PLiM for Paks

The four VVER-440 units of Paks Nuclear Power Plant (NPP) in Hungary were commissioned in the 1980s, with a design life of 30 years. They are now being prepared to operate for up to 20 years beyond their original design life. To support the licensing process for the extended operational period, a sound ageing management programme is necessary, and within this programme the role of in-service inspection (ISI) will be vital. Also, time-limiting ageing assessment (TLAA) is becoming much more important, especially the reactor pressure vessel (RPV) integrity assessment.

For the life extension, the plant owner has decided to adopt the ASME Code requirements for those inspection and operation activities where they are possible and practicable. All these activities involve various aspects of the qualification of the non-destructive evaluation (NDE) systems used for ISI, and thus the inspection qualification activity has become an integrated element of plant life management.

This paper aims to provide an overview of the qualification-related works from the point of view of plant life management. It will present the updating of the qualification programme on the basis of ageing management priorities, and the qualification of eddy-current testing (ET) of the RPV cladding to support the pressurized thermal shock (PTS) analysis, while avoiding costly mitigating measures.

Life extension plans

Paks NPP’s life management programme consists of two major elements: a power uprate and an operational life extension.

For more than a decade, each unit, originally designed for a nominal electric power of 440MW, was operated at about 460MW; the increase was mainly achieved by an efficiency improvement on the turbine side. Recently, an additional 8% power upgrade has been completed. This was achieved by an increase in the reactor’s thermal power, mainly due to modernization of the core control system and introduction of a new type of fuel.

The fundamental aim of the life management programme will be the operation of the plant beyond its design life. Studies have justified the technical and economical feasibility of a 20-year life extension. According to the Hungarian nuclear safety rules the operational life extension has to be based on the following four pillars:

• Ageing management programme

• Environmental qualification

• Maintenance effectiveness monitoring

• Design basis reconstitution

The power uprate has no significant consequences for ISI and inspection qualification; the life extension, however, does, especially the ageing management activity.

The opportunity

The plant life management programme at Paks has provided a unique opportunity for the plant’s ISI programme to be upgraded. The current ISI programme in Hungary is based on the former Soviet normative technical documents [1,2], which represent the technical level of the 1960s and 1970s. Even the later revisions, published in 1989 [3,4], do not adequately follow the technical developments necessary for assessing structural integrity of components (for example, NDE or fracture mechanics). Also, the requirements in [1] and [2] for design, manufacturing, commissioning and operation are fundamentally nuclear safety-oriented; they do not handle the evaluation of pressure-retaining components and piping during operation, i.e. they do not support plant life extension.

As a result, Paks NPP is aiming to adopt the ASME Boiler and Pressure Vessel Code (BPVC) Section XI [5] requirements for activities related to the safety of pressure retaining components. In particular, these activities would include ISI, repair and replacement in case of inadequate ISI results, and strength and fracture mechanics analyses. The Hungarian nuclear safety regulation allows for their adoption, since it does not determine exclusively the codes or standards that must be used during the design and commissioning of a nuclear power plant, or for the ISI to be performed during operation. Instead, its requirements for the ISI programme only prescribe that they shall be specified in accordance with ‘authoritative technical standards’.

The main goals of the adoption of BPVC Section XI are the improvement of the safety and of the cost-effectiveness of plant operation and maintenance. BPVC Section XI’s operation and maintenance technical support will make state-of-the-art implementation possible. It will also enable inspection, maintenance and necessary safety analyses to be compared directly with the most advanced safety requirements and methods. The change has an indirect goal as well, namely to facilitate the international acceptance of the operational life extension plans of Paks NPP. Compliance with the BPVC Section XI requirements provides the opportunity to extend of the current four-year ISI cycle for class 1 components to an eight-year one for the whole operational life of the plant. The intended doubling of the inspection interval has a substantial potential to enhance the cost-effectiveness of future operation and maintenance.

BPVC Section XI and the European guideline document [6] have different ways of determining if an NDE system is able to meet inspection requirements under real inspection conditions. An appendix (VIII) of the BPVC Section XI lays out performance criteria for trials, since the European method focuses on technical planning of the NDE process and on detailed implementation procedures. Therefore the European method puts the emphasis on the technical justification. The most important feature (and advantage) of the European concept is that the preparatory activity of the operator is independent of the inspection itself and is aimed at the preliminary analysis of data such as the physical environment of the inspection location, the damage mechanism, the possible failure, the probable orientation of the flaw, its critical dimension, etc. The Hungarian regulator prefers to follow the European qualification method, so the Section XI Appendix VIII is not being adopted.

Two-level evaluation

The modification of the ISI / NDE programme has been completed after previous studies. The content reached a good agreement with the BPVC Section XI requirements. One of the biggest deficiencies of the Hungarian ISI rule is that acceptance criteria are not expressed in flaw characteristics (such as size, position, ligament), because its flaw evaluation is not based on fracture mechanics principles. Introduction of the ASME acceptance standards overcome this problem. Despite these developments, Paks NPP still decided to continue to use the original acceptance criteria. To accommodate the Paks position, a two-level evaluation process has been drawn up [7]. In this process, NDE results would first be evaluated against Hungarian criteria (Level 1). Only results that cannot meet these criteria would be evaluated according to the ASME Section XI (Level 2). Feasibility studies have shown that ASME acceptance standards are usually less rigorous than Hungarian ones. The first level of this evaluation process meets the expectation for continuity in evaluations of NDE results, while the second level explicitly incorporates the acceptance levels of ASME Section XI. Prior indications that were originally below the Hungarian acceptance level were reevaluated by the Section XI method, but no new pre-service inspections were forseen.

In order to facilitate the application of the two-level evaluation concept, guidelines were written that describe how to transform the results of ultrasonic examinations (UT) between the BPVC and the current rules. A series of experiments was carried out to validate the conversion of data from the different types of reference reflector to each other (side-drilled holes in case of the BPVC and flat-bottom holes in the case of the current Hungarian rule). On the basis of these experiments, the primary reference responses of the two systems were compared. NDE procedures at Paks NPP exist for all types of examinations, such as visual (VT), liquid penetrant (PT), magnetic particle (MPT), ET, UT, radiographic (RT). There are also different NDE procedures for all significantly different testing objects, for example ferritic steel or austenitic steel. NDE procedures also exist for manual and mechanized examinations. The modified NDE procedures contain all ASME BPVC requirements, including calibration, examination implementation, evaluation and documentation.

Paks NPP’s ageing management practice is based on the designer’s specification and on more than 25 years of experience in operation and maintenance. Although these practices seem to be working adequately, the regulatory rules for the extended operating period require a comprehensive ageing management programme. That covers both active components (such as pumps, valves, motors, control rod drive mechanisms, etc.) and passive components (such as pressure vessels, pipelines, heat exchangers, valve and pump bodies, and their supports and welded attachments). As part of the programme, so-called type ageing management programmes were developed to deal with the service-induced degradation processes in passive components such as low-cycle fatigue, thermal ageing, irradiation embrittlement, various type of corrosion (stress, boric acid, global, local, irradiation assisted, microbiological), wear, loosening of threaded fasteners, swelling and erosion. Based on these types of programmes, component-specific ageing management programmes are also under development and execution. The format and content of the component specific ageing management programmes comply with the regulatory requirements and essentially follow the U.S.NRC’s 10-element ageing management programme: scope, prevention, parameters to be inspected, detection, monitoring and trending, acceptance criteria, corrective actions, feedback, administrative control, utilization of operating experience [8]. The new ISI programme was completed with ageing management items wherever necessary.

Systematic inspection qualification-related activities in the nuclear field were started at Hungary in 2001 [9]. The legal basis for qualification is laid down in the Hungarian Nuclear Safety Code. The current version of the regulations prescribes the qualification of NDE systems used for ISI on the reactor coolant pressure boundary. A new version of the code, which is updated every five years, is due soon. This new version determines the scope of NDE systems to be qualified, namely those used for ISI on structures and components important to safety. This is more flexible because the scope of components important to safety seems broader than the reactor coolant pressure boundary. Paks NPP is still in the process of satisfying the regulatory requirements; the current status of inspection qualification can be seen in Table 1. The items market with a ‘+’ in the table have been completed. In the case of mechanized inspection, personnel qualification is not always performed.

Qualification of RPV testing

The reactor pressure vessel (RPV) is a common element of the reactor coolant pressure boundary and the emergency core cooling system (ECCS) in a nuclear steam supply system. The ageing mechanism of primary concern for RPV is the irradiation embrittlement of the beltline region structural materials (including the base metal, weld metal and heat affected zone). RPV structural integrity is jeopardized in transient regimes of the reactor (heat up, cool down) and when the RPV is exposed to pressurized thermal shock (PTS) loading. PTS loading results from activation of the ECCS in response to the detection of a loss of coolant accident (LOCA). In case of LOCA the relatively cold water of ECCS (20-50°C) is injected into the reactor downcomer, and this subjects the RPV inner surface to a severe thermal shock. Indeed, at the same time the pressure may be increased, and the combination of the pressure-induced membrane stresses and of rapid cooling-induced thermal stresses in a crack front result in a high stress-intensity factor. In cases of advanced embrittlement (characterized by low fracture toughness, high ductile-to-brittle transition temperature shift), there is an increased risk of temperature and pressure stresses causing unstable crack propagation (and leading to catastrophic failure). Irradiation embrittlement determines the operational life of the RPV, so PTS analysis is the key element of life management (in particular, time-limiting ageing assessment, TLAA).

Conventionally, a crack type surface defect of 1/4 wall thickness is postulated for the PTS analysis. However, recent international guidelines [10, 11] tend to introduce the use of an embedded defect because systematic investigations on unused RPVs, as well as ISI experience, have shown that surface flaws are extremely unlikely to exist. In accordance with this tendency, the relevant Hungarian regulatory guide allows the postulation of an embedded (under cladding) defect in the case if the cladding integrity is assured by NDE, and the cladding mechanical properties are known. Also, if qualified ISI/NDE is applied, a maximum crack depth of less than 1/4 of wall thickness may also be postulated.

RPVs of the VVER-440 type reactors have a stainless steel cladding with 9mm thickness. Experimental results show that the cladding material after irradiation remains sufficiently ductile [13]. The results of the ISI completed so far has verified that no through-cladding defects exist, which is quite understandable in light of the cladding procedure (which involves an overlapping two-layer strip cladding (two beads in the second layer) with a low depth of penetration). The postulation of an under-cladding crack, and a crack with a size less than the 1/4 of the wall thickness is extremely beneficial when calculating the technically-feasible lifetime of the RPV. The benefit of postulating an under- cladding crack is that the still relatively ductile cladding material above the crack does not allow the crack opening during the PTS overcooling. Also a smaller crack size results a lower stress intensity factor at the crack front. As a consequence of this the qualification of cladding ET becomes a central issue in life extension.

Currently, RPV inspection at Paks NPP is carried out by a Czech ISI vendor in a 4-year cycle. The complete NDE system (UT, ET) used in Hungary has already been qualified for similar RPVs in the Czech Republic (Dukovany NPP). Taking all these factors into consideration, the Hungarian regulator allowed a simplified qualification involving adoption of the Czech qualification results as much as possible. There has been only limited information and little valuable experience available on how to transfer foreign inspection qualification results. After an analysis, I have drawn the following conclusion. For transferring foreign qualification results, each element of the qualification does not need to be the same. However, the major technical, procedural (quality assurance) and legal aspects in the country of qualification and in the country of application have to be identical.

A special procedure was developed for conducting the simplified qualification [14]. The process follows the European qualification guidelines [6] and it is in line with the relevant IAEA methodology [15]. The central element of the simplified qualification is the comparative technical justification (CTJ). The CTJ is prepared with the foreign and Hungarian technical specifications. Its major task is to justify the identity of the components to be inspected, the flaws to be detected and characterized, and the inspection performance requirements.

Qualification of cladding ET

The qualification of inspection techniques at Paks followed the step-by-step process carried out during the Czech qualification, but over a different timeframe. The examinations during the NDE of the RPV cladding and its environment provide authentic information about the inner surface of the RPV and to a depth of 30mm. The VT results provide information on the inner surface, as does the ET in the range 0-5mm from the RPV inside surface, while the results of the UT provide information in the zone of 5-30mm in through-wall extent (TWE). However VT?is not qualified.

Figure 1 shows the different elements of the RPV inspection. In the middle of the split RPV the designations of the circumferential welds can be seen (the critical weld from the point of view of embrittlement is No 5/6). The green areas show the so called outer zone of the circumferential weld and heat affected area within the RPV wall-thickness, the yellow ones the inner zone; the blue one is the interface between cladding and base or weld metal, and the white one is the scope of ET (from the inside surface to ~5mm deep), see bottom left of Fig. 1.

Fig 1. Diagram of the qualification sequence of RPV weld qualification. Phase I qualifies inspection of circumferential welds inner zone; phase II qualifies inspection of circumferential weld outer zone; phase III qualifies inspection of nozzle inner radii; phase IV qualifies the inspection of the RPV cladding. All NDE is carried out from the inner diameter (ID).

It was mentioned before that CTJ primarily deals with the components, the flaws and the inspection performance requirements. As for the components, the CTJ stated that the Czech Dukovany RPV and the Hungarian RPV can be treated as identical; they have the same design, the same manufacturer, even approximately the same operating life. This statement is valid for the flaws, too. For inspection performance requirements the Czech technical specification established six qualification requirements. But Paks NPP limited the qualification requirements to four. The criterion regarding flaw orientation was not adopted because it was judged to be unnecessary in our fracture mechanics analysis. As for the Czech RMS (root mean square) criterion on the flaw depth, the CTJ followed the previous qualification practice, i.e. RMS was not treated as an inspection qualification requirement but discussed in the CTJ, and an internationally-accepted value of 1mm was taken into account.

Table 2 shows both the foreign and the domestic qualification criteria and the results of the Czech qualification. It can be seen that the Hungarian criteria are either less rigorous than the Czech criteria or equal to them. The main reason for the difference is that Paks NPP has determined the target flaw size on the basis of fracture mechanics analysis following ASME BPVC Section XI principles for analytical flaw evaluation, whereas the Czech criteria were determined based on a conservative ISI acceptance level [2], and takes into account the ET system sensitivity. The successful Czech inspection qualification results can guarantee the safety level required for a successful Czech qualification of the ET system for the Paks RPV cladding inspection, taking into consideration that the equipment and procedure are the same as they were applied at Dukovany NPP. No further measures such as practical trials were necessary.

The qualification test block is manufactured from the same base material (15Ch2MFA) as the RPVs at both Dukovany and Paks NPP. Also the test block’s cladding was made with the same strip electrodes (first layer: Sv-07Ch25N13, second layer, first bead: Sv-08Ch19N10G2B, second bead: Sv-04Ch20N10GB). The test block is equipped with PISC A type notches to simulate surface cracks; flaw dimensions and orientations are given in Table 3.

The Paks NPP life management programme is at an advanced stage. The operator has already submitted the preliminary application for licence renewal to the regulator; final application will be submitted in the second half of 2011. Ageing management is one of the key issues of plant operation beyond the design life. Within ageing management, the role and importance of inspection qualification has become more important. Inspection qualification follows the European methodology, and even the adoption of ASME BPVC Section XI requirements are under way.

Since irradiation embrittlement of RPV structural materials determines the technically feasible operating life of the plant, PTS analysis has a great significance. Within the PTS analysis the postulated crack can be either a surface crack (as the conservative version) or a subsurface (under-cladding) crack. To utilize the extreme benefits of the latter one the cladding integrity has to be verified by NDE. Therefore cladding inspection using a qualified NDE system has had a substantial importance. Paks NPP has successfully completed the qualification of the ET system of the RPV cladding inspection. Thus inspection qualification is significant in NPP life management.

Péter Trampus, University of Debrecen, Hungary. The author has been an independent consultant for Paks NPP in plant life management, and team leader in RPV inspection qualification.