THICKET 2008 – Session III – Paper 05 The VVER Code Validation Matrix and VVER Specificities Ivan Tóth KFKI Atomic Energy Research Institute, 1121 Budapest, Konkoly-Th. u. 29-33, Hungary Abstract. CSNI activities to support the safety of the Russian VVER-type reactors are summarised. The most important action was the development of the VVER-specific Code Validation Matrix (CVM) as a supplement to the earlier CSNI CVM for PWRs. Objectives and structure of the CVMs, along with VVER- specific phenomena are described and an overview of selected test facilities and tests is given. Section 3 presents the VVER-related OECD actions: the PSB, Bubbler-Condenser and Paks Fuel projects. Among CSNI’s International Standard Problems (ISP) only one was devoted to VVERs: ISP33 based on the PACTEL facility. Therefore also the earlier IAEA activities in this field are reviewed, with the four Standard Problem Exercises (SPE) based on the PMK test facility. The tests and outcome of the computer code analyses are described. Although not a CSNI action, major conclusions of a series of seminars on horizontal steam generators are also summarised. 1. INTRODUCTION CSNI activities in the field of VVER reactors of Russian design started in the early nineties by forming a Support Group with the mandate to review and collect VVER-specific test facilities and tests in order to support validation of thermal-hydraulic codes. After several years of work the results could be summarised in a report [1]. As a follow-up of this activity, CSNI first decided to support the final stages of construction of the PSB integral-type test facility located in Electrogorsk, Russia and then organised an international project with the aim to conduct tests at PSB that would address phenomena not yet covered in the VVER-specific CVM. In the long series of CSNI ISPs only one, ISP33 [2], was devoted to VVERs: the test was conducted at the Finnish PACTEL facility and addressed the natural circulation behaviour in single- and two-phase conditions. Although directly not supported by CSNI, it is worth-while to mention two activities, where – besides VVER- operating countries – several OECD countries were also involved. Under the umbrella of the IAEA four SPEs were conducted in the period 1985-94 [3-6] based on tests performed at the PMK facility in Hungary. More than 20 countries from all over the world participated in the code validation activity. A series of international seminars was initiated in 1991 by Finland, the sixth of these series being held in these days in Russia. The seminars are devoted to specific issues of horizontal steam generators – as applied in VVER-type NPPs – and address items, like experiments, modelling and structural aspects of such steam generators. 2. THE VVER CODE VALIDATION MATRIX 2.1 Objectives In the early nineties an OECD Support Group was created with the mandate to review the level of validation of advanced thermal hydraulic codes applied for the analysis of VVER reactor primary systems in accident and transient conditions. The aim was to develop a supplement to the existing ITF and SETF CCVMs [7, 8] under consideration of the specific features of VVER reactor systems and their behaviour in normal and abnormal situations. This includes the necessary enlargement of the experimental data base for code assessment with data which were not taken into account in the previous CSNI CCVMs. At present, it is limited to large and small break LOCAs and transients and does not include shutdown transients and accident management scenarios. 61 Ivan Tóth (KFKI, Hungary) The objectives of the OECD Support Group were: • to identify the phenomena relevant in VVER reactor primary and secondary systems during LOCAs and transients. • to compare the phenomena of VVER reactor systems with LWR reactor systems and to clarify similarities. • to describe the phenomena involved in details as the basis for a common evaluation and an assessment by experimental data. • to identify test facilities and experiments that supplement the CSNI CCVMs and are suitable for VVER specific code assessment. • to establish criteria for the quality requirements and completeness of data finally to be used for the VVER specific code validation. The activity included the following steps: 1. Characterisation of the main features of VVER reactor systems that are relevant to the thermalhydraulic design and the safety evaluation. Emphasis was given to hardware and operational features that distinguish VVER from Western PWR-systems. 2. Description of postulated accident scenarios. Again the main effort was to characterize thermalhydraulic aspects and ranges of parameters distinguishing VVER from Western PWR-systems. 3. Identification of facilities and of experiments that supplement the PWR and the BWR SETF and ITF CCVM and are suitable for code assessment. 4. Establishment of the VVER validation matrices that basically include range of conditions already covered by the previous available CSNI matrices, but also include VVER specific phenomena and features. 2.2 Specific Features of VVERs From the hardware point of view the main differences between Western PWR-systems and VVER-systems are the following: VVER-440 • six loops of primary circuit, • loop seals in hot legs, • horizontal steam generator with two headers, • elevation of the top of steam generators tubes related to the top of the active core (about 4 m, PWR about 10 m), • shrouded fuel assemblies with hexagonal fuel rod arrangement, • injection points of ECCS, • secondary side water volume in steam generators compared with nominal thermal core power is larger, • two isolation valves in each primary loop, • special pressure suppression system (bubble condenser), • each control rod consists of two parts: lower fuel assembly and upper absorber, • lower plenum volume larger. VVER-1000 • horizontal steam generators with 2 headers, • ECCS injection points, • secondary side water volume of the steam generators compared with the nominal thermal core power is larger, • lower plenum internal structures, • fuel assemblies with hexagonal fuel rod arrangements. From operational point of view differences are present in relation to: • operational conditions and set points of actuation of ECCS, • working conditions of secondary side of steam generators and set points for the operation of feedwater and steam line. The considered differences may lead to different phenomena or may affect the course of the transients. As an example, natural circulation between core and steam generators may be affected by the difference in the elevations. The presence of the hot leg loop seals may prevent the ’reflux condensing’ natural circulation mode in VVER-440. 62 THICKET 2008 – Session III – Paper 05 Additionally, different condensation rates can be expected inside the upper plenum when accumulators are actuated. Differences in secondary side water volume of steam generators may cause different transient evolution e.g. in a Loss of Feedwater transient. 2.3 Structure of the Cross Reference Matrices Cross Reference Matrices related to LOCA and Transients were drawn up with the objective of allowing a systematic selection of tests suitable for code assessment. Table 1 presents, as an example, the CRM for large break LOCA. Each matrix is composed of six sub-matrices related to the following items: • phenomena covered by CSNI matrix, • phenomena versus plant types, • phenomena versus test types, • test facilities versus phenomena (both system and separate effects tests), • test types versus test facilities (only system tests), • plant type versus test facility. In the term ‘phenomena’ all the important thermal-hydraulic processes expected to occur during an accident are included. In the column “CSNI” it is indicated whether a phenomenon has already been evaluated by experimental investigation within the CSNI frame, “plant type” gives a ranking according to the characteristics of VVER-systems, “type of test” relates to the definition of the experiment, the meaning of “test facilities” is self evident, both system test (integral facilities) and separate effects facilities are included. Principles of the phenomena selection will be discussed in section 4. Test types and test facilities were selected essentially on the basis of personal experience of the participants of the Support Group, and, more specifically, on the basis of the knowledge of the national representative to which the test facility belonged. However, criteria for selection were commonly agreed upon, such as: • general technical suitability, • experimental coverage of phenomena, Table 1: Cross Reference Matrix for Large Break LOCAs in VVERs. Matrix I Plant TEST FACILITY *1 CROSS REFERENCE MATRIX FOR Test type type LARGE BREAKS System Tests Separate Effects Tests - Test facility vs phenomenon - CSNI + suitable for code assessment + covered by o limited suitability o partially covered - not suitable - not covered x expected to be suitable - Phenomenon vs plant type - Test type vs test facility + fully specific to WWER + already performed o partially specific o performed but of limited use - not specific - not performed - Phenomenon vs test type - Plant type vs test facility + occuring + covered by o partially occuring o partially covered - not in list - not covered CSNI WWER-440/213 WWER-1000 Blowdown Reflood Refill PSB-WWER PM-5 SB ISB-WWER bank (EREC) Data GWP REWET-II IVO-CCFL SKN SVD-1 SVD-2 TVC-440 EVTUS KS TOPAZ SG-NPP FLORESTAN Break flow +--+++x +xx Phase separation o+oo++x x o Mixing and condensation during