Apros success story: OL3 commission support

Introduction

Teollisuuden Voima Oyj’s Olkiluoto 3 Nuclear Power Plant is an European Pressurized Water Reactor (EPR) with an electric power capacity of 1600 MW. It is the largest nuclear power plant ever commissioned in Finland. During its commissioning phase in 2022, Fortum provided extensive Apros modelling and analysis services to TVO. In cooperation with Fortum, TVO supported the OL3 commissioning team in progressing with the plant-level commissioning tests, during which serious issues in control loops were observed. With the help of modelling and simulation, the commissioning team was able to understand the root causes and identify possible solutions to these issues.

Case Description

Since 2007, Fortum and TVO have been developing together a dynamic simulation model for TVO's Olkiluoto 3 plant using Apros software. This simulation model has been utilized for various purposes by TVO over the years, and a new use case emerged during the commissioning tests in the spring of 2022. At that time, serious issues in control loops were noticed during the plant-level tests, which threatened to put the commissioning on hold. Given the massive power output of OL3, even an hour's delay in the commissioning schedule could cost tens of thousands of euros in lost income. Therefore, these problems needed to be addressed as soon as possible. Because a comprehensive simulation model of the entire EPR power plant existed already, the investigation of the root causes could start immediately.

Plant-level commissioning tests are a crucial part of the commissioning phase, where plant-level systems are tested at partial loads, including both normal operation and safety systems. During these tests, it was noticed that when the second main feedwater pump started at low partial loads, the main feedwater control valve position and steam generator water level oscillated divergently. At partial load, the temperature of the supplied feedwater is lower than at full power, causing the steam generator level oscillation to also affect the reactor core thermal power level, which was highly undesirable. This issue was investigated with high priority, and the existing OL3 Apros simulation model was quickly utilized as a tool for this analysis. During these investigations, it was possible to reproduce the divergent oscillation in the steam generator water level using the detailed simulation model and identify the root cause. It was essential that the relevant simulation capabilities and high-accuracy simulation model had been developed beforehand.

Analysis Results

The root cause of the oscillations was found to be the PID controller related to the steam generator water level. The controller had too high derivative term for the partial load operation conditions, which produced too coarse control output for the valves controlling the feedwater flow and, consequently, the steam generator water level. Based on the analysis, it was proposed to modify this derivative term.

During the commissioning phase, it was found that the Apros simulation model with a detailed 3D reactor model was the best way to accurately reproduce the original plant behavior (the divergent oscillation) and was therefore a very important tool for verifying the proposed control parameter changes. It is important to note that the investigated controller parameters are defined as a function of the reactor power. Therefore, it is crucial that the simulation model contained the detailed 3D neutronics reactor core model as well as accurate process models for both reactor and turbine islands and detailed control loop system models.

The proposed control modifications were verified with the Apros simulation model using several relevant transient cases, including part load and full load initial conditions. For example, the following transients were simulated and reported to TVO by Fortum:

• Stopping of one reactor coolant pump
• Load rejection to house load
• Turbine trip

End Results

Based on the Apros analysis results, TVO was able to demonstrate the root cause of the undesired oscillations and suggest a proper modification of the control systems to the system supplier. The corrective actions included adding a separate input signal for the controller’s derivative term and filtering it appropriately. After these corrective actions, the oscillation of the steam generator water level did not occur anymore, which enabled the commissioning team to progress further with the commissioning phase.

Lessons Learned

One lesson learned is that it is very important to have an extensive and accurate simulation model of the whole power plant available as soon as possible. Time and effort should be invested early in the project to ensure that the simulation model includes all necessary equipment and control systems at a detailed level, so that these kinds of engineering analyses can be carried out whenever necessary. When the simulation model is ready to be used before the commissioning phase, it reduces the time needed to find the root cause of problems and provides a way for engineers to find and verify solution proposals.

Future

Fortum and TVO will continue to cooperate in developing the Apros simulation models for all three Olkiluoto reactors to further ensure the safety and optimal operation of the nuclear power plant. TVO plans to further develop this model with the help of Fortum experts, so that it can be used to calculate deterministic safety analyses for the OL3 unit in the future.

"The use of Apros software and the expertise of Fortum's team were instrumental in helping us reach milestones in the OL3 project. Their support allowed us to quickly identify and resolve issues, ensuring the smooth progression of our commissioning phase. This collaboration has been a significant asset to our project's success." Juha Poikolainen, Head of Nuclear Safety Design, TVO