Materials management strategies for VVER reactors

T. J. KATONA, MVM Paks Nuclear Power Plant Ltd, Hungary

DOI: 10.1533/9780857097453.3.335

Abstract: The strategic goal of the VVER operator is to extend its operational lifetime beyond the design life. Here, technical and regulatory conditions and methods for ensuring long-term operation of the VVER plant are presented plus an overview of the basic technical design features of VVER relevant to long-term operation. Degradation mechanisms of structures and components which limit the operational lifetime of the plants are identified. The method for evaluating ageing of the plant, a review of existing plant activities for ensuring the required performance of safety-related systems, development of ageing management programmes and other related plant programmes are described. The integration of plant programmes into a system that ensures safe long-term operation is shown through examples. Trends and need for future research are presented.

Key words: VVER, ageing mechanism, ageing management, long-term operation, in-service inspection, maintenance, environmental qualification, time-limited ageing analyses.

7.2 Introduction

The VVER reactors (Vodo-Vodyanoi Energetichesky Reaktor, which trans­lates as Water moderated Water Cooled Energetic Reactor or WWER) are light water moderated and cooled, that is, pressurized water reactors (PWRs). A summary of basic features of VVER reactors is given by Katona (2010, 2011). VVERs were developed in the 1960s. The first three were built in Russia and Eastern Germany in the period 1964-1970, and operated up to 1990. There are 52 Russian-designed, VVER-type, pressurized water nuclear power plants operating in the world today, out of a global total of 443 nuclear power plants (for the latest operational statistics on VVER plants, see IAEA PRIS database) (IAEA PRIS, 2011). The cumulative time of safe operation of VVER reactors currently exceeds 1200 reactor-years.

The first standard series of VVERs had a nominal electrical capacity of 440 MW (and are therefore referred to as 440 units, 440 reactors, 440 designs, etc.) and reactors in the second standard series have a capacity of 1000

MW (and are thus referred to as 1000 units, etc.). There are two basic types of VVER-440 reactors, which are based on different safety philosophies. The VVER-440/230 design comprise the Generation I reactors, while the VVER-440/213 represents the Generation II reactor design with reduced pressure containment. There are two specific VVER-440 designs currently in operation: the Finnish Loviisa NPP with reduced pressure western-type containment and the Armenian Medzamor NPP. In the VVER 1000 MW series, there was a gradual design development through the five oldest plants (small series), while the rest of the operating plants represent the standard­ized VVER-1000/320 model. More VVER-1000 units were commissioned recently and those currently under construction are improved versions of the VVER-1000/320. For example, the Tianwan (China) plant with AES-91 type units and the Kudankulam (India) plant with AES-92 type units. New VVER models, such as the AES-2006 design, are being considered for future bids; these new evolutionary models of large VVERs already exhibit Generation III features.

The design operational lifetime of the VVER plants is generally 30 years, with the exception of the new VVER-1000 type units which have 50 or 60 years of designed operational lifetime. A great majority of VVER plants are quite old, nearing the end of their design lifetime, except for some in Russia. The VVER operating countries are dependent on nuclear power production, for example the Paks Nuclear Power Plant in Hungary pro­vided 40% of domestic production in 2010. The nuclear power capacities in these countries ensure the necessary diversity of power generation and contribute to the security of supply. Therefore, the VVER owners in Central and Eastern Europe are keeping their plants in operation via implement­ing plant lifetime management (PLiM) programmes, with the intention of ensuring a safe and financially viable operation in the long term. The PLiM practice of VVER plants is presented by Katona (2010) and Katona and Ratkai (2008,2010).

The possibility of extending the operational lifetime of VVER-440/213 plants was recognized in 1992. It was based on an assessment of the robust­ness of the design, good technical condition of the plants and synergy between safety upgrading measures and overall condition of the plants (Katona and Bajsz, 1992). In all VVER operating countries, lifetime man­agement had the explicit goal of ensuring the extension of operational life­time (Rosenergoatom, 2003).

The operational licence of the four VVER-440/213 units at Paks NPP in Hungary, is nominally limited to the design lifetime of 30 years. Extension of the lifetime of this particular plant by an additional 20 years is feasible. The first formal step of licence renewal of the Paks NPP was made in 2008 and the relicensing process is still ongoing. In Ukraine, the nuclear share of domestic production of electricity is approximately 48%, while this nuclear power plant comprises 26.6% of total installed capacity. There is a keen interest in extending the operational lifetime of all Ukrainian NPPs. The operational licence of the VVER-440/213 type Units 1 and 2 at Rivne NPP in Ukraine has been renewed by an additional 20 years with the condition of performing a safety assessment after ten years of prolonged operation. The extension of operational lifetime is a generic strategy of operators of VVER-440/213 plants in the Czech Republic and Slovakia. The Loviisa NPP in Finland (a non-standard VVER-440 design) has been allowed to prolong operation up to its next Periodic Safety Review (10 years).

The operational lifetime of the VVER plants in Russia will be extended by 15-25 years. The four oldest VVER-440/230 units, Novovoronezh NPP Units 3 and 4 and Kola NPP Units 1 and 2, have already received a 15 year licence for extended operation. The VVER-440/213 type units (Kola NPP Units 3 and 4) are also prepared for 15 years extension to the operational licence. Among VVER-1000 plants, Novovoronesh Unit 5 is prepared for a 25-year extension of operation, after an extensive safety upgrading and modernization programme.

The VVER operators performed a comprehensive assessment of plant condition and safety, while making their decisions about the extension of operational lifetime. A decision on the preparation of feasibility studies for long-term operation (LTO), was based on the recognition of the following VVER features and experiences:

• robust design of VVER plants

• good plant condition due to well-developed maintenance, in-ser­vice inspections, careful operation and extensive modernization and reconstruction

• implementation of safety upgrading measures, resulting in an acceptable level of safety.

Safety of the plants and compliance with international standards has been considered as the decisive precondition for LTO. The comprehensive modernization and safety upgrading programmes (Vamos, 1999) imple­mented by the VVER operators during the last two decades, resulted in gradual decreases in the CDF of these plants. The level 1 probabi­listic safety analysis (PSA) study establishes the resulting CDF for all VVER-440/213 units at Dukovany NPP of 1.47-1.67 x 10-5/a, as stated in national reports compiled under the Safety Convention (Czech National Report, 2010). The same achievements are published for other VVER plants. Extensive modernization and safety upgrading programmes have been implemented in Ukraine (2011), Russia (Rosenergoatom, 2003) and Bulgaria (Popov, 2007). The safety deficiencies do not inhibit the LTO of the VVER plants; the VVER operators have a strong commitment to continuous improvement of safety and are ready to meet the future chal­lenges in this respect.

One of the issues related to the current licensing basis at VVER plants out­side of Russia was the inadequate knowledge of the design basis. The design of VVER-440/213 and the older VVER-1000 plants was generally based on the former USSR regulations of the early 1970s, the General Requirements on Safety of NPP Design, Construction and Operation (OPB-73) and the General Safety Rules for Atomic Power Plants (PBYa-74). OPB-73 marked the beginning of a transition to the generally accepted international practice in nuclear safety (e. g. defence in depth, single failure criterion). Knowledge of the design base is absolutely critical for the preparation of LTO and licence renewal, especially for the review of time-limited ageing analyses. Operators of VVER-440/213 units have to perform a specific project for design base reconstitution. In many countries, the design base has to be entirely recreated, taking into account all essential changes in the licensing requirements. For example, in the case of the Paks NPP, seismic loads had not been considered in its design. The current design/licensing base includes safe shutdown during an earthquake with 0.25 g horizontal acceleration. Availability of a state-of-the-art Final Safety Analysis Report (FSAR), and regular updating thereof is required for the control of compliance with the current licensing basis and configuration management.

The condition of the plant and appropriate plant programmes are also preconditions for LTO, especially surveillance of reactor pressure vessel (RPV) embrittlement and monitoring the condition of long-lived pas­sive structures and components. The most important ageing management (AM) activities are performed at the VVER plants from the very begin­ning of their operation. The early AM activity was focused on known degradation of the main systems, structures and components (SSCs), like the RPV embrittlement, or on the early recognized issues, for example leaking of the confinement due to the liner degradation, outer surface corrosion of the steam generator heat-exchange tubes. Most of the early AM programmes were state-of-the-art, for example the RPV surveillance programme. In the course of the first periodic safety reviews, the defi­nition of the most critical SSCs for operational lifetime and the domi­nating ageing mechanisms were explained. Adequate assessment of the aged condition and forecast of safe lifetime of structures and components (SCs) can only be performed if the ageing process is monitored properly from the very beginning of the operation. The operational history of SCs has to be documented in sufficient detail for the trends in ageing to be discovered.

There are several non-technical conditions which affected the strategy of VVER operators and can be considered as motivation for the decisions on LTO. The positive international tendencies, with regard to LTO of existing nuclear power generation capacities, stimulated the LTO of VVERs too. (This tendency might be changed by the Fukushima nuclear accident fol­lowing the Great Tohuku earthquake in Japan March 2011.) Accumulation of the experiences and scientific evidence for justification for longer than designed operation of NPPs, provides a good basis for LTO of the VVER. Good market positions of NPPs overall in the VVER operating countries, with high levels of public acceptance and positive public attitudes, help in supporting the operation of NPPs in these countries.

Considerable progress has been achieved at VVER plants with respect to the improvement of the performance and plant reliability. The load factor of the majority of VVER plants is over 80%; in some places for example at Paks and at Dukovany NPP it is around 90%.

The national regulation for allowing the approval of an extension beyond designed operational lifetime is also a condition of the LTO. According to Svab (2007) and IAEA (2006, 2007a), there are two prin­cipal regulatory approaches to LTO, depending on the legislation for the operational licence. The operational licence in VVER operating countries may be either limited or unlimited in time. In countries where the oper­ational licence is not time limited, the basis of regulatory approval is the periodic safety review (PSR). In those countries where the operational licence has a limited validity in time, a formal renewal of the operational licence is needed.

The internationally accepted rules and requirements regarding PSR are documented in the IAEA Safety Guide NS-G-2.10 (IAEA, 2003). One of the objectives of the PSR is to review the condition of the SSCs, and whether it is adequate to meet their intended safety functions. This includes knowledge of any existing or anticipated ageing and obsolescence of plant systems and equipment. In particular, the objective of the review of PSR Safety Factor 4: ‘Ageing,’ is to determine whether the ageing of SSCs is being effectively managed. This means whether or not the required safety functions are main­tained, and whether an effective ageing management programme is in place for future plant operation (NS-G-2.10 para 4.21 of IAEA, 2003). The design lifetime is a technical limit for the operation, which is based on assumptions by the designer regarding time limit of performance and functionality of systems, structures and components due to ageing. The PSR used for jus­tification of extension of operational lifetime beyond the design lifetime has to demonstrate that the prolonged operation is safe, despite expiration of the design lifetime. It means the PSR has to review all the time limiting analyses made by the designer. When reviewing the ageing of the plant, both programmatic aspects and technical aspects of ageing management should be evaluated. Rules for developing and establishing and attributes for ade­quacy of ageing management programmes are given in the IAEA Safety Guide NS-G-2.12 (IAEA, 2009).

Examples of the licence renewal approach are the Russian and Hungarian cases. For licence renewal, the regulations require the perfor­mance of integrated plant assessment, focusing on the review of plant con­dition, effectiveness of ageing management programmes and validation of time-limited ageing analyses for the extended period of operation. In Hungary, the national rules for licence renewal have been developed on the basis of 10CFR54, the licence renewal rule of the U. S. Nuclear Regulatory Commission. In Russia, the rules are defined within the context of national regulation.

In this chapter — after an overview of the basic technical features of VVER plants — the basic issues and methods for ensuring LTO of VVER plants will be presented. The dominating degradation mechanisms of structures and components limiting the operational lifetime of the plants will be identified, on the basis of operational experience and research results. The method for evaluating the condition of the plant; review of existing plant activities for ensuring the required performance and functionality of safety-related sys­tems, structures and components; and development of ageing management programmes and other related plant programmes are described. Integration of particular plant programmes into a system that ensures safe LTO is shown on the basis of particular examples. Trends and needs for future research are also presented.

The presentation of the ageing issues will focus on the older VVER — 440/213 and VVER-1000 plants. The VVER-440/230 plants (Kozloduy NPP, Bulgaria and Bochunice V1 NPP, Slovakia) are already on permanent shutdown. In contrast to this, the Kola 1 and 2 and Novovoronesh 3 and 4 units in Russia have already received licences to operate for a further 15 years. This was after implementation of modernization and safety enhance­ment programmes (Rosenergoatom, 2003) to cope with the safety issues relevant to this design (IAEA, 1992). The LTO and plant lifetime manage­ment of VVER-440/230 is not a generic practice and will be discussed below, although only to a limited extent. The LTO of the VVER-440/213 plants requires specific engineering effort and will be discussed in detail. From the point of view of LTO, the newly designed and constructed VVER plants are also of less interest. Obviously, they have been designed and manufactured taking into account the ageing lessons learned from operational experience. The question about the need and possibility of longer than designed opera­tion of these plants is not on the agenda today.

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