1 Abstract—Nuclear power is one of the main electricity production sources in Sweden today. Maintenance management is one tool for reducing the costs for operation of a power plant. Driving forces for cost-efficiency has pushed the development of new methods for maintenance planning and optimization forward. Reliability Centered Asset Management (RCAM) is one of these new approaches, and maintenance optimization is one way to perform quantitative analysis which is a feature of RCAM. This paper proposes a model for opportunistic maintenance optimization where replacement schedules for shaft seals in feed-water pump systems in nuclear power plants are constructed. The feed-water pump system is important for the availability of the entire nuclear power plant. Results show that the optimization model is dependent on e.g. the discount interest and a limit for when the optimal solution goes from non- opportunistic to opportunistic is calculated. The circumstances for which opportunistic maintenance could be used have been investigated given different values of discount rates and remaining life at start of the planning period. Index Terms—Maintenance management, nuclear power, opportunistic maintenance, optimization, reliability. I. INTRODUCTION sustainable energy system must involve several electrical production sources. Local conditions are decisive for determining the electrical production system that should be dominating. About 90% of the electricity production in Sweden is today from nuclear power and hydro power, however there are large increase in renewable energy production from e.g. wind. This work was supported by the Swedish Centre of Excellence in Electric Power Systems (EKC2) at KTH. Julia Nilsson is with the School of Electrical Engineering at the Royal Institute of Technology (KTH) in Stockholm, Sweden (e-mail: julia.nilsson@ee.kth.se ). Adam Wojciechowski, Ann-Brith Strömberg and Michael Patriksson are with the Department of Mathematical Sciences at Chalmers University of Technology and the Department of Mathematical Sciences at the University of Gothenburg, in Göteborg, Sweden (e-mail: wojcadam@chalmers.se , anstr@chalmers.se , mipat@chalmers.se ). Lina Bertling is with the Department of Energy and Environment, and Division of Electrical Power Engineering at Chalmers University of Technology, in Göteborg, Sweden (e-mail: lina.bertling@chalmers.se ). One benefit with nuclear power is that the operation is almost free from climate-affecting pollution. A major disadvantage is the waste and its final storage. Nuclear power is associated with several risks. Risk is defined as the probability of failure multiplied by the consequences of failure. The probability of failure is a reliability measure. The performance of operation and maintenance must always be safe and structured. Reliability-centered maintenance (RCM) is a structured approach that focuses on reliability when planning maintenance [1]. The deregulation of the electric power system and the introduction of the electricity market have generally led to lower investments in maintenance. As a result, new approaches for maintenance management have been developed, such as Reliability Centered Asset Management (RCAM) [2]. RCAM is a development of RCM into a quantitative approach with the aim to relate preventive maintenance to total maintenance cost and system reliability. This paper focuses on RCAM using a maintenance optimization approach for nuclear power plants. Moreover, an opportunistic approach is used. A general optimization model, applied for example to the replacement of components in aircraft engines [3], is used to gain a maintenance schedule for replacements of subcomponents. The system observed is the feed-water pump system, which is a system in the nuclear power plant critical for the availability of the plant. This system is necessary for maintaining a stable level of water in the reactor, and it must therefore keep a steady flow in the tank. Minimizing costs for replacing shaft seals and costs for production loss would gain an optimal maintenance schedule. As a case study, the Boiling Water Reactors (BWR) at Forsmark nuclear power plant are studied. II. THEORY A. Availability Availability is an important measure of reliability, and has several different definitions. One definition is found in [4]: An Opportunistic Maintenance Optimization Model for Shaft Seals in Feed-Water Pump Systems in Nuclear Power Plants J. Nilsson, Graduate Student Member, IEEE, A. Wojciechowski, A-B. Strömberg, M. Patriksson and L. Bertling, Senior Member, IEEE. A Paper accepted for presentation at 2009 IEEE Bucharest Power Tech Conference, June 28th - July 2nd, Bucharest, Romania 978-1-4244-2235-7/09/$25.00 ©2009 IEEE