Typically, the PLiM programme has an initial focus on a relatively small set of critical structure and components. As mentioned, for the most critical SSCs, life assessment processes are typically used.

The CA process can be applied to less critical components and commodities. Some utilities are using an on-going CA process, and using CA outcomes as an input into their plant/utility business planning processes.

PLiM assessments have typically focused on passive major components. However, a comprehensive PLiM programme will address all SCCs that represent a potential risk to the plant that warrants mitigation as evaluated in the screening process discussed above. However a large number of components subject to degradation may still require some level of assessment from the point of view of the functionality of the system. These components are best dealt with in the context of a system assessment.

The development of a comprehensive PLiM programme will also ensure that degradation mechanisms are assessed as they relate to specific ageing management strategies, executed as the preventive maintenance (PM), condition based maintenance (CBM), predictive maintenance (PdM), surveillance, inspection and testing programmes of the plant. To achieve this goal, two assessment strategies are available; namely systematic maintenance planning (SMP) assessments and condition assessments of system.

These two processes are both capable of dealing with degradation, however, CA is usually applied to components subject to longer term degradation (passive components) while the SMP assessment methodology is usually more efficient at assessing components subject to short term degradation (typically active components).

Even with the best secondary side water chemistry control, corrosion products from the secondary side systems will continue to be carried into the steam generator tube bundle during its lifetime. A large percentage of these corrosion products come to rest in the steam generator as deposits, mostly on the tube surfaces. Those deposits that end up on horizontal surfaces, and particularly the tubesheet, can be difficult or impossible to remove while the steam generator is operational. While considerable effort is made to maintain good bulk water chemistry in the steam generator itself, and advanced chemistry control methods are now available to reduce such fouling, these deposits, if allowed to accumulate, become hard (consolidated) and create crevice conditions at the tubesheet-to-tube interface, as well as fouling the tube-to-support gaps. Feedwater impurities diffuse to these crevices and, as a consequence of boiling in the crevice, can concentrate by factors of up to 106. Hence, a proactive ageing management programme for SGs should include secondary side cleaning (particularly tubesheet flushing or lancing), with regular application, even before the presence of significant SG deposit is detected.

Although there is considerable world experience to support this activity, plant operation and maintenance staff sometimes question the benefits of cleaning because of the considerable costs involved. To provide enhanced tools (and the science behind them) that will aid cleaning decisions, models are being developed to predict tubing corrosion damage using a variety of laboratory and field data. It is known that Alloy 800, similarly to all other SG tubing, is not immune to pitting corrosion or stress cracking, particularly under deposits (like tubesheet sludge piles) where aggressive contaminants such as chlorides and lead may have concentrated. Pitting is likely the highest concern for CANDU SG tubing. Pitting potential

modelling is being coordinated with the crevice/SG chemistry modelling to provide a qualitative guide for operators to assess the adequacy of current chemistry control and to provide input to plan tubesheet sludge lancing.

PIE of pressure tubes and other core components periodically removed from the reactors is very important for lifetime management of coolant channels of Indian PHWRs. A large lead cell has been set up in BARC for carrying out PIE of full-length pressure tubes and other irradiated components from Indian PHWRs. The basic examinations done on a pressure tube include visual examination, measurement of diameter and sag profiles, hydrogen content and oxide layer thickness, tensile properties and fracture toughness. Other examinations include metallography, eddy current and ultrasonic tests, micro-structural evaluation and neutron radiography. The PIE facilities also include advanced NDT instruments for monitoring oxide layer and hydride blisters, facilities for hydrogen estimation of scrape or trepanned samples from pressure tubes and a computerised remote cutting, milling and drilling machine for preparation of specimens for fracture toughness evaluation.

Hydrogen content is measured using either bulk samples or scrape samples. The bulk samples are removed from different axial locations of pressure tube removed from reactor and hydrogen content is measured by inert gas fusion (IGF), differential scanning calorimetry (DSC) and hot vacuum extraction quadruple mass spectrometry (HVEQMS) techniques. Scrape samples are removed using the sliver-sampling tool and hydrogen content is measured using DSC or HVEQMS techniques. As on today, hydrogen measurement has been carried out on more than 100 bulk samples and more than 300 scrape samples.

The ductile-brittle transition temperature has been estimated for a number of irradiated pressure tubes having different levels of hydrogen concentration. The tests have been carried out on rings cut from the pressure tubes and doing tensile tests at different temperatures to determine the fracture strain. Fracture toughness is estimated using an empirical correlation.

The results of PIE are used for deriving the necessary inputs for refining the models of the degradation mechanisms. Feedback taken from the results generated out of the [H/D] concentration measured have resulted in taking several safety related decisions related to the health of the coolant channels and helped in understanding the hydrogen pickup behaviour and validation of the computer codes against data relevant for Indian PT material.

In addition to the pressure tube, other core components like calandria tube and garter spring spacers have also been examined. So far, twenty-three garter spring spacers have been

Key attributes of an effective plant life management programme include a focus on important SSCs which are susceptible to ageing degradation, a balance of proactive and reactive ageing management programmes, and a team approach that ensures the coordination of and communication between all relevant NPP and external programmes. Continued plant operation, including operation beyond design life (usually called long term operation), depends, among other things, on the physical condition of the plant. This is influenced significantly by the effectiveness of management ageing process.

Most HWR NPP owners/operators use a mix of maintenance, surveillance and inspection (MSI) programmes as the primary means of managing ageing. Often these programmes are experience-based and/or time-based and may not be optimized for detecting and/or managing ageing effects. From time-to-time, operational history has shown that this practice can be too reactive, as it leads to dealing with ageing effects (degradation of SSCs) after they have been detected. Reactive ageing management (i. e. repairing or replacing degraded components) may be cost effective for some, in particular, small replaceable components. However, for most important SSCs, utilizing proactive ageing management is generally most effective from both the safety and economic perspective.

Premature ageing of NPP SSCs implies ageing degradation that occurs earlier than expected. It can be caused by pre-service and service conditions (fabrication, installation, commissioning, operation, or maintenance) that are more severe or different than assumed in the design.

For instance, frequent pressure/ temperature transients, particularly those that have been not considered in the design basis, might lead to premature component fatigue. Even small changes, particularly those that affect the chemistry of the circuits, may induce premature degradation several months or years later. Excessive testing and/or routine maintenance can accelerate wear-out of components without additional benefit. Such conditions may not have been taken into account in the usual MSI programmes, unless there is a systematic and comprehensive assessment of ageing effects.

In many cases premature and/or undetected ageing cannot be traced back to one specific cause or an explicit error. The root cause is often a lack of communication, documentation and/or coordination during design, fabrication, commissioning, operation or maintenance. This lack of effective communication and interfacing frequently arises because, with the exception of major SSCs, such as the fuel channels or steam generators, there is a lack of explicit responsibility for achievement of specific SSC lifetime. Lack of effective communication and coordination can be remedied by the implementation of a systematic ageing management process.

Some of the emerging regulatory issues that need to be addressed for long term operation include:

• Potential increased requirement for safety analysis.

• Incorporating or accounting for all ongoing ageing degradation in the safety analysis.

• Resolution of a number of generic action items requiring possible modifications.

• Possible need to maintain an updated PSA.

• Evolving requirements for human factors engineering analyses.

While the codes, standards and regulations in effect at the time of plant design and construction continue to be the basis for licensing, some changes and upgrades to meet new requirements may be needed to support LTO. As the plants establish the scope for LTO, it is important for the utilities to:

• Maintain accurate records of the design basis including the modifications installed since in service (configuration management).

• Obtain a documented agreement with the regulator for the licensing basis for operation beyond design life (such as 30 years).

• Clearly identify any modification or enhancement to satisfy the licensing basis.

Analysis of the combined effects of ageing is necessary to ensure that the unit is operating within the original design envelope, to demonstrate that there has been no deterioration of the operational or safety margins, and to ensure mitigation methods are effective in managing ageing. A specific example of the integrated safety/performance assessment part of the PLiM programme is the effort to predict and manage performance of the HTS, as plants age.

When the “first generation” CANDU 6 stations were originally designed, the need for a detailed thermal hydraulic predictive modeling capability for the CANDU Heat Transport System (HTS) was recognized. A number of ageing mechanisms were anticipated and margins were provided to cater to the in-service ageing degradation that would occur. Accurate predictions of HTS thermal and hydraulic parameters were recognized as an important capability. Hence predictive codes were not only developed but also extensively validated with both commissioning and operational data from the early CANDU 6 experience. This programme of code development and refinement for prediction of HTS ageing behaviour has continued throughout the operational period. In parallel, the supporting R&D programme developed tools to predict the thermal and hydraulic behaviour of deposits that accumulate on various surfaces in the HTS, including the primary and secondary sides of the steam generators. This effort provided additional important data and modeling parameters that were subsequently incorporated into the prediction codes. Also, a specialized eddy current interpretation method has been used to measure the extent and distribution of SG tube primary side fouling.

The result of the integrated programme is an enhanced HTS ageing predictive capability and proven mitigation techniques. The refined codes can both provide an accurate reflection of the current HTS condition of the plant and also be used as an important aid to plant management to predict the benefit of ageing mitigation techniques. An example is an assessment of the effectiveness of a cleaning process, prior to that technique being applied at the plant. Primary side mechanical cleaning at one HWR NPP restored ~+5% of core flow and decreased the reactor inlet header temperature (RIHT) by ~ 3°C.

These values were very close to the improvements predicted prior to the cleaning, using the refined HTS performance codes. Recent experience, including that mentioned, is with a mechanical method for cleaning the inside diameter of the Steam Generator tubes during a shutdown. The system employs robotic manipulators to visit several tubes at one time. A suitable grit material is propelled through the tubes cleaning the oxide from the ID surfaces by abrasion. By carefully controlling a number of parameters (including application time), the amount of oxide removed and overall cleaning efficiency can be optimized.

Hence, there are two important uses of these refined codes. First, they can be used to accurately reflect the current HTS condition of the plant for operational/safety margin assessment. Second, they can also be used as an important PLiM technique (to assist operators, maintainers and plant technical staff) to predict the benefit of ageing mitigation processes and plan when the mitigation implementation is needed.

Korea Electric Power Research Institute (KEPRI) had worked a comprehensive Plant Lifetime Management (PLiM) project for a CANDU plant Wolsong Unit 1 in corporation with Korea Hydro and Nuclear Power (KHNP). The project has been performed to understand the ageing status of major components screened from the plant and to address provisions for the continued operation beyond its design life. A feasibility of the continued operation was reviewed in the aspects of technology, economics, and regulatory environments. And detail life evaluation and development of ageing management programme for continued operation are on-going. This section introduces general approach of ageing assessment in Korea, screening of critical structures and components, and an experience of ageing assessment for an example of fuel channel that is the most critical component in CANDU plant.

A. II.1.1. INTRODUCTION

A CANDU6 nuclear power plant in the Republic of Korea has been operating about 22 years since 1983, which is more than two-thirds of design life. As time passed, systems, structures, and components (SSCs) can be degraded by various modes of ageing phenomena although good operation and maintenance practices have been implemented to the field. Korea Electric Power Research Institute (KEPRI) has worked a comprehensive Plant Lifetime Management (PLiM) project for a CANDU plant Wolsong -1 in corporation with Korea Hydro and Nuclear Power (KHNP). The project had been performed to understand the ageing status of major components screened from the plant and to address provisions for the continued operation over its design life. A feasibility of the continued operation was reviewed in the aspects of technology, economics, and regulatory environments. And detail life evaluation and development of ageing management programme for continued operation as of the second phase of PLiM programme are carried out until 2007. This article introduces general approach of ageing assessment, screening of critical components and an experience of ageing assessment for an example of fuel channel that is the most critical component in CANDU plant.

Figure A. II.1 shows a schematic diagram of the PLiM feasibility study. On and off-shore licensing requirements and current practice for continued operation of CANDU plants beyond design life are reviewed and used for a reference of ageing assessments. Prior to assessing ageing and life of the SSCs, KEPRI screened the critical SSCs that are passive and long-life components and can limit the continued plant operation. An example of the screened major critical components and groups are listed in the centered box of Figure 1. Collected data of design, manufacturing, test and inspection, maintenance, replacement, drawings, material, and operation history are used as technical fundamental of the assessment. And they are stored into the PLiM database with the results of technical ageing and life evaluations. Including PLiM recommendations from assessing ageing of each SSC, PLiM cost and investment strategy can be established. Based on the cost and strategy economic evaluation is performed in the way of various economic parameters, like comparison of continued operation cost, generation cost change, and income per kW with alternative power sources that will used instead of the PLiM plant. In this study 1000MWe Korean standard nuclear power plant was assumed as an alternative power source based on the national policy of electricity power resources.

Korean nuclear industry follows the periodic safety review practice. In normal operating period before the plant design life, general PSR is reported to regulatory body every 10 years in accordance with IAEA safety guideline 50-SG-12 of PSR. For the long term operation beyond the design life, PSR should review plant safety including the ageing management for the continued operating period. Korea regulatory authority is trying to combine LR and PSR approach to take the synergy effect.

2.4.1. Regulatory approach in India

After completion of the safety review, the license for the nuclear power plant is issued by Atomic Energy Regulatory Board (AERB) for its design life which typically is in the range of 30 to 40 years. Within the operating license, the Regulatory Body grants initial authorization for a specified period and renewal of authorization for further specified periods after assessment of PSR. AERB requires HWR owner/ operators to conduct PSR for renewal of authorization which include the following:

• Cumulative effects of plant ageing and irradiation damage

• Results of in-service inspection (ISI)

• System modifications

• Operational feedback

• Status and performance of safety systems and safety support systems

• Revisions in applicable safety standards

• Technical developments

• Manpower training

• Radiological protection practices

• Plant management structure, etc.

This process of PSR for renewal of authorization is to be carried out every nine years within the design life of the NPPs. These PSRs are intended to further ensure a high level of safety throughout the service life of the plant. AERB has already prepared relevant guidelines in this connection as safety guides as follows:

• In-service Inspection of Nuclear Power Plants (AERB/NPP/SG/O-2)

• Surveillance of Items Important to Safety in NPPs (AERB/SG/O-8)

• Renewal of authorization for Operation of NPPs (AERB/SG/O-12)

• Operational Experience feedback for NPPs (AERB/SG/O-13)

• Ageing Management of NPPs (AERB/SG/O-14)

Sag occurs by irradiation creep from the weight of the fuel and heavy water in the pressure tube. Gross sag deformation of the fuel channel is primarily controlled by the relatively cool calandria tube. There are several limits to pressure tube/fuel channel sag that must be monitored:

• Calandria tube contact with horizontal structures that are perpendicular to the fuel channels (such as the liquid injection shutdown nozzles and horizontal flux detector guide tubes);

• Pressure tube to calandria tube contact leading to blister formation;

• Pressure tube sag leading to fuel bundle passage problems.

To maximize the life of the channels with respect to potential contact with horizontal mechanisms, the current strategy is to perform in reactor gap measurements so as to determine when contact could occur and to identify which channels would be affected. To address channels predicted to be in contact prior to the design life, the following remedial actions could be implemented depending on when contact is predicted to occur relative to the design life:

Perform testing to demonstrate that fretting between the components would be acceptable,

Defuel the channel in contact with the liquid injection nozzle or the horizontal flux detector to remove contact,

Adjust the tension on the liquid injection nozzle to increase the sag rate of the nozzle, Replace the liquid injection nozzle with an offset design, and Replace the calandria tube

Potential pressure tube to calandria contact resulting from movement of the loose fitting spacers is addressed by inspections to detect the spacers and reposition them, if required. To ensure that pressure tube to calandria tube contact does not occur, the repositioned spacers must be both adequately loaded so that they do not move after being repositioned and appropriately located so that the pressure tube will not sag onto the calandria tube.

Fuel bundle passage, as proven by tests using predicted end of life curvature is not impaired during the fuel channel design life.

The following sections summarize the different ageing assessment methodologies that are applied to components at multi-unit CANDU plants, depending on the component criticality. [5]

For major components that were considered potentially life limiting for a plant, such as fuel channels, steam generators and feeders, life cycle management plans (LCMPs) have been prepared. These component LCMPs are updated annually.

II. 2.1. CONDITION ASSESSMENTS

Component, system and plant condition assessment (CA) procedures were written in [Ш.2]. The focus of the CAs is to identify actions to address component ageing in the short term, and to also consider actions required to ensure the component reaches the target plant life. These steps are summarized below:

• Plausible degradation mechanisms are identified systematically by assessing material, design and environment. Components are grouped according to type for efficiency, provided the environment was similar. Degradation assessment matrices, such as shown in Table 3 below are used. •

• Programme costs are identified, programme activities/milestones scheduled, and assigned to a responsible individual.

• Condition assessment results are reviewed annually, updated as necessary based on inspection and operating experience over the year, and recommended actions input to other managed processes such as business planning, strategic planning and generation planning.