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Tieftemperaturverhalten und Alterung von Elastomerdichtungen - Aktuelle Tätigkeiten und Ziele
(2012)
The long time-frame of the necessary safe enclosure of radioactive waste in containers gains interna-tionally an increasing attention. As well extended storage periods as a usually necessary subsequent transportation after storage have to be evaluated.
In Germany, the concept of dry interim storage in dual purpose metal casks before disposal is being pursued for spent nuclear fuel (SNF) and high level waste (HLW) management. The initially planned and established dry interim storage license duration of up to 40 years will be too short and its exten-sion will become necessary since there is no repository available in due time.
The main safety relevant components are the thick-walled dual purpose metal casks. These casks consist e.g. of a monolithic cask body with integrated neutron shielding components (polymers, e.g. polyethylene) and a monitored double lid barrier system equipped with metal and elastomer seals. The metal seals of this bolted closure system guarantee the required leak-tightness whereas the elastomer seals allow for leakage rate measurement of the metal seals.
At BAM we focus on the long-term performance of the sealing system and neutron shielding materials as their application conditions have to be concerned as rather demanding.
Therefore, BAM began to investigate the overall function and long-term behavior of elastomer and metal seals and also the irradiation influence on neutron shielding materials.
Recently a new project was initiated to expand our investigations to get more and in some context better applicable data.
This paper describes the approach of our new research project with the aim of further closure of existing knowledge gaps and presents an overview of investigations concerning the low temperature behavior of elastomer seals, of running long-term tests on elastomer seals at different temperatures under static conditions and the influence of gamma irradiation on neutron shielding materials.
Rubber is widely used as sealing material in various applications. In many fields the function of seal materials at low temperatures is necessary.
Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling the material changes from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour, that allows nearly no strain or retraction, due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely, what can cause problems during application. Therefore the lower operation temperature limit of rubber seals should be determined in dependence of the material properties.
In the past we investigated the material properties and sealing behaviour of elastomeric seals at low temperatures. Additionally we started to study the effect of a small partial release of elastomer seals at low temperatures. To study the influence of dynamic events on seal performance and to enhance the understanding of occurring seal failure, a setup for a fast partial seal release was designed.
In this contribution we focus on the correlation of material properties and component behaviour under different application conditions. It is desired to widen the understanding of the process of seal failure at low temperature.
As a Federal institute for materials research and testing it is part of our tasks to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the investigation of elastomeric seals and polymeric neutron shielding materials used in the containers.
In this contribution we would like to present our research focused on the long term performance and associated changes of material properties of elastomeric seals (e.g. EPDM, FKM) and neutron shielding materials ((Ultra) High Molecular Weight Polyethylene) employed in storage casks. Our approach comprises accelerated ageing investigations at elevated temperatures and to some extent investigations of irradiated materials.
As the requirements for the extended use of cables in nuclear power plants and for the long term use of cask components are to a certain extent similar, our work might be of interest for the workshop on “Cables ageing in nuclear power plants; R&D current status and forecast”.
Ein wesentliches sicherheitstechnisches Kriterium für Transport und Lagerbehälter ist das Langzeitverhalten von verschraubten Deckel-Dichtsystemen unter Verwendung von Metall- und Elastomerdichtungen sowie von im Behälterkörper integrierten Polyethylenen zur Neutronen¬abschirmung.
Die ordnungsgemäße Funktion der Metalldichtungen ist Voraussetzung für die Einhaltung der geforderten Dichtheit des Deckelsystems, während die Elastomerdichtungen für die Messung der Leckagerate an den Metalldichtungen benötigt werden. Darüber hinaus kommen Elastomer-dichtungen als Primärdichtungen in Behältern für radioaktive Abfälle mit vernachlässigbarer Wärmeentwicklung zum Einsatz.
Zur Bewertung des Dichtungsverhaltens sind die alterungsabhängige verbleibende Rückstellkraft für die Metall- und Elastomerdichtungen sowie die sog. nutzbare Rückfederung bei Metalldichtungen von entscheidender Bedeutung. Für Elastomerdichtungen ist zusätzlich eine radiologisch bedingte Materialdegradation zu bewerten.
In Bezug auf das Alterungsverhalten von Polyethylen zur Neutronenabschirmung sind neben zeit- und temperaturabhängigen Materialveränderungen insbesondere auch durch Gammastrahlung induzierte Degradationseffekte wie Wasserstoffabspaltung, Vernetzung und Abbau der Polymer¬moleküle zu berücksichtigen. Über die eigentliche Bewertung der Abschirmeigenschaften hinaus sind somit zusätzlich Auswirkungen von Degradationsprodukten auf andere Behälterkomponenten zu diskutieren.
Die an der BAM im Fachbereich 3.4 Sicherheit von Lagerbahältern durchgeführten und aktuell laufenden Untersuchungen werden vorgestellt und diskutiert.
Rubber is widely used as sealing material in various applications. In many fields the function of seal materials at low temperatures is necessary. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling, due to the glass transition the material changes from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour, that allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition. But as the minimum working temperature limit of elastomers cannot be defined globally and precisely, the lower operation temperature limit of rubber seals should be determined in dependence of the application conditions and the most relevant material properties.
In this paper, wesummarize results of our temperature dependent investigation of seal material properties by classical thermal analysis as Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA), combined with measurements of standardized tests as compression set and the seal performance determined in component tests. To reduce the test time of compression set tests a faster technique was developed and applied.
To study the influence of dynamic events on the seal performance and to enhance the understanding of occurring seal failure, a setup for a fast partial seal release was designed.
For many sealing applications rubbers are applied due to their special material properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to low temperatures during operation. Therefore, it is important to know the material behaviour at those low temperatures to determine the temperature range that allows a safe operation of the seal.
In this work, we focus on the behaviour of elastomer seals at low temperatures with regard to potential decrease of leak-tightness or catastrophic seal failure of O-ring rubber seals. This is required as material properties of rubbers are strongly temperature dependent but their temperature application range is not always clearly defined.
Based on previous investigations which considered the physical material properties and the seal behaviour under purely static conditions we widen the focus on the sealing performance after a fast partial relief of compressed seals and additional materials.
For the investigations, different typical rubber seal materials were used as e.g. fluorocarbon (FKM), ethylene propylene diene (EPDM) and hydrogenated acrylonitrile-butadiene (HNBR) rubber.
The correlation of the physical material properties with seal performance is studied. This includes as well purely static sealing applications as the Performance during/after fast partial relief of seals at low temperatures.
Characterisation of degradation and determination of end-of-lifetime criterion for O-ring seals
(2017)
The ageing of polymers, including elastomers, is an undesirable but inevitable process leading to a limited lifetime of rubber products such as seals. Independent of the application, as e.g. automotive, piping or container applications, a long lifetime or at least a timely exchange of a seal before occurence of critical failure is desired.
In order to investigate the degradation of material properties and to determine the lifetime of elastomers used as seals, an ageing programme was started with hydrogenated acrylonitrile butadiene rubber (HNBR), ethylene propylene diene rubber (EPDM) and fluorocarbon rubber (FKM) [1-3]. Both O-ring seals (uncompressed and compressed by 25 %) with a cord diameter of 10 mm as well as sheets with a thickness of 2 mm were aged at 75 °C, 100 °C, 125 °C and 150 °C for up to two years. The changes of material properties during ageing were characterised for each material using the sheets by means of e.g. dynamic-mechanical analysis and tensile tests, while sealing properties such as compression stress relaxation (CSR), compression set (CS) and leakage rate were determined on O-rings. The experimental results indicate that while material properties, CSR and CS show considerable degradation effects, the static leakage rate stays constant or even decreases before failure occurs quite suddenly at advanced degradation levels. This reveals that static leakage rate has only limited sensitivity for the degradation of the seal material.
Our approach to determine a suitable end of lifetime criterion, which involves a partial decompression of the seal during the leakage test, is presented and discussed.
Elastomer seals are widely used as barrier seals in containers for low and intermediate level radioactive waste and in some countries for spent fuel transportation casks. Depending on the area of use, the rubber materials have to demonstrate proper sealing performance with regard to mechanical, thermal and environmental conditions as well as irradiation during the entire operation period.
In Germany the low and intermediate level radioactive waste is planned to be disposed in the Konrad repository in standardized containers. Many of these containers use an elastomer seal between lid and container body. Depending on the radiactive inventory and a possible containment of the waste in inner barrels and/or concrete structures, such containers may require a specific leak tightness. In such cases, specified leakage rates are shall not be exceeded by the the seals to keep potential activity releases from the inventories well below defined limits.
Generally, all waste containers have to fulfill the waste acceptance criteria for the Konrad repository as defined by the reports BfS SE-IB-29/08 and SE-IB-30/08. From these criteria several requirements for seal performance are derived. The seals have to:
- ensure full functionality until final container disposal,
- maintain its function in case of a thermal or mechanical accident,
- ensure encapsulation also at -20 °C.
This combination of requirements is challenging especially as a seal exchange, which is typical for many other applications, is hardly possible and thus a sound understanding of how to perform safety evaluation of containers is required. Therefore BAM has initiated investigations on elastomer seal behaviour. This work focuses on low temperature behaviour and sealing performance concerning static conditions as well as partial loss of compression and ageing of elastomer seals. Concerning ageing, property changes caused by long-term thermooxidative conditions as well as gamma irradiation are investigated.
This contribution presents an overview and explanation of the aims of the ongoing research and gives an update of the most relevant results.
For many sealing applications rubbers are applied due to their special material properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to low temperatures during operation. Therefore, it is important to know the material behaviour at those low temperatures to determine the temperature range that allows a safe operation of the seal.
In this work, we focus on the behaviour of elastomer seals at low temperatures with regard to potential decrease of leak-tightness or catastrophic seal failure of O-ring rubber seals. This is required as material properties of rubbers are strongly temperature dependent but their temperature application range is not always clearly defined.
Based on previous investigations which considered the physical material properties and the seal behaviour under purely static conditions we widen the focus on the sealing performance after a fast partial relief of compressed seals and additional materials.
For the investigations, different typical rubber seal materials were used as e.g. fluorocarbon (FKM), ethylene propylene diene (EPDM) and hydrogenated acrylonitrile-butadiene (HNBR) rubber.
The correlation of the physical material properties with seal performance is studied. This includes as well purely static sealing applications as the performance during/after fast partial relief of seals at low temperatures.
Dichtungsverhalten bei niedrigen Temperaturen - Untersuchung der unteren Temperatureinsatzgrenze
(2018)
Elastomere werden vielfach in Produkten eingesetzt, die bei verschiedenen Temperaturen verwendet werden. Dabei sind O-Ringe kostengünstige Dichtelemente, die allerdings für die Funktionsfähigkeit und Sicherheit eines Produkts entscheidend sind. Die untere Temperatureinsatzgrenze ist dabei durchaus relevant und aus wissenschaftlicher Sicht nicht allgemeingültig zu definieren, da diese einerseits durch die Änderungen der Materialeigenschaften im Zuge des Gummi-Glas-Übergangs definiert werden, die Temperaturlage dieses Übergangs jedoch sehr stark von den jeweiligen Einsatzbedingungen (z.B. Frequenz) abhängt. Dadurch ergeben sich große Unterschiede zwischen statischen und dynamischen Anwendungen. Verschiedene Untersuchungen sollen vorgestellt und interpretiert werden, um das Auditorium auf verschiedene Herausforderungen aufmerksam zu machen und Lösungsansätze vorzustellen...
Elastomers show a high versatility which makes them ideal materials for sealing applications in various fields. Especially under changing application conditions the high recovery potential of this class of material is beneficial to compensate temperature or pressure fluctuation, and geometrical changes resulting from mechanical loads in e.g. accident conditions. Out of these reasons elastomers are also used in containers for low and intermediate level radioactive waste and for spent fuel transportation casks. In cask designed for low and intermediate level waste elastomer seals can act as primary seal responsible for the containment function whereas in spent fuel storage and transportation casks (dual purpose casks (DPC)) elastomer seals are used as auxiliary seals to allow leakage rate measurements of metal barrier seals. An inherent prerequisite for this kind of application is the long time-scale of operation without or with limited possibility of seal replacement. In Germany an interim storage license for DPC`s is typically issued for 40 years, a timeframe which might increase in future due to challenges of the final repository siting procedure. For low and intermediate level waste, also long time periods are required before final disposal can be achieved. Therefore, the performance of elastomer seals over extended time periods is, as for other applications, of high importance.
A typical approach to ensure long-term functionality is to perform accelerated aging tests to calculate an estimated lifetime by assuming e.g. Arrhenius like equations for the time-temperature relationship. This approach requires a suitable end of life criterion considering the application of interest. This often can represent a challenge on its own.
As BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues we initiated several test programs for investigating the behavior of elastomer seals. Experiments concerning the low temperature performance down to -40°C and the influence of gamma irradiation have been started first. Currently the thermal aging behavior of elastomer seals, which is the topic of this contribution, is examined.
For our aging investigations we use a broad approach to first determine the property changes in different elastomer materials due to thermo-oxidative aging at elevated temperatures and secondly we test how the typical methods of lifetime extrapolation can be applied to these results. This approach enables us to detect and exclude undesired side effects which very often influence lifetime estimations. In this contribution, our recent results are discussed.
Low temperature performance of rubber seals - Influence of crystallite formation on seal performance
(2018)
Rubbers are widely used as sealing materials in various applications. In many fields the function of seal materials at low temperatures is necessary. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that rubbers possess strongly temperature dependent material properties. At low temperatures high changes in the material properties are observed which can be caused by the rubber-glass transition (abbr. glass transition) or for some rubbers also by partial crystallisation at lower temperatures. During continuous cooling the material changes during rubber-glass transition from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour. In case of (thermal-) crystallisation the formed crystallites act as additional (physical) crosslinks within the material but the molecules in the amorphous phase can still behave like a rubber. Hence, rubbers are normally used above rubber-glass transition and (thermal-) crystallization temperature but the minimum working temperature limit is heavily depending on the application conditions. Therefore, the lower operation temperature limit of rubber seals is under investigation.
In the past we have investigated material properties and sealing behaviour of rubber seals at low temperatures. As the initial work was dedicated to purely static sealing conditions we started to investigate the effect of a small partial release of rubber seals at low temperatures to study the influence of dynamic events on leak tightness. In addition, other groups report on the influence of counterpart surface and adhesion.
In this contribution we focus on the difference between purely amorphous elastomers and elastomers which become partially crystalline at low temperatures. The influence of partial crystallization on the low temperature performance of rubber seals is discussed.
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung (BAM). Our investigations comprise investigations on metallic and elastomeric seals and covers experimental investigations to get a database on the component/material behaviour, work on analytical descriptions and numerical analysis. Our aim is to understand the long-term behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation.
Elastomere werden vielfach in Produkten eingesetzt, die bei verschiedenen Temperaturen verwendet werden. O-Ringe sind kostengünstige Dichtelemente, die allerdings für die Funktionsfähigkeit und Sicherheit eines Produkts entscheidend sein können. Die untere Temperatur-einsatzgrenze ist dabei relevant und aus wissenschaftlicher Sicht nicht allgemein gültig zu definieren, da diese einerseits durch die Änderungen der Materialeigenschaften im Zuge des Gummi-Glas-Übergangs definiert werden, die Temperaturlage dieses Übergangs jedoch sehr stark von den jeweiligen Einsatzbedingungen (z.B. Frequenz) abhängt. Dadurch ergeben sich große Unterschiede zwischen statischen und dynamischen Anwendungen.
Verschieden Untersuchungen sollen vorgestellt und interpretiert werden, um das Auditorium auf verschiedene Herausforderungen aufmerksam zu machen und Lösungsansätze vorzustellen.
For many sealing applications rubber O-rings are applied due to their special properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to a variety of conditions during operation as e.g. low temperatures, elevated temperatures and different media. Therefore, it is important to know the material behaviour and the seal performance under those conditions. Additionally ageing of seals is an important factor which has an influence on the long term reliability.
In this contribution we would like to discuss different aspects affecting the seal performance, lifetime estimation and address open questions which should be addressed to come to a deeper understanding of these seals commonly understood as typical machine elements.
Results of running investigations at the Bundesanstalt für Materialforschung und -prüfung (BAM), Division 3.4 “Safety of Storage Containers” are presented and discussed.
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung. The behaviour of metallic seals, which are employed in interim storage casks for used fuel and high active waste, over time is of high importance for ensuring safe enclosure. Therefore, investigations on these systems were started at BAM to get a general understanding of the relevant processes. Our investigations comprise investigations on different parameters which influence the seal performance and the main part is focussed on the time and temperature dependent behaviour.
In this contribution an overview of the performed tests and their respective results will be given. A focus will lie on a comparison between seals with large torus diameter in comparison to small diameters.
Our aim is to understand the long-term behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation.
Understanding the failure mechanisms that lead to leakage of rubber seals at low temperatures is important. Rubber properties are highly temperature dependent. Changes in behaviour at low temperatures can result from the rubber-glass transition and for some rubbers by partial crystallisation. Experimental results on the low temperature behaviour of different rubber seals were discussed to help understand seal failure processes.
In this work we present results for three typical sealing materials (EPDM, HNBR, FKM) after up to 100 days ageing at high temperature under hydrogen. To allow for a better evaluation of the occurring changes the results are compared with effects of ageing in air on the material properties. Despite the common assumption that ageing under hydrogen atmosphere is expected to be less severe for the material in com-parison to air ageing, this is not seen for every material.
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung. Metallic seals are in use in interim storage casks for used fuel and high active waste. Their change in performance over time is of high importance for maintaining safe enclosure. Therefore, investigations on these systems were started at BAM to get a general understanding of the relevant processes. Our investigations comprise investigations on different parameters which influence the seal performance and the main part is focussed on the time and temperature dependent behaviour.
In this contribution an update on the performed tests and their respective results will be given in respect to the overview presented in 2020.
The understanding of the long-term behaviour of the sealing systems and the performance evaluation during interim storage and subsequent transportation are our goals.