As there is no legal or administrative limitation as regards the establishment of the operating life of NPPs, with consequently no fixed operation period established, the NPP operating licenses/permits are renewed periodically through on-going assessments and PSRs. The PSRs provide a global evaluation of the plant safety (including the analysis of aspects such as compliance with the standards in force, plant specific and industry operating experience and
updating the safety evaluation and improvement programmes), and also the PSR is the main tool used by the nuclear regulatory authorities to establish the additional requirements for any new operating license period.

In Spain, PSR is used as a basis for renewal of the operating licence for all NPPs. Safety Guide 1.10 of the Spanish nuclear regulatory authority (CSN) establishes the safety issues to be reviewed and resolved to obtain a new operating license for the next period.

The regulatory process, based on on-going assessment and PSRs, includes the necessary mechanisms, and provides a reasonable guarantee that all aspects potentially affecting plant safety or public health are incorporated into the plant licensing bases.

Conditions associated with an operating license in place state that, in the case of applying for a 10 additional years of operation, a new PSR must be performed and submitted to CSN for approval three years before the end of the current operating term. If the new operating period pertains to the operation of the NPP beyond the design life, the main objective of the new PSR will be to determine whether ageing of certain SSCs is being effectively managed so that required safety functions are maintained, and whether an effective AMP is in place for continued operation. Additionally, Time limited ageing analysis(TLAA) must be identified and evaluated in order to demonstrate that it will remain valid for the new period of operation.

The scope of the PSR has been reviewed by the CSN in order to incorporate other aspects related with the continued operation beyond the design life. In order to accomplish this, it has been concluded that the best basis and most detailed international reference for establishing the fundamental requirements for renewal of the operating license beyond the design life is the US regulation 10CFR54 (LR). Therefore, the LR methodology constitutes the supplementary process that has been incorporated to the specific PSR to be performed when applying for a renewal of the NPP operating license exceeding the original design life.

Extensive analysis and studies of fuel channels have already been completed, including a TECDOC on HWR pressure tubes [I.1]. Comprehensive PLiM for pressure tubes are established for inspection and maintenance to ensure plant life attainment. These plans are updated periodically (typically every 3-4 years) as part of the plant life management programme for this component.

Under normal operating conditions the pressure tubes are exposed to an operating environment of high temperature (250 to 315o C), high internal pressure (9 to 11 MPa) and high flow rate D2O coolant. The tubes also experience a fast neutron flux of up to 3.5 x 10 n. m.’ s’ . These conditions result in the following ageing mechanisms being experienced by the tubes.

Creep and growth

Thermal creep, irradiation creep and irradiation growth, resulting from the above operating conditions, cause axial elongation, diametral expansion and wall thinning of the pressure tubes. In addition, since the fuel channels are horizontally oriented, the previous factors, along with the weight of the fuel and D2O coolant, also result in creep sag of the channel.

Corrosion

The internal surfaces of the pressure tube and the stainless steel end fitting are exposed to and corroded by the slightly alkaline (pH10) D2O coolant. A fraction of the deuterium released by the corrosion process is absorbed and retained by the pressure tube. Lithium Hydroxide (LiOH), used to control pH in the Primary Heat Transport System (PHTS), can concentrate under the fuel bearing pads due to local boiling effects. This concentration of LiOH under some fuel bundle bearing pads, mainly in the outlet half of the fuel channel, has resulted in crevice corrosion in some tubes. Examination of removed tubes has shown that the pits are wide and very rounded. These are not considered to be sites for the initiation of Delayed Hydride Cracking (DHC).

NPP operation has a significant influence on the rate of degradation of SSCs. Exposure of a SSC to operating conditions (e. g. temperature, pressure, water chemistry) outside prescribed operational limits could lead to accelerated ageing and premature SSC degradation affecting plant safety and availability. In particular, it is prudent to attempt to control the operating environment of inaccessible SSCs where detection and repair of degradation would be difficult and costly.

Since operating practices influence SSC operating conditions, NPP operations staff have an important role to minimize SSC ageing. They can do this by maintaining operating conditions within prescribed operational limits. Examples of such operating practices are:

• Operation within the prescribed pressure and temperature range and rate of change during startup and shutdown to avoid undue transient stresses and the risk of over-stress (this risk varies, depending on the material’s fracture toughness)

• Performing maintenance according to procedures designed to avoid contamination of metal components with aggressive contaminants (such as lead or other reagents containing halogens)

• Maintaining plant heating, ventilation, and air conditioning in a state that keeps plant environments within prescribed (design basis) conditions

• Maintaining thermal insulation on high temperature lines and equipment in good order.

Moreover, good understanding of SSC ageing facilitates optimization of operating conditions and procedures to reduce the rate of normal ageing degradation of some SSCs, hence increasing safety margins and/or service life.

Cernavoda Unit 1 which is a newer plant having been in operation only since December 1996, started recently the development of Plant Life Management (PLiM) Programme. Due to its complexity, the programme plan has been divided in several subprogrammes and pilot projects and integrated with other initiatives for improvement in the long term strategy of Cernavoda NPP (2004-2008), and managed effectively by annual Station Technical Programmes.

A. III.1. MAINTENANCE ENHANCEMENT PROJECT

As a result of recommendations made by WANO/IAEA Mission in April 2000, Cernavoda is transitioning from “find & fix” maintenance to a “predict and prevent” strategy. The Mission recommended that Cernavoda NPP follow the EPRI3 recommendations while striving for a well integrated and optimized maintenance programme. Cernavoda NPP joined EPRI in March 2002, the required documentation (EPRI-NMAC4 Preventive Maintenance Basis Reports and similar) was secured and the Maintenance Enhancement Project was committed.

The process, as described below, is based on expert panel approach and depends on close collaboration of various departments within the power plant:

• Identify key equipment and components from critical systems (contributors to systems functional failures) and create a database

• Group key components by type and technical characteristics and identify applicable EPRI — PM basis/templates, manufacturer recommendations and current practice.

• For each type of equipment assemble an expert panel and carry out a comprehensive and systematic review of PM tasks and frequency.

• Review existing call-up work requests and maintenance procedures accordingly

During the expert panel interviews the System Engineer’s provided the worst-case effects on their systems and the station (if applicable) for failure of each of the “key” components they identified. In addition, failure modes and effects for components and equipment included in the Level 1 PSA study were also considered.

Over the last 4 years, as the Maintenance Enhancement Project progressed, the result was not only the enhancement of the maintenance programme but also a significant increase of the number of preventive and predictive maintenance tasks scheduled for the next few years and it became apparent that the programme must be optimized.

Therefore, in parallel with Maintenance Enhancement Project, the critical equipment list has been systematically screened to determine the essential equipment and components which by the virtue of their single functional failure would cause a unit transient/shutdown, a level 1 or 2 impairment and/or an unacceptable risk of asset damage.

The outcome of this initiative was identification of about 500 essential equipment and [6] [7] components throughout the station which represent the first priority for implementation of enhanced maintenance programme.

The next action carried out was the prioritization of maintenance activities based on the results of the vulnerability analysis of the unit to single failures of essential equipment. The vulnerability analysis was performed for so far for common equipment and components (valves, motors, pumps, electrical equipment, etc), with a software application developed by EPRI5 and the results was used for defining the scope of 2006 planned outage.

The general approach implemented in the vulnerability analysis software module is to benchmark the current surveillance, monitoring, inspection and maintenance tasks, time intervals and failure rates achieved since commissioning for each family of equipment and components, with an industry standard, state of the art programme and reliability targets developed by EPRI (EPRI PM Basis templates) and built into the PM database software.

Training and retraining based on knowledge management is necessary to ensure that NPP personnel remain highly qualified to do their tasks. Programmes for initial and upgrades in training, including the use of simulators should be in place. These should also include the following aspects: Training in safety culture, particularly for management staff, the adoption of a questioning attitude, etc.

Many NPPs will be entering their new licensing period, which may be regarded as being beyond the nominal design life, but not their technical life, thanks to PLiM measures. In particular, these highly experienced personnel not only possess detailed knowledge concerning the particular SSC they were responsible for, but also have a good general appreciation for the characteristic behaviour of the NPP as a whole. A factor in a NPP’s PLiM strategy must therefore be to ensure that sufficient replacement personnel are available and that the transfer of knowledge is guaranteed via adequate on-the-spot training and comprehensive documentation.

Despite the excellent record at CANDU6 NPPs, it is well known that steam generators provide challenges for the assurance of continued good health, through to design life and particularly for a significant period of extended operation. This includes components other than the tube bundle, which typically has been the most-inspected component to date. Many important secondary side internal components are very difficult to inspect and as a result, little is known about their current condition. However, as the SGs age there have been several instances of secondary side component degradation, typically support plates which are most readily inspected and therefore most likely to be so, which can have significant impact on tube bundle life. Subtle changes to plant operation, especially chemistry control, may have a significant impact on the tubing corrosion potential under deposits that have built up, and in crevices between the tubing and support structures.

As an outcome of the SG work at a number of CANDU plants, it has been concluded that each plant and its steam generators have unique aspects that could impact on life attainment or extended operation. The Life Assessment recommendations typically focus on specific aspects of chemistry control, proactive inspection and monitoring and periodic cleaning. While the prognosis for life attainment and for extended operation of CANDU 6 Steam Generators is good, it has also been found that this conclusion is very dependent upon implementation of the recommended programme enhancements of inspections, effective maintenance, good chemistry control and detailed assessment of the future field data. It is also dependent on assumptions about the condition of un-inspected components, particularly those on the secondary side of the SG.

From the studies undertaken to date, a typical proactive SG age management strategy for life extension would include the following elements.

The Periodic Safety Review normally sets the implementation schedule of recommended improvements. The safety significance, PSA, techno-economic considerations govern the implementation plan. However, the older plants up to KAPS 1 had used Zircaloy2 pressure tubes in the original design. The en-masse pressure tube replacement period is being used in a big way to implement major retrofitting jobs. RAPS 2, MAPS 1 and MAPS 2 have already undergone this exercise. In MAPS units apart from safety up-grading, SG replacement, portions of feeder lengths replacement at all outlet end and plant life management activities have also been completed. Safety upgrades have also been completed in RAPS 1. At present NAPS 1 has been taken up for pressure tube replacement. Feeder length replacement is however being considered for NAPS units. It is seen that NAPS and onwards will not need many retrofits as they have been designed with prevailing safety standards and there are no major signs of degradations. The following paragraphs summarize the experiences of life management in Indian PHWRs.

Feeders pipes can face an unacceptable wall thinning, due to flow accelerated corrosion for this reason periodic measurements of the wall thickness are necessary.

The chemistry control, as for the steam generators, can mitigate the degradation process and extend their service life.

• For the two-inch outlet tubes, this trend was not noticed clearly. This group, of two inches of diameter, had not shown any significant change of wall thinning rate between 2000 and 2002. Maybe this was due to the low amount of tubes inspected up to 2002 of this group that were not enough to calculate a reliable value for the average of the mentioned wall thinning rate.

• Of the 379 outlet feeder bends (first ones) inspected until the last programmed outage (May 2004), all they would reach the Design Life of 30 years (24 EFPY, keeping in mind a gross capacity factor of 80%). However, some outlet feeders (around 39) have minimum wall thickness which are near to the minimum allowed values and need to be re-visited during next outages for assuring that all they are fit for service, or to discover some of them that could need a substitution before the end of the Plant’s Design Life. Figure 5 shows thickness evolution.

A. V.2.1.3. Secondary side piping

CNEA development a code to determine the areas of the secondary piping circuit that could be susceptible of FAC, being determined 40 new inspection points for the next programmed outages.

The following IAEA terminology and definitions have been adopted throughout the TECDOC:

• Ageing is defined as the continuous time dependent degradation of SSC-materials due to normal service conditions, which include normal operation and transient conditions; postulated accident and post-accident conditions are excluded. It is emphasized here that ageing is a wide term, and may even be extended to include the extent and current level of personnel training and even the status of updated documentation used in the NPP.

• Ageing management (AM) is defined as engineering, operations and maintenance actions to control within acceptable limits ageing degradation of systems, structures and components (SSCs).

• Ageing management programme (AMP) is defined as any programme or activity that adequately manages the effect of ageing on SSCs (e. g. maintenance programme, chemistry programme, inspection or surveillance activities, etc.).

• Condition assessment (CA) is defined as an ageing assessment methodology applied to systems, as well as components and structures, or groups of components with similar characteristics (commodities).

• Life assessment (LA) is defined as an ageing assessment methodology applied to critical and/or complex components and structures that involve mainly passive functions and typically are not expected to be replaced within the original design life of the plant.

• Plant life management (PLiM) is the integration of ageing 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 long term operation (LTO).

• LTO of an NPP so long as all safety requirements are fulfilled and that economic viability, taking into account all costs, including PLiM measures is guaranteed. LTO is a utility/owner policy of intent to operate beyond the original design life.

Therefore, PLiM for NPPs is a methodology whereby all expenses are optimized to favour commercial profitability and competitiveness, while providing safe and reliable supplies of electrical power. In general, PLiM may be defined as the continuous operation of the plant, with an acceptable level of safety, beyond a licensing period established following a safety assessment.

While the word “refurbishment” is often applied to actions taken at any point during HWR service to return a SCC to its original functional capability, in this report “refurbishment” means those actions taken near end-of-design life, for HWR PLiM.

The key element for HWR PLiM is fuel channel replacement (FCR), as there are known degradation mechanisms that will limit the life of the pressure tubes. The overall approach is to perform both the FCR work as well as any other necessary refurbishment work. The FCR work is also an opportunity to refurbish other HWR systems or components to ensure that an extended service life will be achieved without the need for another extended outage.

During the planning period, assessments and plans are made to identify the detailed scope of those specific systems, components and structures for which the FCR outage provides the most economic opportunity to inspect and maintain or replace. It is important to clearly define the required work scope upfront in order to ensure that the FCR outage duration is not lengthened or burdened with the cost of maintenance work, which could otherwise be accomplished during future station outages.

PLiM planning establishes the timing of replacement of major pieces of equipment so that estimates can be made of the expenditures to be expected during the refurbishment project and subsequent operation. One process to identify what work is required consists of the following steps: [2] [3]

(3) Confirmation of SSC lists — The equipment database from the NPP is used as the basis for defining all the component parts that are addressed for each CA report. However, experience has also shown that these databases may not be sufficiently developed in all areas to be used as a reliable basis for the CA. Therefore, confirmation of the SCC list for each system may be required based on Design Manuals, Operating Flowsheets and other relevant information.

(4) Screening — The next step is to screen out from the CA process all items that are normally replaced in the plant as part of current maintenance programmes. Items were removed from further consideration if:

• The devices can be out of service for short periods of time without requiring plant shutdown

• Work order history revealed no problems resulting in unit outages

• Devices can be isolated for easy refurbishment/replacement

• Devices are readily accessible

• Replacement of devices does not require significant capital or maintenance costs

• Replacement of devices is possible on power or during a normal outage

• Replacements/spares are part of existing inventory or readily available

(5) SSC health prognosis — Items that are not screened out are subjected to a detailed assessment process consisting of the following items:

• Review of the SSC design basis

• Review of historical operational and maintenance data, primarily the work order history, supplemented by system engineer interview

• Identification of ageing related degradation mechanisms and evaluation of the SSC against each relevant mechanism.

• Identification of any known obsolescence issues

•Generation of conclusions about the health prognosis for the SSC and recommendations.

Any decisions made are retained in a database for later implementation. Using a systematic process such as the one described above should result in a highly effective and auditable process to scope the refurbishment work.