Critical Studies in support of the Ageing Management of NPP Concrete Infrastructure

The ageing management of nuclear power plants (NPP) structures, systems and components should be implemented proactively throughout the plant’s lifetime. It is initiated during the design, and follows through fabrication and construction, commissioning, operation including long-term operation (LTO) and all the way through extended shutdown, and decommissioning. Many of the decisions related to ageing management are made early in the process, during the phases of design, construction and commissioning [1].
Current NPP have been designed for 50 years of operation and the concrete structures are constructed before the starting of the NPP operation, i.e. concrete structures’ age is older than 50 years. There is evidence that ageing effects in some cases has been underestimated during the original design, construction and commissioning or have not been accurately taken into account during operation. It has also been recognized that the ageing of plants needs to be assessed, and an effective management strategy developed in a timely manner, to ensure the necessary technical basis for maintaining safety margins throughout the NPP operation.
The project CONAGE addresses these ageing aspects of NPP structures, systems and components. This project identifies special issues that are needed for LTO within the scope of ageing management. The topics of research addressed in this project are of interest not only for the concrete infrastructure of existing NPPs, but also especially for those in the design phase. According to an IAEA study [2], 90 % of all ageing problems are initiated in design phase.
NPP ageing management programmes should have access to relevant concrete related R&D. This is fundamental since new developments may contradict assumptions made during a plant’s design [2] or highlight new issues previously unknown. Even though concrete has been extensively used as a construction material, the concrete used 40 years ago differs significantly from that commonly used today. The reinforced concrete structures of NPP (e.g. containment, spent fuel pools, water intake/out-take structures, foundations) perform multiple safety related functions (e.g. load carrying, radiation shielding and leak tightness). For many of these structures, it is neither technically nor economically feasible to have them replaced. For this reason, it is important that a comprehensive understanding of all possible ageing mechanisms, and their degradation consequence for the safety function of the structure, is achieved. This way, adequate considerations can be made during the design of existing NPPs, while mitigation measures can be planned for existing NPPs.
CONAGE addresses research topics related to the ageing mechanisms and age-related degradation, condition assessment and key aspects of ageing management (i.e. inspection, monitoring, assessment and remedial measures). The research topics are divided into three Work Packages.
Work Package 1: Non-destructive evaluation of NPP concrete infrastructure, addresses condition assessment and inspection of concrete structures. This work package focuses greatly on competence development with concrete NDE, and on the assessment of existing and emerging NDE technologies using the full-scale mock-up of a NPP containment wall developed in the SAFIR2018 - WANDA project [3]. Basic non-destructive testing (NDT) techniques such as rebound hammer, concrete cover meter and ultrasonic pulse velocity (UPV) were used to characterise the common properties of the reinforced concrete mock-up wall. Advanced NDT techniques such as ground penetrating radar (GPR) and ultrasonic pulse echo (UPE/MIRA) were used for reinforcement rebar mapping, reinforcing steel location and concrete imaging surveys for identifying/quantifying the sub-surface defects.
One goal of this task was to familiarize students with the field of assessment of NPP reinforced concrete structures using basic and advanced NDT techniques commonly used in construction engineering. To achieving this goal, a part of student training course (CIV-E2030 - Experimental Methods in Building Materials) at Aalto University School of Engineering was arranged. The course included a researched based teaching which used the research result of CONAGE project, also a national seminar about the field of NDT techniques for the Finnish NPP industry was arranged during the course. The students learn and practice the basic and advanced NDT techniques through (i) lectures given by experts in the field, (ii) excursions to a companies providing NDT services and (iii) laboratory demonstrations for common NDT devices on the full-scale mock-up of a NPP containment wall.
Research results from the NDT techniques show that the combination of several NDT techniques can mutually strengthen individual assessment, however, the diagnosis of the results requires additional expertise to interpret NDT combined measurements. The use of Ultrasonic pulse echo (UPE) as MIRA and Monolith can enhance and automate the assessment of reinforced concrete structures. Ground penetrating radar (GPR) is useful for locating post tension cables, steel reinforcement and detecting the construction defects.
Work Package 2: Assessing the risk of internal expansive reactions for NPP concrete infrastructure, addresses ageing mechanisms and age-related degradation of concrete structures. This work package focuses studying the potential of NPP infrastructure to become damaged due to internal expansive reactions. Initial experimental work, never conducted before in Finland, characterises the performance of new Finnish concrete compositions (study critical alkali content values for concrete since Finnish concrete codes only recently have acknowledged the occurrence of this degradation mechanism in Finland), and will study the residual expansion potential of existing NPP concrete (cracking potential) subject to accelerated alkali aggregate reactions. This WP initiates the necessary work to understand how Finnish concrete (new and from existing structures) perform. The main goals for Work Package 2 are:
• Study reactive and non-reactive aggregates and typical Finnish structural grade concrete (varying cement types) and quantify their reaction potential.
• Contribute significantly to the current state of knowledge concerning AAR performance in Finland.
Research results from all the different accelerated tests shows that the same aggregate may be evaluated either unlikely to be reactive or potentially reactive based on the test method. The concrete tests RILEM AAR3 and RILEM AAR4 are considered more reliable than the mortar bar test RILEM AAR2. It should also be noted that the aggregate is partly of different size fraction and may thus contain fragments of different reactivity. It is, therefore, not quite so unexpected to see differences between the mortar bar and concrete prism tests. Also, there is some uncertainty, whether the certain aggregate in both the mortar bar tests and the concrete tests is in fact the same aggregate, which would explain the noticeable difference in the expansive behaviour between the mortar bar test and the concrete prism tests.
Work Package 3: The CONAGE project, “Critical Studies in support of the Ageing Management of NPP Concrete Infrastructure” in SAFIR2022 programme addresses the ageing aspects of nuclear power plant (NPP) structures, systems and components (SSCs). Of particular interest of WP3 of the project is the corrosion of nuclear containment liners.
Corrosion of nuclear containment liners embedded in concrete has been observed at several occasions and at various NPPs globally. Instances of steel liner corrosion have been reported as well in Sweden and Finland. Steel liners can be embedded inside the concrete containment or pool walls, or they form the inner surface of the wall. The liner materials are usually carbon steel, galvanised steel or stainless steel. Corrosion has been detected where the plates are in direct contact with concrete (outer surface). In many cases it has been associated with foreign material left embedded in the concrete, such as wood pieces or distance holders left in place at the time of concrete casting. Sometimes the cause of the corrosion has been unclear, but delamination of the steel plates from the concrete or liner bulging has been observed. Corrosion has appeared to penetrate the thick plates of the liner, or led to a significant decrease of the liner cross-section. These motivated us to propose two scenarios for the corrosion experiments based on the presence of a 1) foreign matter and 2) delamination gap between steel and concrete.
In 2022, the tests with regular electrochemical measurements, initiated in earlier years, were continued. The tests finished in September 2022 after the duration of approximately 2 years. Before finishing the experiments, the specimens were subjected to nonlinear ultrasonic measurements by a colleague, Ph.D. student Markus Nilsson from Lund University, Sweden.
After finishing the experiments, the electrochemical data was analysed. The specimens were opened, i.e., concrete was removed from the steel liner specimens, enabling the examination of steel specimens. They were photographed, and the photographs were subjected to image analyses in order to determine the extent of corrosion. Additionally, the specimens were subjected to profilometry studies in order to reveal the extent of material losses in the specimens. The obtained results were analysed and compared to findings from literature.
Assessing steel liner and anchor corrosion, addresses age related degradation of steel components embedded or in direct contact with concrete structures. This work package focuses on competence development and closing significant knowledge gaps with respect to the corrosion of steel liners and anchors in contact with concrete and its detection. Corrosion experiments will help to identify the electro-chemical conditions and the mechanisms that allow for the corrosion of the steel liner plates at the interface of steel and concrete, i.e. in an area where the liner should be passive, thus protected from corrosion.
Additional experiments will focus on corrosion and bond of anchors in concrete. Non-destructive examination (NDE) techniques will be assessed that can be used to identify and quantify locations and extent of corrosion damage. The full-scale mock-up of a NPP containment wall developed in the SAFIR2018 - WANDA project [3] will help to further develop feasible NDE technologies as well as assess and verify their applicability in detecting steel liner corrosion. The main goals for Work Package 3 are:
• Examine effects of material ageing, structural design, construction, ambient conditions and operation of NPPs on steel liner and anchor corrosion mechanisms and the roles of parameters promoting or preventing corrosion.
• Identify the electrochemical conditions and the mechanisms that allow for the corrosion of the steel liner plates and anchors in contact with concrete.
• Assess the feasibility of NDE technologies for detecting locations and quantifying the extent of corrosion damage. Identify and define measures for further development of NDE techniques capable of detecting liner corrosion.