87055-08-0312 - Incorporating Ageing Effects into PSA Applications – Phase 2

Country:

Canada

City/Locality:

Canada

Notice/Contract Number:

can:154977

Publication Date:

Jul 26, 2008

Deadline:

Sept 16, 2008

Buyer:

Canadian Nuclear Safety Commission Commission canadienne de sûreté nucléaire

Original Language:

English

Address:

Daniel R Pilon
280 Slater Street P.O. Box 1046, Station B
Ottawa , ON K1P 5S9
Canada

Telephone:

(613)947-3775

Fax:

(613)995-5086

Email:

Click here

Web Site:

http://www.nuclearsafety.gc.ca/

87055-08-0312 - Incorporating Ageing Effects into PSA Applications – Phase 2 – R322.2

1.0 Background

Probabilistic Safety Assessment (PSA) is one of the most effective tools for the risk-informed decision-making. Thus, it is important to have credible and defensible PSAs representing adequately the actual risk profile of the plants.

The current standard PSAs do not address important ageing issues. For instance, reliability models of components are based on the "component constant failure rate" assumption, which is not valid in the long term. Consequently, ageing-related models and the effects of test and maintenances in controlling the ageing of safety components have to be applied.

PSA is a tool to evaluate plant safety, and CNSC should ensure that licensees maintain the plant safety at an acceptable level for the whole plant life. However, no Canadian PSA developed to date models ageing effects; therefore, the PSAs validity is limited in time. Furthermore, existing PSAs traditionally neglect some components (e.g. cables, structures) as having very low failure probability, but they may have an increasing contribution due to ageing effects.

The major problem with modelling of ageing effects is that the experience in this area is limited worldwide, there is no commonly accepted approach, all studies are performed by relatively isolated organizations, and related publications are scarce. Therefore, CNSC does not have presently expertise in assessing importance and adequacy of incorporating ageing effects, as well as effects of plant life management activities, into PSAs. Accordingly, the CNSC staff cannot require or deny soundly the need for addressing the issue in the regulation. Also, the CNSC staff does not have expertise in evaluating advantages and shortcomings of different ageing-related modelling approaches. This may undermine credibility of PSAs developed by licensees according to requirements of the current regulation and used by CNSC for its risk-informed decision-making.

This project will benefit from other researches done in CNSC and worldwide and will disseminate results to the international nuclear community.

The work, which will be performed according to this contract, is a continuation of the work performed in Phase 1 of the project. The contractor will receive all reports developed and information collected in Phase 1 of the project, which includes:
• Overview of similar researches performed in Canada and abroad and associated reports if available;
• Relevant information from CNSC and Canadian plants related to PSA, degradation mechanisms and ageing management;
• Detailed results of the process for selecting Systems, Structures, and Components (SSCs) for consideration in Phase 2.

For purpose of this statement of work, we will refer to a PSA, which explicitly incorporates ageing effects and is capable of generating age-dependent risk profile of the plant, as Ageing PSA (APSA).

2.0 Objectives

The overall objective of the project is to determine an appropriate regulatory approach and to acquire an expertise in addressing aging effects in the Probabilistic Risk Assessment, which will provide CNSC with a credible tool for risk-informed decision-making in the area of ageing management.

The objective of this phase of the project is to develop methods for evaluating effects of ageing and plant life management activities on reliability of the selected SSCs, and assist CNSC staff with understanding advantages and deficiencies of each method and data requirements for performing ageing-related reliability studies.

3.0 Scope of Work

The contractor will be required to perform a preliminary screening of degradation mechanisms, operational stressors, ageing management activities and develop the final list for detailed modelling. The contractor shall then develop generic time-dependent reliability models for selected degradation mechanisms, operational stressors, and ageing management activities.

4.0 Tasks to be Performed

4.1 For each degradation mechanism in Table 1 perform a preliminary simplified qualitative and, if possible, quantitative evaluation to screen out those having a negligible impact on the APSA results.

For instance, this screening process may conservatively evaluate the maximum possible degradation of a SSC over plant life due to a specific mechanism, identify related failure modes, and conservatively estimate the worst potential impact of these failure modes on PSA results. If such simplified screening is impossible for a specific degradation mechanism, this conclusion needs to be documented and these degradation mechanisms will be addressed in the detailed modelling. If a detailed model cannot be developed for any reason, this degradation mechanism will not be considered further in this phase of the project and requirements for additional experimental work and research shall be established.

The contractor may propose additional SSCs and/or degradation mechanisms to be considered in the APSA if they are deemed necessary from the APSA importance point of view.

4.2 For each selected degradation mechanism, identify applicable operating stressors and ageing management activities, which can be modelled in the APSA and can have significant impact on the results.

The list of degradation mechanisms, associated operational stressors and ageing management activities shall be split on three approximately equal from the efforts point of view sets with decreasing priorities from first to the last set. This will be used as basis for completing intermediate deliverables to CNSC.

4.3 For each selected degradation mechanism, develop generic time-dependent reliability models, which can be further incorporated into plant-specific PSA and adjusted to plant-specific equipment and environment. The contractor may propose several approaches for modelling and discuss advantages and drawbacks of each.

These models shall:
• Consider impact of applicable operational stressors (e.g., vibration, static and dynamic loads, duty cycles, flow, temperature, pressure, chemical interactions, etc.) and ageing management activities (e.g., inspection, testing, preventive and corrective maintenance, etc.)
• Consider applicable failure modes and their impact on PSA. At least, impact on probability of failure on demand, probability of failure during mission, and frequency of initiating event shall be considered.
• Be coded as plug-in modules to SAPHIRE and have sufficient number of adjustable variables to cover most of the specific plant operational conditions. However, contractor may propose an alternative approach how to calculate time-dependent reliability parameters for different SSCs and supply them into SAPHIRE for running plant PSA model for a specific year of the plant life.
• Establish requirements for input data.
• Account for aleatory and epistemic uncertainties.
• Include sufficient description to allow PSA analysts incorporating the models into plant-specific PSA and adjusting them to plant-specific conditions.

Existing models developed in other studies shall be used as much as possible, but their applicability shall be evaluated and appropriate source documentation shall be referenced. The information on related studies has been collected in the first phase of this project, R322.1, and will be provided to the contractor.

The models will be delivered to CNSC in three packages according to the preliminary division in the previous task.

4.4 Document the results of the project insuring that the following is included:
- description of all available models whether developed within the project or known from literature with updates if necessary;
- when it is concluded that a model(s) cannot be developed with a credible accuracy using the current level of knowledge, the basis for this conclusion and requirements for additional research and/or data collection to cover the gaps; and
- requirements for additional experimental work and research shall be established.

*** Please contact MERX to obtain documents ***

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87055-08-0312 - Incorporating Ageing Effects into PSA Applications – Phase 2 – R322.2

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