Currently, there are 16 VVER-440/V213 reactors operating in the Czech Republic, Hungary, Russian Federation, Ukraine and Slovak Republic.
Unlike western PWR reactors, the standard VVER-440/V213 reactors are equipped with a confinement including the reactor coolant system (RCS) hermetic compartments and passive pressure suppression system called a bubbler-condenser (BC). The BC has been designed in place of a standard containment to depressurize and confine the primary/secondary coolant and non-condensable gasses (possibly radioactive) leaking from the RCS or main steam lines in a loss-of-coolant accident (LOCA).
The BC is located in a BC tower connected by a corridor with hermetic compartments of RCS components. The BC structure consists of 12 levels of trays (enclosed steel tanks with capped slots at the bottom). Each tray rests on 16 supporting steel I-beams anchored in opposite BC tower walls. The supporting beams divide each tray in 17 parallel sections with 9 transversal slots per section. The trays are filled with 12 g/kg water solution of boric acid to a level of 45-48 cm, which is below the top of the capped slots forming water seals at the slots. The BC further includes four gas traps (vertically laid out hermetic rooms adjacent to the BC tower) and associated check valves of 500 or 250 mm inside diameter. Every three trays are interconnected with one gas trap through the 500 mm check valves. The 250 mm check valves connect the internal volume of the BC trays with the BC tower shaft (a free space between the tray system and the tower outer walls, which is open to the confinement corridor). At the entrance from the corridor into the BC tower, an impact shield of perforated steel sheet is installed to reduce the strongest steam impact in the initial phase of the LOCA. The schematic BC layout is shown on the Figure 1.
Normal absolute pressure in the tray internal space above the water solution level is p2 = 98 kPa. In case of a primary/secondary coolant leak, the confinement pressure p1 initially increases and a differential pressure p12 (p12 = p1 – p2) forms between the BC tower shaft and the water trays. If the p12 exceeds 5 kPa, the water seals open allowing the air-steam mixture to enter the trays. Most of the steam then condensates in the water solution. Non-condensable gasses and not condensed steam accumulate above the water level increasing internal pressure inside the trays. If the differential pressure p23 (p23 = p2 – p3) between a water tray and a gas trap exceeds 5 kPa, the related 500 mm check valve opens releasing the air/steam mixture in the traps. Once the p12 over the BC trays becomes negative (pressure in the BC shaft decreases below the pressure in water trays) by more than -0.2 kPa, the 250 mm check valves release the air-steam mixture from the trays in the BC shaft. However, if the BC shaft (confinement) overpressure still exceeds 655kPa (1655kPa absolute), the 250 mm check valves lock in closed position by a special locking mechanism. The overpressure in the trays then drives the water solution out of the trays through the capped slots, additionally spraying the shaft and decreasing the pressure in the confinement. The flow of the spraying water is directed by a special perforated baffle at the edge of each tray. The operating principle of the BC has been illustrated on the Figure 2.
The PH2.13/95 project was implemented within the EC PHARE/TACIS programmes and was financed from both PHARE and TACIS funds. The main purpose of the project has been to study in-depth, both experimentally and analytically, functional, thermal-hydraulic and structural behaviour of the BC pressure retaining structures in design basis accident (DBA) conditions. The aim has been to verify that the BC systems work as designed and no structural or other damage would occur in DBA conditions. As the BC structures at Dukovany/Jaslovske Bohunice NPP (Czech/Slovak Republic), have been found less robust than the BC structures at the other VVER-440/V213 plants, two main test programmes were carried out:
- A test programme to asses the thermal-hydraulic performance and fluid-structure interaction of BC system under DBA conditions at an experimental facility modelling the Paks NPP BC.
- A test programme to verify the static structural integrity of the pressure retaining steel structure (water trays) at Dukovany and J. Bohunice NPP under the maximum differential pressure at DBA conditions at another experimental facility modelling the BC system at Dukovany and J.Bohunice NPP.
These two main test programmes have been complemented by additional analytical support and small-scale tests.
According to the above objectives, the project has been divided in four tasks:
- Task 1: General project management
- Task 2: Thermal-hydraulic and fluid structure interaction tests
- Task 3: Static structural tests
- Task 4: Small-scale tests and additional analytical support
The project was implemented by the BCEQ Consortium consisting of Siemens, EdF and Empresarios Agrupados. The consortium was supported by local subcontractors: EREC Elektorgorsk (Russia), VUEZ Levice (Slovakia) and SVUSS Prague (Czech Republic).
The Task 2 was aimed at assessment of thermal-hydraulic performance and fluid-structure interaction of Paks NPP BC system. The work was performed at the Elektrogorsk Research and Engineering Centre (EREC), Russia. It included:
- Building of a test facility at EREC replicating the Paks NPP BC system. The test facility has modelled two sections of a BC tray and the adjacent gas trap of the original Paks NPP BC system in scale of 1:100
- Performance of pre-test TH computational analyses of TH behaviour of the test facility in two DBA scenarios - Large Break (LB) LOCA (guillotine rupture of the 500 mm RCS loop pipe) and a main steam line break (MSLB). The objective of these analyses was to verify adequacy of the test facility design, structures and parameters for the intended experiments. ATHLET and DRASYS computational codes were used for the analyses.
- Performance of three experiments at the EREC test facility focused on integral performance of the BC during the LBLOCA and performance of BC in initial phase of LBLOCA for two different break locations. The test conditions were chosen to truly simulate Paks NPP LBLOCA maximum loads at BC.
- Performance of post test analysis comparing the pre-test calculations with test results.
It followed from the DRASYS calculations that the maximum confinement overpressure due to the LBLOCA (MSLB) was 107 kPa (69 kPa), while the maximum p over the BC trays was 20 kPa (9 kPa). The calculations determined that LBLOCA would generate the highest risk to the BC structures requiring verification of functionality and integrity of the BS in the accident conditions.
In the subsequent experiments, the maximum overpressure in BC reached 109 kPa, while p over the BC tray was 20/19 kPa depending on the initial LBLOCA conditions. The measured deformations, deflections and stress due to the increased pressure during the experiments did not pose any risk to functionality and integrity of the BC structures. Visual inspections of the experimental facility following the test did not reveal any deformations, cracks or other damage either. The tests demonstrated proper function of the BC system.
The post test analysis verified fair agreement between the analytical predictions and experimental results.
The purpose of Task 3 was to verify static structural integrity of the Dukovany and J.Bohunice NPP BC trays considering the maximum p occurring during LBLOCA, as indicated by the Task 2 tests. The Task 3 tests were performed at a Bubble-Condenser Test Prototype (BCTP) and a Separate Model Test Prototype (SMTP) at the Research Institute of Power Facilities (VUEZ) Levice, Slovak Republic.
The BCTP has comprised two full-size BC tray sections (2x9 capped slots) including the supporting I- beams and surrounding walls with man-doors and other internal structures. The tray sections were placed in an outer container simulating the BC tower. This facility has truly replicated the trays at levels 2-11 of BC tower at Dukovany and J.Bohunice NPP including mechanical characteristics.
The SMTP represented a reduced BC tray section in a steel container including three capped slots of different lengths in full scale. It has been designed to test performance of all 3 types of capped slots of different lengths, as used at various levels of the BC tray, under maximum postulated differential pressure.
Before the BCTP test, computational analysis using COSMOS 1.75 code was carried out to evaluate maximum deformations and stresses of the BC tray at p of 22 and 30 kPa. The calculations showed acceptable deformations and stresses. During the subsequent experiments, the facility was pressurized in three steps up to 22 kPa, 23,7 kPa and 30 kPa, which was conservatively well above the overpressure determined during the Task 2 tests. The BCTP experiments have verified that wall deformations, rib buckling, component displacement and stress occurring at p up to 30 kPa did not pose any risk to functionality or integrity of the corresponding BC trays at J. Bohunice and Dukovany NPP. These results have been relevant also to the Paks and Rovno NPP, where the BC structures are more robust.
The post test analysis confirmed good agreement between the pre-test calculations and experimental results.
Within the Task 4, supporting small scale tests and parallel thermal-hydraulic analyses were performed.
The small scale tests were carried out at a BC experimental facility in the Research Institute for Machine Building (SVUSS) Prague, Czech Republic. The experiments were focused on design, development and qualification of visualization equipment (mostly underwater camera tests) and specific instrumentation (stress and air concentration measurement devices) to be used in the Task 2 tests at the EREC experimental facility.
The parallel thermal-hydraulic (TH) analyses were performed at SVUSS Prague in addition to the Task 2 TH analyses done at EREC. The parallel pre-test analyses were performed to verify adequacy of the EREC experimental facility for testing the Paks NPP BC in postulated DBA scenarios (large, medium and small break LOCA, MSLB) and asses relevance of the Task 2 test results also for the other NPP concerned (Dukovany, J.Bohunice, Kola, Rovno). DRASYS and CONTAIN codes were employed in the pre-test analyses. The diverse CONTAIN code was used to eliminate code or user effects on the numerical results received from the DRASYS calculations (performed also at EREC within the Task 2). The analyses confirmed the LBLOCA was the most severe DBA for the BC (maximum confinement overpressure 117 kPa and BC differential pressure 25 kPa). It has resulted from the calculations that the EREC experimental facility has been fully adequate for modelling of Paks NPP BC system. The calculations for the other plants have shown that the BC pressure difference at these plants would be by about 10% higher than at the Paks NPP.
The parallel post-test analysis was performed to verify consistency and reliability of experimental data, to estimate some uncertain characteristics of experimental facility and gain additional insight in certain phenomena occurring during the BC operation.
An extensive and comprehensive experimental and analytical work was carried out at EREC, VUEZ and SVUSS within this project. The analyses and experiments verified that the overpressure and differential pressure occurring in BC system following DBA (LBLOCA) remained well bellow the maximum design and postulated values (150 kPa designed confinement overpressure and 30 kPa postulated maximum BC differential pressure). The actually achieved values would not pose any risk to functionality and integrity to the BC system at any NPP concerned.
It followed from the Task 4 analyses that the EREC test conditions and results have been reliable, consistent and relevant for the Paks NPP. The test results can be also used for both DRASYS and CONTAIN code improvement and validation.
Based on static structural test results and computational analyses, however, the following modifications to the BC structures have been recommended:
- Minor modification of the tray level 12 should be carried out at Paks, Dukovany, J. Bohunice and Rovno NPP to improve stability of the tray walls;
- Structural modifications of the tray level 1 should be carried out at the Dukovany and J.Bohunice NPP to improve stability of the tray walls;
- A modification of the tray man-door at J.Bohunice NPP has been proposed to improve tightness.
Further investigations have been recommended for the Kola NPP due to lack of relevant input data questioning validity of the comparative computational analyses performed for this NPP.