectively [17]. (1) where W is the water injection flow rate, ˙Q 0 is the decay power level before shutdown, t is the time elapsed since shutdown, c Pw is the water specific heat, ∆T sub is the difference between the saturation temperature at the RPV pressure and the water injection temperature, and h is the enthalpy of gas (g) and fluid (f), respectively. The temperature of the injected water is assumed to be 311 K (100 F) as the initial condition. For the minimum flow rate, the injected water is assumed to change completely to vapor. If the subcooling of the incoming water in Equation 1 is ignored, about 10 kg/sec is determined as the minimum required value. Alternatively, if only sensible heat removal is allowed (ignoring the enthalpy change term in Equation 1), 30 kg/sec is required to cool the core by single phase flow. For sufficient subcooling margin, 50 kg/sec is considered as the maximum flow rate in this analysis. Referring to the document related to the in-vessel injection of FLEX from NEI, there is a commercial pump that can supply up to a range of 50 kg/sec [18]. It should be noted that for each 3 Onset of SIT injection 3.2 hr (11404 sec) 3.7 hr (13205 sec) 4.2 hr (15005 sec) 4.7 hr (16805 sec) 5.1 hr (18226 sec) | | Tab. 2. Analysis Results according to Depressurization Timing. 4 Time of first relocation to RPV LP 9.2 hr (33188 sec) 6.3 hr (22748 sec) 6.7 hr (24000 sec) 6.5 hr (23393 sec) 4.8 hr (17395 sec) 5 Time of vessel failure 11.1 hr (39786 sec) 7.8 hr (28097 sec) 7.9 hr (28468 sec) 7.8 hr (28045 sec) 10.3 hr (37036 sec) of the assumed flow rates, the adequacy of onsite water resources have been checked for 72 hours. 2.5.3 Assessment of phenomenological uncertainties The most important phenomenon during the in-vessel phase is the core melting and relocation process given their potential impact on reactor vessel integrity. Therefore, it is necessary to examine the uncertainty about the parameters affecting these phenomena. Two parameters are important in this regard, EPSCUT, which is the cutoff porosity below which the flow area and the hydraulic diameter of a core node are zero, and TCLMAX, which is the temperature that will lead to cladding rupture. MAAP5 simulates the melting process as a progression between five different configurations (types), each having a characteristic shape, porosity, and surface area as shown in Figure 5. | | Fig. 5. Types of Solid Core Geometry during Melt Progression [15]. 6 Maximum molten mass in core 7 Mass of total debris bed in RPV lower plenum 8 Period from the onset of severe accident to first relocation to LP 40.7 ton 63.5 ton 6.7 hr (24185 sec) 55.7 ton 104.4 ton 3.8 hr (13745 sec) 65.6 ton 100.1 ton 4.2 hr (14997 sec) 64.4 ton 102.4 ton 4 hr (14390 sec) 43.4 ton 116.2 ton 2.3 hr (8392 sec) As the type number increases, the porosity and the flow area decrease. While type 1 denotes an intact fuel rod, type 5 means the molten pool has completely melted. When the core melting begins, the mass of each core node moves gradually to the bottom (candles down). When the porosity of the core node becomes 0.1, which is the default value obtained by benchmarking the TMI accident in MAAP5, it becomes a block node that is no longer mass transferable as type 4 [19]. Therefore, changing this cutoff porosity value (EPSCUT in MAAP5) can change the core melting and relocation process. The default value of EPSCUT is 0.1, the minimum value is 0, and the maximum value is 0.25 according to MAAP5 manual. The clad rupture also affects the core melting and relocation process. When the clad temperature reaches to 2,500 K, the core clad rupture could occur. After the clad rupture, the mass relocation to the lower plenum of vessel throughout the bypass region or downcomer occurs radially in the blocked node as shown in Figure 6. | | Fig. 6. Breach of Side Crust of Molten Pool [15]. 9 Period from the onset of severe accident to vessel failure 8.6 hr (30783 sec) 5.3 hr (19094 sec) 5.4 hr (19465 sec) 5.3 hr (19042 sec) 7.8 hr (28033 sec) ENVIRONMENT AND SAFETY 749 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
Page 1: nucmag.com 2017 12 ISSN · 1431-525
Page 4 and 5: atw Vol. 62 (2017) | Issue 12 ı De
Page 68: Atkins Energy Germany GmbH Wir habe

atw 2017-12

Create successful ePaper yourself

Delete template?

Save as template?