Ageing PSA incorporating effectiveness of maintenance and testing

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Highlights

  • New approach to Ageing Probabilistic Safety Assessment (APSA).

  • Ageing, maintenance and testing addressed simultaneously and explicitly in APSA.

  • APSA for Risk Informed Decision Making (RIDM) to support NPP Safety Review.

  • RIDM on changes to maintenance management programs.

  • RIDM on changes to surveillance requirements.

Abstract

This paper proposes a new approach to Ageing Probabilistic Safety Assessment (APSA) modelling, which is intended to be used to support risk-informed decisions on the effectiveness of maintenance management programs and technical specification requirements of critical equipment of Nuclear Power Plants (NPP) within the framework of the Risk Informed Decision Making according to R.G. 1.174 principles. This approach focuses on the incorporation of not only equipment ageing but also effectiveness of maintenance and efficiency of surveillance testing explicitly into APSA models and data. An example of application is presented, which centres on a critical safety-related equipment of a NPP in order to evaluate the risk impact of considering different approaches to APSA and the combined effect of equipment ageing and maintenance and testing alternatives along NPP design life. The risk impact of the several alternatives is quantified and the results shows that such risk depends largely on the model parameters, such as ageing factor, maintenance effectiveness, test efficiency.

Introduction

One of the main challenges for 2020 horizon is to maintain fission technologies competitiveness by research and technological development necessary in order to allow a safe Long Term Operation (LTO) of the nuclear reactors in operation nowadays (GEN II and III) [1], [2], [3]. For example, in 2020 most of the Nuclear Power Plants (NPP) in Spain, which are Light Water Reactor (LWR) type, would be operating for around 40 years, which is the plant design life, and they will face the Periodic Safety Review (PSR) to obtain, eventually, life extension approval for the LTO from the regulatory body (usually for 10 more years).

During the design life, licensee of NPP must perform the necessary activities for ageing management of Structures, Systems and Components (SSC), which include consideration of measures for monitoring, control and mitigation of the mechanisms of physical ageing of SSC adopting the necessary maintenance in scope of asset management programs and surveillance testing activities within NPP technical specifications.

An important part of PSR involves the evaluation of risk impact of how current life management of ageing equipment, e.g. maintenance and testing programs, is successful in achieving the objective of allowing a safe operation along NPP design life. In particular, PSR close to the end of the NPP design life should involve the evaluation of at least, but not only, how ageing management along NPP design life would jeopardize NPP safety for the extended period. In addition, if necessary, it should be also evaluated the effectiveness of adopting an alternative ageing management plan, e.g. maintenance policy, or even, studying the risk impact of changing technical specification requirements, such as surveillance requirements, for the extended life in order to achieve the safety standards required to get approval from regulatory bodies for NPP long term operation.

Section snippets

Probabilistic safety assessment and its application

Probabilistic Safety Analysis (PSA) is an essential tool for assessing, maintaining, assuring and improving the NPP safety. Nowadays, PSA is an efficient tool for evaluation of risk impact of changes to licensing basis [4] and in particular to surveillance requirements of NPP technical specifications within the framework of the Risk Informed Decision Making (RIDM) according to R.G. 1.174 principles [5], [6].

In a similar way, PSA can become also a relevant tool for the evaluation of the

Effectiveness of maintenance and testing

Ref. [17] reviews briefly the role of maintenance and testing activities to achieve appropriate levels of reliability, availability and safety of safety related equipment at NPP. It illustrates basic concepts in relation to natural reliability, reliability degradation (e.g. ageing as degradation mechanism), inherent reliability, maintainability (e.g. maintenance types such as preventive, corrective and overhaul maintenance), availability, maintenance effectiveness (e.g. imperfect maintenance)

Failure rate model

Traditionally, standard PSA of NPP assumes constant failure rate to modelling the reliability of components normally in standbyλ(t)=λ0

In Ageing PSA, failure rate models must be formulated using age-dependent models, i.e. component reliability depends on its age, to account for the effect of component ageing. Different models of age-dependent reliability of components have been proposed in the literature [8], [9], [18], [22], [23].

The selection of the most adequate reliability model depends on

Unavailability modelling

As introduced above, Surveillance Requirement involves periodic tests, e.g. monthly or quarterly, with positive and negative effects [5]. The positive effect of testing is its capability to detect hidden failures and this way limiting the risk of undetected downtimes of the safety component, i.e. the “test-limited” risk, which depends on the equipment reliability, i.e. equipment failure rate, test efficiency, etc (see Section 4.4). Often, a very important adverse effect of testing, to be

Risk modelling

There are different approaches that can be used for transforming a standard PSA to an age dependent PSA, or an APSA. Ref. [9] introduces three basic approaches. Herein, only two are introduced.

The first approach involves carrying out a standard PSA evaluation a number of times with different component failure rates used in each evaluation. The different component failure rates which are used are stepwise approximations to time dependent component failure rates. This approach is straightforward,

Problem description

This section presents a simple example of application that focusses on analysing the risk impact of ageing of a single critical component of a NPP depending on several maintenance and surveillance alternatives along the last ten years of a NPP design life.

A motor-operated valve (MOV) of the Auxiliary Feed Water System (AFWS) has been selected based on several arguments. This valve is normally open and its function is to control the flow from AFWS until Steam Generators on the secondary of a

Concluding remarks

This paper presents a new approach to Ageing PSA (APSA) modelling to be considered in the framework of the Risk Informed Decision Making according to R.G. 1.174, which is intended to be used to support risk-informed decisions on the effectiveness of asset management programs and technical specification requirements of critical equipment of Nuclear Power Plants (NPP). This APSA can help in performing the Periodic Safety Review to support risk-informed decisions required to obtain approval from

Acknowledgements

Authors are grateful to the Spanish Ministry of Science and Innovation for the financial support of this work (Research Project ENE2013-45540-R) and the Doctoral Fellow (BES-2011-043906).

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