Computer-Aided Analysis of Bypass in Direct Vessel Vertical Injection System

Transcription

1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1220 Computer-Aided Analysis of Bypass in Direct Vessel Vertical Injection System Yong H. Yu 1, Sang H. Yoon 2, Kune Y. Suh 1,2* 1 PHILOSOPHIA, Inc. San 56-1 Sillim-Dong, Kwanak-Gu, Seoul, , Korea 2 Seoul National University San 56-1 Sillim-Dong, Kwanak-Gu, Seoul, , Korea Recent advances in three-dimensional computer-aided design has rendered it practicable to lend itself to commercial computational fluid dynamics and structural mechanics finite element analysis codes to cross-communicate detailed geometric data. The main focus is placed on application of this powerful design tool in examining performance of the emergency core cooling system (ECCS) employing the direct vessel injection (DVI) in the APR1400 (Advanced Power Reactor 1400MWe). In this work a new method of direct vessel vertical injection (DVVI) is proposed as an advanced type of DVI for the APR1400. In view of the ECCS function to efficaciously cool down the core during a loss-of-coolant accident (LOCA), for example, the concept of DVVI is geared to increasing the downward momentum of the injected water so as to maximize penetration into the downcomer versus bypass through the break. Furthermore, one needs to pay a special attention to the structural integrity of the injection nozzle on account of the radioactive primary system environment. In particular, vibration and pressure induced by the safety injection (SI) flow will tend to exert predomination forces onto the DVVI nozzles having an elbow. Therefore, in this work, focus is placed on the flow regime at the tip of the DVVI nozzle and the structural impact caused by rapid injection of the emergency core cooling (ECC) water in the APR1400. The current work consists of a three-dimensional (3D) computer-aided design (CAD) modeling using CATIA V5R9, CFX 5.5 and ANSYS 6.1 to determine the most vulnerable spot of the DVVI nozzle, and to check on the performance efficiency of DVVI during the ECC injection period. In this study it was considered that the 3D CAD model for the DVVI nozzle was submerged in a high-velocity steam. The effect of the rebounding force of the injected water has also been simulated. The 3D CAD model for the DVVI nozzle simulates its mass, texture and constitution material. In this work the DVVI nozzle is digitally mocked up using CATIA. This model is then converted to a proper file format compatible with the computational fluid dynamics code CFX to compute the flow regime during the ECC injection. The result of the CFX analysis lends itself to phenomena identification of the DVVI system, and provides with the boundary conditions needed for the structural analysis. Finally, using the boundary conditions provided from the CFX analysis and the geometrical model from CATIA, quantitative assessments are made utilizing ANSYS to determine the degree of damage on the DVVI nozzle caused by the vibration, pressure and mass during the ECC injection. KEYWORDS: Direct Vessel Vertical Injection, APR1400, LOCA, CFX, ANSYS, CAD I. Introduction The present study relates to a direct vessel injection (DVI) system of a nuclear reactor for injecting the emergency core cooling (ECC) water using a vertical injection pipe, and more particularly to a system for injecting the ECC water into the reactor vessel downcomer utilizing a horizontal safety injection (SI) pipe with a vertically downward elbowed tip, rather than on the side thereof, for the purpose of enhancing the ECC penetration capability into the core in a pressurized water reactor (PWR), a boiling water reactor (BWR), or an advanced reactor using a DVI system for which the ECC water is injected through the SI pipe installed separately from a cold leg. Conventionally, the SI trains are used for injecting the ECC water through a cold leg by connecting the SI nozzles to each one of the cold legs. When designing the * Corresponding author, Tel , Fax , kysuh@snu.ac.kr conventional cold leg injection (CLI) system, the SI system consists of two SI trains, onto each of which a high-pressure safety injection (HPSI) pump and a low-pressure safety injection (LPSI) pump are arranged, so that the ECC water can be injected through the remaining unbroken cold legs even if one of the cold legs is broken or one of the two SI trains fails. According to the probabilistic safety assessment (PSA) for a nuclear power plant, it has been found that when the SI system is designed as a mechanical four SI train system, the safety in a nuclear power plant is significantly improved as compared with a two SI train system. If the SI system is designed as a four train system and if the cold leg injection is applied, the number of the HPSI and LPSI pumps required is four, respectively, in order to supply the sufficient amount of coolant required for core cooling. On the contrary, if the DVI is adopted in this case, it is possible to design the system with pumps of a smaller capacity than the conventional SI capacity for an accident in which the cold legs are broken. In case the SI nozzles are

2 connected to the cold legs as in the conventional emergency core cooling system (ECCS), when the cold legs are broken, the ECC water that must flow into the reactor core through the SI nozzles may flow out of the break prior to reaching the downcomer. That is, since the ECC water flowing in through the SI nozzles is mixed with the coolant that flows backward through the cold legs due to a pressure differential between the reactor core and the break and thus flows out through the break in the cold legs together with the coolant, the ECC water cannot flood the core. Recently, in view of such aforementioned problems, as shown in Fig. 1, a method has been proposed to connect the SI pipes directly to a nuclear reactor vessel to allow for the ECC water to flow directly into the reactor vessel, so that the ECC water does not flow out of the primary system through the break in the cold legs. Therefore, the ECC water has a higher potential to submerge the core. To summarize, the SI pipes are connected directly to the reactor vessel to allow for the ECC water to be injected directly into the vessel in the DVI method. attached to the cold leg. Hence, the objectives of the direct vessel vertical injection (DVVI) system are not only to preclude the PTS by injecting the ECC water through a vertical pipe, but also to increase the amount of the ECC water penetration into the lower plenum without bypassing due to the steam injected from the intact cold legs by increasing the downward momentum of the cooling water through direct vertical injection. The concept of the DVVI system appears to be promising. Nonetheless, there is a potential concern about the additional structure to be introduced in the reactor downcomer. Thus, the present study has mainly to do with structural analysis, using the ANSYS code, and fluid dynamics analysis, employing the CFX code, of the newly propose DVVI system. II. Modeling Considerable amount of time and efforts are expended in constructing the geometrical model to run ANSYS and CFX for analysis of the DVVI system. Thus, approximations and simplifications are essential to render the model tractable. Instead, use of a professional computer-aided design (CAD) program can not only drastically reduce the total time and efforts required to create the model, but help obtain a more reliable result owing to the more accurate model. In this work, CATIA V5R9, a CAD program, is used to create more accurate models to run ANSYS and CFX. The DVI nozzle of the APR1400 is located above the hot legs and the cold legs, 15 off the cold legs, as portrayed in Fig. 2. The CATIA-processed ANSYS and CFX models are respectively presented in Fig. 3 and Fig. 4. Additional modules are required to directly import the model generated by CATIA to ANSYS and CFX so as to ensure efficient coupling of CAD and analysis. Fig. 1 Safety injection systems However, it was found that the ECC water may bypass through the break even in the aforementioned DVI system. Namely, if the SI pipes are situated higher than the cold legs, a considerable amount of the ECC water injected through the SI pipes may bypass through the break in the cold legs together with the coolant which flows backward into the cold leg, while flowing down the internal wall of the reactor vessel when the cold leg, which is an inlet pipe for the coolant, is broken. If the SI pipes are positioned lower than the cold leg, however, the ECC water at a low temperature injected through the SI pipes suddenly strikes onto the reactor vessel at a high temperature and high pressure, causing a pressurized thermal shock (PTS) so that the reactor vessel may be damaged. In particular, when the SI pipes are broken, even more coolant may flow out through the SI pipes than the case in which the injection nozzle is Fig. 2 Nozzle arrangement in APR1400 CATIA is a 3D CAD program developed by Dassault Systemes, Inc., France.

3 Fig. 5 DVVI conceptual view Fig. 3 CATIA model for ANSYS (a) Horizontal type (b) Vertical type Fig. 6 Two types of injection pipe Fig. 4 CATIA model for CFX III. Analysis 1. Flow Analysis Using CFX Fig. 5 illustrates a system for injecting the ECC water, in which a vertical pipe 4 is attached to a SI pipe 3 horizontal with a reactor vessel to allow for the cooling water injected through the SI pipe 3 to flow into the reactor downcomer 5 in a vertically downward direction, in a PWR, a BWR or an advanced reactor 1 with direct injection of the ECC water through the SI pipe 3 installed separately from a cold leg 2. The vector distribution of the ECC water for two types of (a) the horizontal injection and (b) the vertical injection shown in Fig. 6 is presented in Fig. 7. Observe that the downward velocity distribution is more pronounced in the vertical injection than in the horizontal direction. (a) Horizontal type

4 (b) Vertical type Fig. 7 Downward velocity vector distribution Fig. 8 depicts the contour of the iso-velocities in a flow field on the assumption that the initial injection velocity of the ECC water is 12m/s, wherein (a) shows the iso-velocity contour for flow velocity of 2.5m/s in the horizontal injection, (b) shows the iso-velocity contour for flow velocity of 2.5m/s in the horizontal injection; (c) shows the iso-velocity contour for flow velocity of 5m/s in the horizontal injection and (d) shows the iso-velocity contour for flow velocity of 5m/s in the vertical injection. In the vertical injection, a high downward velocity contour extends more broadly than in the horizontal injection. Result of the analysis for the vertical and horizontal injection of the ECC water can be evaluated for their comparative integral performance only when performance of individual injection pipes are examined and compared a priori. (b) Vertical type (2.5m/s iso-velocity) (c) Horizontal type (5m/s iso-velocity) (a) Horizontal type (2.5m/s iso-velocity) (d) Vertical type (5m/s iso-velocity) Fig. 8 Contour of iso-velocities

5 2. Structural Analysis Using ANSYS The elbow tip of the DVVI nozzles takes a large force caused by the injected water and high pressure steam in the reactor downcomer. There may consequently be a potential for structural failure. Now, let us account for the relevant factors that may attack the elbow tips. Fig. 10 is a model for the ANSYS analysis simplified from Fig. 3. Note that the vertical section has been removed from the DVVI system based on the CATIA analysis yielding the stress concentrated at the horizontal section as shown in Fig. 9. F total = F F2 F3 F : Reaction force caused by the injected water 1 F : Rising force of water and steam in the reactor 2 F : Other relevant forces affecting the nozzles 3 The velocity of the injected water is 12 m/s, which leads to force tantamount to 5000N. The vertical rising force of water and steam in the reactor is about 3000N. There are a number of other forces that may hit the elbow tip, which is approximated to 2000N. The rising force and other forces are taken from a simple hand calculation. Thus, the total force ( F total ) imposed on the elbow tip is about 10000N. Fig. 9 shows the linear Von Mises stress distribution. Result of this simple pre-analysis could simplify the model required for detailed analysis using ANSYS. Fig. 10 Modified model for ANSYS Fig. 11 illustrates the stress and strain distributions analyzed using ANSYS. The yield strength of steel is about 235MPa, while the maximum stress applied at the elbow tip is merely about 1.115MPa. Thus, the DVVI nozzle satisfies the structural constraint during the ECCS operation. Fig. 9 Linear Von Mises stress distribution per CATIA (a) Stress distribution

6 vertical and horizontal injections appears to demonstrate the advantage of the vertical injection over the conventional horizontal injection in terms of performance and safety in that the ECC water by the vertical injection reaches the reactor core more efficiently than by the horizontal injection. It was also shown that the DVVI system will safely operate maintaining the structural integrity during its operation. The proposed DVVI system exploits a vertical pipe for which the gravity is used when injecting the ECC water into the core. Therefore, according to the present study, it is possible to prevent degradation of performance on the current DVI system in the APR1400 due to such thermal hydraulic phenomena as impingement, breakup, and bypass taking place in the downcomer of the reactor vessel. The DVVI allows for the ECC water to flow more efficiently and stably into the lower plenum than by the conventional DVI horizontal injection. (b) Strain distribution Fig. 11 Stress and strain distribution per ANSYS IV. Conclusion Notwithstanding the uncertainties and simplifications embraced in this work, comparison of results between the Acknowledgment This work was performed under the auspices of the Korean Ministry of Commerce, Industry and Energy. The authors also thank Dr. C. H. Song of the Korea Atomic Energy Research Institute for his valuable assistance provided during the course of the work.