This TECDOC deals with organizational and managerial means to implement effective PLiM into existing plant in operating HWR NPPs. The guidance provided is applicable also to future HWR NPPs.

The objective of a PLiM programme is to effectively integrate ageing management programmes and economic planning to maintain a high level of safety, optimize the operation, maintenance and service life of SSCs, maintain an acceptable level of performance, maximize return on investment over the service life of the NPP; and provide NPP utilities/owners with the optimum pre-conditions for PLiM.

This TECDOC is primarily addressed to both the management (decision makers) and technical staff (engineers and scientists) of NPP owners/operators and technical support organizations, and will be also of interest to NPP regulators and designers. It is intended to be a living publication to be periodically updated and supplemented as new knowledge is gained.

The specific objective of the report is to assist HWR NPP owners/operators with PLiM programmes by providing guidance on:

• Typical processes and methodologies in HWR PLiM programmes, including plant organization considerations, technology infrastructure and supporting data management,

• Component-specific technology considerations for several of the most important HWR SSCs,

• Planning for long term operation (refurbishment/life extension),

• Strengthening the role of proactive ageing management, and

• Implementing a systematic ageing management process.[1]

3.8.1. Overview of integrated safety performance assessments

To properly set up a PLiM driven action plan, compounded effects of interfacing ageing components and systems must be considered. For example PT creep must be considered in concert with the other PHTS ageing mechanisms and effects such as boiler fouling, increased resistance in the boilers, effects on reactor overpower (ROP) and bundle power and channel power limits, evolution of the inlet feeder temperature, heat transport system asymmetries, effects on Steam Generator pressure and on critical channel power limits. A corrective maintenance plan should be devised in view of the overall objectives. For example PLiM results may suggest boiler divider plate change out, steam generator cleaning, ultrasonic flow measurement (USFM) both cold & on line, trip calibration, use of high performance fuel to restore original design margins, but all these suggestions must be vetted in the context of overall performance, safety parameters and operating margins, etc.

Stations may be required to incorporate effect into ROP set point and perform detailed assessment for ROP, time to contact studies, studies to determine the effect on Reactor control, on bundle power and channel power (BP/CP) affecting uncertainties & limits, shutdown system effectiveness for LOCA and in-core breaks.

A comprehensive programme has been developed covering cables themselves (control and power), terminations, penetrations, junction boxes, panels and associated termination. Such a programme would include the effects of normal ageing as well the effects of design basis events for which relevant cable systems have to be qualified to meet the design requirements. It should be noted that PLiM ageing assessments of these components need to be integrated with Environmental Qualification programmes, where the cable system components have a safety function. Also extensive condition assessment studies of cable systems have been performed as part of PCA studies and are useful for LTO.

Results from these programmes will determine the extent, if any, of remedial work such as partial cable replacement, for life attainment or long term operation.

Particular attention has also to be paid to cable containment penetrations through the reactor building wall. These penetrations are one component in the overall cabling life management programme. While sealing performance is confirmed by the reactor building pressure test, the PLiM programme recommends additional condition assessment by supplementary tests specific to the penetrations themselves.

Procedures were developed to both examine these components in situ and to remove “aged” penetrations from the reactor building and replace, all during a planned outage. This removal will allow a number of potential ageing mechanisms, internal to the cable penetrations, to be assessed via direct tests on internal sub-components of these “aged” spare penetrations. For instance, at one Canadian HWR NPP, two “aged” penetrations were successfully removed and new ones installed. The overall duration of the activities was approximately one day. A programme of tests and analysis on these penetrations was defined and then implemented.

Some of the general hardware, used in C&I poses the greatest challenge to tackle in ageing management. Irrespective of the category of the system for which it is used, the ageing study for this general hardware is required to ensure healthiness for the continued operation of the station. Some of these items are:

• Teflon wires used in control panels

• Terminal blocks of CDF& junction boxes

• Hand Switches

• Transmitter power supply units

• Indicating lamps

• Relays

• Control cables

Failure or deterioration in the condition of these items during reactor operation can jeopardize the trouble free operation of the station. The foremost reason for this is that replacement of these items requires detailed planning and long shutdowns. Hence residual life assessment of these items, during an up gradation job is of prime importance.

Historical evolution

By the end of the 1980’s programmes were in place related to ageing, however a more comprehensive and systematic ageing management strategy was needed. As a result, in 1990, Canadian Nuclear Safety Commission (CNSC) staff requested licensees to demonstrate that:

• Potentially detrimental changes in the plant condition are being identified and dealt with before challenging the defence-in-depth philosophy;

• Ageing related programmes are being effectively integrated to result in a disciplined overall review of safety;

• Steady state and dynamic analyses are, and will remain, valid;

• Review of component degradation mechanisms is being conducted;

• Reliability assessments remain valid in light of operating experience; and

• Planned maintenance programmes are adequate to ensure the safe operation of the plant.

The IAEA guidelines were accepted as an appropriate framework for such a programme. As a result of the above request, the Canadian nuclear industry put systematic ageing management programmes in place that were based on the IAEA guidelines. The specific processes and procedures developed in support for the ageing management plan varied from plant to plant, though a summary of the general approach is presented below.

Using the guidance provided by the IAEA publications, utilities undertook efforts to identify gaps in their operating policies and procedures with regards to the ageing management of critical components. Initially discussions focused on the selection of critical components. Generally, economically “critical” components were incorporated as well as the safety critical ones into an overall plant life management programme. The CNSC accepted either approach provided the safety critical components are sufficiently addressed.

Programmes were developed that considered the known degradation mechanisms of the selected components. Industry also considered operating experience to ensure that all mechanisms that had previously caused failures were addressed. The programmes already in place to deal with known degradation mechanisms were evaluated to determine their effectiveness.

Coincident with the above activities, generic procedures for evaluating component and system ageing were developed, often in conjunction with the plant designer. Along with these, condition assessments of the major plant components were and are being performed. These assessments evaluated the feasibility, from a safety standpoint, of continued use of the components.

Vibration of the pressure tubes caused by installation activities, such as rolling the pressure tube into the end fittings, commissioning and operation has been found to cause migration of some loose fitting spacers away from their design locations if the spacers are not sufficiently pinched between the pressure tube and the calandria tube. This displacement, if sufficient, allows the pressure tubes to sag into contact with their calandria tubes. If the hydrogen equivalent concentration at the point of contact is above a threshold value then hydride blisters can start to form.

Material property changes

Irradiation of the Zr-2.5 Nb pressure tube material causes hardening of the metal structure, an increase in the yield and tensile properties and a decrease in ductility and fracture toughness.

A variety of preventive and corrective actions are available to manage ageing effects detected by inspection and monitoring in SSCs. Decisions on the type and timing of the maintenance actions are based on an assessment of the observed ageing effects, the understanding of the applicable ageing mechanism(s), the predictability of future degradation, available decision criteria, and the effectiveness of available maintenance technologies.

For ageing mechanisms with low predictability, such as stress corrosion cracking, it is appropriate to use preventive maintenance to prevent the onset of this type of degradation. For example, proactive steam generator secondary side cleaning, implemented on a regular basis before in-service degradation is detected, is an example of preventative maintenance that can prevent or delay tubing corrosion.

APPENDIX III

The overall PLiM programme is designed to meet the needs of Cernavoda Unit 1 for a

structured work programme and will be implemented in phases. This phased approach will

provide the information required to input to its cost model for plant economic assessments.

There are four major phases, as follows:

• PLiM pilot project phase

• PLiM assessment phase provides the methodologies and studies dealing with critical systems, structures and components, which could affect life attainment or extension

• Plant life attainment programme which is essentially the implementation phase of the PLiM programme.

• Plant extended operation (or plant life extension) programme.

The pilot project phase has been started effectively in 2004 and is intended to be completed

by the end of 2006. The general objectives of PLiM Pilot Projects are:

• To demonstrate the application and usefulness of the key elements of physical plant life management programme to ensure that Station goals for safety and performance are met,

• To develop the necessary governing documents related to physical plant life management,

• Reconciliation of plant equipment databases and system definitions, which is a mandatory prerequisite for any large scale screening process during development of engineering programmes

• To refine the long term PLiM programme plan and procedures based on the experience gained during the pilot tasks for maximum benefit to various plant programmes.

Successful completion of the pilot projects would give a significant level of experience and learning that will enable the Cernavoda Unit 1 plant team to implement the next phase of the PLiM programme, similarly to those being implemented by other power plant owners and CANDU operators. For next steps, it is planned to assess the pilot project results and review accordingly the PLiM programme action plan and specific procedures, have in place specific organization infrastructure (by the end of 2006) and to have fully implemented the PLIM Programme governing framework until the end of 2008.

Effective PLiM involves the integration of ageing management and economic planning. As plants age and as HWR owner/operators make decisions on age management programmes and on investments to enhance plant reliability and predictability, economic planning must consider the current and future condition of the plant with regard to ageing.

Most PLiM programmes that have been integrated with the economic planning decision making, utilize an economic model to evaluate operating alternatives for their HWR plants. For example, the following situations would typically be assessed with an economic model that includes the technical costs generated from the systematic ageing assessments:

• An evaluation of outage strategies — is it cost effective to add additional manpower (OM&A) to shorten the planned outage length (increase production)?

• An evaluation of the cost effectiveness of capital upgrades that can increase capacity or MCR (maximum continuous rating) and possibly increase the economic viability of a refurbishment.

• An assessment of when increasing operating and capital costs make a nuclear plant no longer financially viable.

• An evaluation of the additional value that can be realized by maintaining the option for continued operation.

• An assessment of the optimum economic strategy for a new plant design.

For HWRs, economic models specific to HWR technology, licensing practice and electricity pricing practice, have been developed and are being used to optimise decisions on plant projects that involve PLiM. For instance, economic models have been developed with capabilities to handle single unit or multi unit sites, the pressure tube style reactor, re-tubing, and outages not linked to refuelling.

Although repair/replacement for most SSCs is technically possible, ageing of most plant SSCs can indirectly impact on plant costs, because ageing-related behaviour can lead to forced outages, unplanned extensions to planned outages, reduced availability, and increased production costs. Measures to minimize ageing that involve system or component backfitting may be very expensive. For such measures, cost-benefit analysis will be necessary before making a decision. It must be noted that, when the plants get older, this type of cost-benefit analysis will require some assumptions about the planned plant service life.

An effective PLiM programme should ultimately improve capacity factors by reducing the number of unplanned shutdowns and assist in defining strategies to lengthen operating cycles. PLiM can also be a major force in optimizing and even reducing plant OM&A costs when applied properly and early in the life of a plant. An effective PLiM programme also provides rigorous end-of-life estimates and long life strategies needed for economic and risk evaluation of repair/refurbish/replace options.

PLiM is an important input into the long term strategic plan. This plan typically contains alternatives for long term operation, including shutdown at design life, or retube and extend life. Cost of other major component replacement and/or refurbishment is estimated and input to asset evaluation for each plant. Benchmarking (using experience from other plants) and/or system maintenance predictive models are used to estimate the change in maintenance costs with age. Alternative operating scenarios are analyzed, using discounted cash flow, to determine the alternative that creates the maximum value for the HWR owner/operator. Uncertainties (such as cost of licensing issues, major refurbishment, electricity pricing, future capacity factors) are addressed via an economic sensitivity analysis.

In summary, the PLiM programme should be linked to the station business plan. While the primary aim of most PLiM programmes is to improve the availability and assure safety throughout HWR NPP service life, PLiM can have a strong influence (and likely improve) NPP profitability.

An important proactive life management technique in many CANDU’s is a programme of regular tube removal and subsequent metallurgical evaluation. For instance, examination of removed tubes is a requirement of the Canadian Standards Association (CSA) standard. Such examinations are an important supplement to the NDE inspections and provide confirmation of tube wall condition and an insight into the local operating environment on the tube surface. This is particularly useful for the secondary side tube surfaces that have been exposed to under-deposit conditions (such as in tube sheet sludge piles).

Secondary side internals

The importance of many key internal components to successful long term operation of the SGs, and the relative lack of information on in-service condition, are typical outcomes of the life assessment work. A detailed risk assessment of these non-tube components based on design function and operational experience, leads to identification of those specific internals that should be subject to inspection in a proactive and comprehensive age management programme for life extension. There have been several instances, both in CANDU SGs and those of PWRs, of degradation of internals structures resulting from flow assisted corrosion. This experience indicates that the secondary side internals are an area where some inspection for component integrity is essential for assurance of life extension.