The Common Project for Completion of Bubbler Condenser Qualification (Bohunice, Mochovce, Dukovany and Paks NPPs)

Transcription

1 International Conference Nuclear Energy for New Europe 2003 Portorož, Slovenia, September 8-11, The Common Project for Completion of Bubbler Condenser Qualification (Bohunice, Mochovce, Dukovany and Paks NPPs) Holubec Jaroslav and Baumeister Pavol Slovenské elektrárne a.s. NPP Mochovce Mochovce Slovak Republic 1 ORIGIN OF THE COMMON PROJECT 1.1 Background Intensive discussions within the OECD Support Group on "VVER Bubbler Condenser Containment Research Work" between 1991 and 1994 revealed the need for supplementary research work to achieve sufficient level of basic knowledge about BC behaviour in course of design based accidents (DBAs). In 1994 the European Commission (EC) asked for a complementary "Bubbler Condenser Qualification Feasibility Study". The Phare Project NUC was finished early in 1996 confirming the need for additional research in this field. The Feasibility Study formed the basis for the Bubbler Condenser Experimental Qualification Project (BCEQ) with two separate experimental activities to be executed within the frame of the PHARE/TACIS 2.13/95 project of the European Commission. During the nineties the IAEA has published a series of reports describing the background and characteristic design features of VVER-440/213 NPPs. In this context the mechanical strength of some existing BC containment systems was also reassessed. This work was undertaken to assist operators in the re-evaluation of the BC performance and to identify the need for supplementary experimental studies to better understand the specific problems of the BC. Separate IAEA working group reviewed BC operational indicators and certain possible problems of mechanical strength of BC structure during LB LOCA were identified. However sufficient documentation was not available for the IAEA, confirming that the BC structure is designed in such way that it could withstand pressure differences in the initial phase after an instantaneous rupture of primary circuit with discharge from both sides

2 403.2 In the IAEA document listing generic safety issues for VVER-440/213 reactors five generic containment related issues have been identified and ranked. These issues are : Containment 1 BC strength behaviour at maximum pressure difference possible under LOCA conditions. Containment 2 Bubbler Condenser thermal-dynamic behaviour Containment 3 Containment leak rates (containment tightness) Containment 4 Maximum pressure differences on walls between compartments of hermetic boxes. Containment 5 Peak pressure in containment and activation of sub-atmospheric pressure after coolant blow down. 1.2 PHARE/TACIS PH2.13/95 BCEQ Project Conclusions of PH 2.13/95 project Further analyses of the obtained results is recommended to study the - Non-uniformity in temperature distributions - Non-uniformity in the flow velocity distributions In summary it was stated: The tests and analyses present that the BC systems for Paks, Dukovany, Bohunice and Rovno NPPs are capable to withstand the induced loads and they are capable to keep their functions" (during whole period of the max DBA). The same conclusion was defined at analyses and tests containment Mochovce NPP by Siemens, SVUSS Řež, All-Rusian Thermal Engineering Institute Moscow and VUEZ Levice 1.3 OECD-NEA Bubbler Condenser Steering Group BC-SG Based on the OECD NEA initiative the BC-SG (Bubbler Condenser Steering Group) was established in December Its activities are arranged in form of project with the following main objective: To provide convincing proof that the V-213 type containment acts in compliance with the design in course of design basis accidents. To provide assistance in planning of new tests and in interpretation of their results. To provide qualified experimental results, which will serve as a base for validation by computational tools for the best estimate analyses. The Steering Group under the auspices of OECD NEA has been established in Paris in December 7 th, The Project PR/TS/17 The Project PR/TS/17 with the title "TSO Support to CEEC Nuclear Regulatory Authorities and their TSOs in the safety related evaluation of the VVER 440/213 Bubble Condenser Experimental Qualification" relates to the project TSO Phare SK/HU/CZ/TS/08 and it provides assessment of the Project PH 2.13/95 (BCEQ). It should provide assistance to state regulatory bodies in licensing. Its duration is 18 months and the project started in June 2002 and should be completed in September The main beneficiary in the Slovak Republic is the State Office for Nuclear Safety.

3 THE COMMON PROJEKT FOR COMPLETION OF BUBBLER CONDENSER QUALIFICATION (BOHUNICE, MOCHOVCE, DUKOVANY AND PAKS NPPS) 2.1 Origin of the common project The Atomic Question Group (AEQ) and relevant partners and the state regulatory bodies started discussion on problems concerning completion of Bubbler Condenser Qualification during the meeting of VVER Users Club at the beginning of The managements of four NPPs (Bohunice, Mochovce, Dukovany and Paks) decided on this meeting to perform together additional experiments at the experimental equipment EREC and to invite the national TSOs into supporting work. The Agreement among individual parties was developed for this reason. Meeting on the technical contents of this Agreement was held at Paks Nuclear Power Plant in August There was agreed to perform common additional experiments (in sense of the AQG requirement) as follows: 1. MSLB (Main Steam Line Break simulation) under conditions proposed originally in the PHARE Project (pressure 4.7 MPa, water level 500 mm, duration 1800 sec, simulated break location medium distance from the BC). 2. MBLOCA (Medium Break LOCA simulation) Size of the simulated break was proposed to be 200 mm, in order to prevent total water discharge from tray (pressure MPa, water level 500 mm, duration 1800 sec, simulated location of break as close as possible to BC). 3. SBLOCA (small break LOCA simulation according to the EREC test facility limitation). The necessary time for accident simulation was determined based on the pre-test calculations (pressure MPa, water level 500 mm, duration 3600 sec, simulated break location medium distance from the BC). The specific test conditions were elaborated during consultations with the national research institutes. The necessary administrative and professional issues were negotiated during the common meetings of involved NPP representatives at Paks NPP in August 23 rd, 2001 and in Pieštany in February 8 th, The entire project management was agreed on the following way. 2.2 Provision of project development and its implementation The agreed principles as well as the technical contents of Project provided base for definition of "Agreement on co-operation and implementation of additional tests at the EREC facility Electrogorsk and on development of report on bubbler condenser serviceability". Directors of Dukovany, Bohunice, Mochovce and Paks NPPs signed this agreement in April 25 th 2002.

4 The project management scheme and relation to BC-SG BC-SG (Steering International technical validation of project progress group) OECD-NEA Operating Officer Basic data negotiation in compliance to SG program COMMON PROJECT More detailed progress Leading group (LG) Project manager EDU - 1 vote Paks - 1 vote EBO and EMO - 1 vote Basic data approval Paks NPP contract Work management EREC Experiments according to contract MSLB, MB LOCA, SB LOCA supervision and support TG task definition Basic data processing Technical Group (TG) local contracts EBO, EMO, EDU and Paks Representatives Supporting organizations i Objectives and purpose of the " BC Qualification" National supporting organisations NRI ŘEŽ, SVUSS Praha, VÚJE, VEIKI The intention of the "Agreement on co-operation in preparation on Bubbler Condenser Function Capability" participants were to comply the EU requirements as well as requirements of individual national regulatory bodies in area of BC qualification through the way of mutual co-operation in common experiments, analyses and supporting activities. Participants agreed on that they will reach the above mentioned intention based on the contract between EREC company and with Paks NPP on implementation of tests with scenarios simulating MSLB and MB- and SB LOCA on the test facility built within the framework of the PHARE Project 2.13/95. Subsequently it was agreed to co-operate in development of a common Final Report. The technical support will be provided by the Technical Support organizations, which will compare and provide validation of applied computational codes through exchange of their results. The presentation provides summary of all identified facts about BC behaviour both from the analyses results of common experiments and the PH 2.13/95 Project conclusions. This report is developed with intention to use gathered information to proof the BC function capability for individual national regulatory bodies of participating countries. The conclusions and recommendations are determined also in order to be applied during the OECD NEA SG discussion and as the response to AQG recommendations.

5 DESCRIPTION OF THE BUBBLER CONDENSER SYSTEM 3.1 Brief description of the VVER 440/213 containment design The VVER 440/213 containment presents the 3 rd barrier against the release of radioactive fission products into environment. It consists of the following main parts: Steam Generator Compartment Corridor BC building with accident confinement shaft System of BC with air traps Ventilation systems Spray system The scheme of containment is shown in the Fig.3.1 Air traps R eactor hall Bubble condenser SG boxes reacto r c o rridor cavity door Fig. 3.1: VVER 440/213- Containment building with the Bubble Condenser The containment consists of hermetically sealed compartments, housing reactor equipment and the equipment of air-conditioning systems, spray system and a BC building, which is connected to hermetically sealed compartments by corridor. The BC building contains 12 staggered floors of passive condenser and air traps equipped with check valves. The gas volumes above the water level (beyond the water sealing) are connected to the air traps through dual check valves. There are four air traps to which uncondensed gases from containment are forced to flow - each connected to three floors of BC trays. Between the volume beyond water sealing and the shaft of bubble condenser there are two self-closing check valves on each floor whose function is to prevent a reverse flow of water from trays in case of small LOCA accident. 3.2 Course of maximum design accident in VVER 440/213 containment The BC system is operated only in accident conditions in conjunction with the loss of coolant of either primary or secondary circuit, resulting in pressure and temperature increase in the containment. It is put under operation automatically by the pressure difference arising

6 403.6 between hermetically sealed and retaining areas. Operation of BC is clearly passive. Details of BC building with several floors of trays are given Fig. 3.2 Principle of passive pressure reduction Passive reduction of the quick pressure increase in hermetically sealed areas at design basis accident is enabled by two basic functions of BC: a) steam condensation through the bubbling of steam in 12 floors of trays with water (2 ) b) capture and retention of air and uncondensed gases in four air traps (5) 1 Corridor 2 Bubble condenser shaft 3 Perforated protected wall 4 Gasroom 5 Perforated collectors 6 Lockable check valve DN Tray with water solution (H 3 BO 3 ) 8 Gap for inlet of steam-air mixture 9 Cap forming water seal 10 Check valve inlet protection duct 11 Dual check valve DN Damper device 13 Air trap Fig.3.2 Details of the part of containment bubble condenser system The BC is arranged in such way that the air-steam mixture is passed through the corridor to the BC where the flow is distributed in BC shaft (1) to individual floors and through the volumes between ceilings and bottoms of the floors the mixture enters in 1806

7 403.7 gap-cap water seals (2). After the expulsion of water column in the inlet cap of seals (3) the mixture bubbles through the water layer where steam condenses with transfer of part of its thermal energy and concurrently its volume is reduced significant by way. Air and uncondensed gases are then cumulated above water level and due to arising overpressure this mixture flows through the dual check valve DN 500 (4) to the air traps (5). The time history of the presented process is governed by the pressure difference arising between the BC shaft (1) and air traps (5). In case of design basis accident it means a very quick process (see graph in Fig.3.3.) with significant dynamic impacts of jet flows on all technological devices, as well as on the structure of BC and hermetically sealed area. Dynamic effects of steam-air mixture flow are at the inlet to BC captured by a special reflexive wall anchored to the bearing structure of BC trays and in this way to the reinforced concrete building. In the further course of the accident the pressures above water level (3) and in air traps (5) equalise while check valves DN 500 (4) automatically close retaining compressed air in trap chambers. The flow of hot water and steam from primary circuit continuously decreases and pressure in hermetically sealed area begins to fall due to steam condensation and heat transfer to the walls, also due to operation of an active spray system. Reverse pressure difference, when the pressure above water seal (2) is greater than the pressure in the BC shaft (1), causes a reverse water flow from trays to BC shaft. Water flows through the same path where the steam air mixture flowed up, along the ceiling of the lower floor, flows to the perforated collectors on the front wall of the BC and sprays the volume of shaft. This passive spraying causes a further reduction of pressure in hermetically sealed volumes. Spilled water from trays is collected on the bottom of the BC shaft and spontaneously flows through the corridor to the SG compartments. From this room the water is together with used spray and primary (secondary) leak water transferred to the emergency system pumps. Accidents with small release of coolant are similar but slower in course with lower achieved pressure. In order to prevent an undesirable reflux of water in case of small accident, there are two special check valves DN 250 on each floor (6) which allow pressure equalisation before and behind the water seal. These check valves are fitted with a special blocking system which, depending on pressure in the volume before the hydraulic seal, automatically locks or unlocks the valve. The blocking system is set to the value of 165 ± 5 kpa of absolute pressure. Above this value the valve is automatically locked and does not allow the pressures to equalise. If pressure in the shaft during an accident localisation does not exceed the limit value of 165 kpa the valves remain unlocked and, in case of pressure drop before water seal, the pressures before and behind the seal equalise thus water remains in trays. The presented passive function of BC system causes a spontaneous decrease of pressure in hermetically sealed areas with continuous cumulating of significant amount of released thermal energy. Full localisation of the accident is accomplished by active spraying of SG compartments which gradually reduces pressure in containment to the minimum value of 80 kpa (absolute) when the spray system is automatically switched off. A moderate vacuum in the sealed area will prevent the release of radioactive substances; the vacuum is maintained by controlled actuation of active spray systems.

8 403.8 P [kpa] Break of the MCP Safety margin in the containment pressure Spray system in operation and water back flow from the trays Spray stops Spray starts , P atm [100 kpa] 20 kpa 0 s 10 s 200s 15 min Fig.3.3 Pressure history in VVER 440/213 containment at maximum design accident 4 ASSESSMENT OF BC FUNCTIONALITY 4.1 Test facility and methodology description Short description of the EREC experimental facility BC V-213 The test facility BC V-213 (Fig. 4.1) has been designed and built-up specially for investigating thermal-hydraulic and fluid-structure interactions of the BC system under conditions typically expected during DBAs. It is located in a separate building and consists of the following main systems and components: a simplified room system simulating the hermetic compartment system of the Paks NPP containment upstream of the BC tower; a BC model consisting of 2x9 original sized gap-cap systems, corresponding side walls, bottom and ceiling parts with mechanical properties identical to the Paks Nuclear Power Plant and a corresponding air space above the BC water volume of the trays; an air trap connected to the aforementioned air volume by a check valve; an relief valve to the BC shaft and a spray system providing the simplified room system with spray water; a blow down system consisting of 5 interconnected pressure vessels, pipe systems and blow down nozzles to provide the necessary mass- and energy reservoir to simulate the anticipated DBA blow down rates at three different locations inside the compartments; the necessary auxiliary equipment including instrumentation and the data acquisition system.

9 Dead-end volume (V 0 ) 5 BC module (V 4 ) 2 SG box (break node, V 1 ) 6 Air trap (V 5 ) 3 SG box (V 2 ) 7 High pressure vessel system 4 BC shaft (V 3 ) 8 DN500 check valve Fig. 4.1 General view of the EREC test facility BC V-213 Other items characterising the test facility are: dimensioning of the blow down nozzle derived on the basis of the scale results of ATHLET calculations for the anticipated failure conditions of the reference NPP; preservation of the mechanical properties of the tray and the gap-cap systems, closely linked to the existing configuration of the Paks Nuclear Power Plant; preservation of scale characteristic main volumes and/or flow cross section areas of the prototype plant with limited modelling of the corridors between the steam generator boxes and the BC shaft; scaling factor 1/100 for the design of the test rig volumes and the necessary mass-and energy reservoirs to simulate the variety of DBA conditions Description of methodology followed during the test preparation and execution Scope of the analytical works performed A. Pre-test calculations Plant blow-down and containment calculations for a representative scenario These calculations have been performed to have basis for preparation of the test scenario and for comparison with the EREC pre test blow-down proposal. EREC TF blow-down For the EREC facility the blow down calculations of each specific case have been performed with ATHLET code to define the blow down rates and enthalpy in order to define and agreed initial and boundary conditions of each test to be as compliant as possible with the NPP scenario. The report made by EREC provided good background for these calculation

10 assessing the differences between he measured and calculated values of blow-down rates during the previous experiments. Plant containment calculations These calculations prior to the tests (also with proposed EREC data) have been done to have a basis for comparison and possible requirements to EREC to modify test conditions in order to keep representative ness and/or conservatism of the tests being prepared. Wide discussion was concentrated especially on MSLB test preparation, where several proposals have been treated to reach a technical consensus. EREC TF containment calculations It was appropriate to perform these calculations prior the tests, in order to be ready for the discrepancies caused by the modelling. B. Post-test calculations The goal was to perform additional validation of containment codes and provide both industry and regulators with well-validated codes serving as best estimate calculational tools for any type of LOCAs. 5 OVERALL CONCLUSIONS Bubbler condenser qualification has been performed in the following experimental and analytical projects: LBLOCA tests and post-test calculations performed in the PHARE Project PH 2.13/95, MSLB, MBLOCA and SBLOCA tests and post-test calculations sponsored by VVER utilities, Post-test calculations for the resolution of the open issues of LBLOCA tests, performed by VEIKI in In previous chapters we have reviewed the test results, post-test calculation results and NPP calculation predictions obtained in the projects. We have come to the following conclusions concerning main parameters of NPP containment during accidents (including LBLOCA): peak containment pressures are below the design value (0.25 MPa) with a safety margin, maximum pressure difference on the BC structure is below the limit (30 kpa) with a safety margin, temperature of the BC water is in the anticipated range, far below the saturation level with a safety margin, non-uniform temperature distributions in the BC do not lead to local maximum values that would deteriorate condensation of the steam, pressure oscillations with a capability for impairment of BC function would not occur. On this basis the appropriate operation of BC containment under accident conditions can be positively confirmed. Results of all tests should be used also for validation of codes applicability and for improvement of codes in modelling of phenomena which occur in BC.

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