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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.
Recovery is an important measure for seal applications representing to which extent the elastomer regains its initial shape after deformation and release of an applied force. Compression set (CS) indicates the degree of recovery. Ethylene propylene diene rubber (EPDM) was aged at 75 ◦C, 100 ◦C, 125 ◦C and 150 ◦C for different ageing times up to five years and compression set measurements were performed at different times after disassembly and after additional tempering. Short- and long-term recovery up to one year after release for samples aged at 125 ◦C and 150 ◦C was also studied. To assess the curvature in the Arrhenius diagram that may occur due to non-sufficiently aged specimens, a degradation-rate based model was fitted to the CS data after tempering. For each ageing temperature, two decay fit functions were proposed, each with an activation energy and a corresponding degradative process. The influence of ageing on the leak-tightness after fast small partial release is investigated and estimated through the analysis of the shift factors from time temperature superposition (TTS) of CS measurements at different times after disassembly. Shift factors of CS measurement after 1 s and after additional tempering are in good agreement.
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.
At the Bundesanstalt für Materialforschung und -prüfung (BAM) in Division 3.4 Safety of Storage Containers, one of our tasks is to evaluate the safety of containers designed for disposal of low and intermediate radioactive waste. As such containers might be transported before and stored until disposal, safe enclosure of the radioactive inventory is important for this time span.
Elastomer O-rings are widely used as barrier seals in these containers. Thus, as for many other applications, an understanding of the practical effects of ageing degradation on elastomer seals during long-term exposure is mandatory for predicting the lifetime of such components. According to a long-term test programme on three kinds of rubbers (EPDM and FKM (relevant for application), HNBR (for comparison)), over several years we have studied the degradation and the change of mechanical properties (e.g. hardness, strain at break) at four different ageing temperatures (75 °C, 100 °C, 125 °C and 150 °C) as well as the change of sealing properties. Continuous and intermittent compression stress relaxation (CSR) measurements were performed in order to investigate the respective contribution of crosslinking and chain scission to the observed degradation effects. Thus, the degradation kinetics and mechanisms could be resolved more clearly. For assessing the seal performance, compression set (CS) and leakage rate measurements were conducted. The experimental results showed that the O-rings remained leak-tight under purely static conditions even when CSR, CS and mechanical properties already indicated far advanced degradation. For this reason, a modified leakage test involving a small and rapid partial decompression of the seal was developed that enabled determining an end-of-lifetime criterion for O-rings with a safety margin for thermal shrinkage and vibrations.
The containment systems of transport and storage casks for spent fuel and highlevelradioactive waste usually include bolted lids with metallic or elastomeric seals. The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of such containment system.
Scaled cask models are often used for providing the required mechanical and thermal tests series.Leak tests have been conducted on those models.
It is also common practice to use scaled component tests to investigate the influence of deformations or displacements of the lids and the seals on the standard leakage rate as well as to study the temperature and time depending alteration of the seals.
In this paper questions of the transferability of scaled test results to the full size design of the containment system will be discussed.
The containment systems of transport and storage casks for spent fuel and highlevelradioactive waste usually include bolted lids with metallic or elastomeric seals. The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of such containment system.
Scaled cask models are often used for providing the required mechanical and thermal tests series.Leak tests have been conducted on those models.
It is also common practice to use scaled component tests to investigate the influence of deformations or displacements of the lids and the seals on the standard leakage rate as well as to study the temperature and time depending alteration of the seals.
In this paper questions of the transferability of scaled test results to the full size design of the containment system will be discussed.
Packages for the transport and storage of radioactive materials are often sealed with elastomer or metal seals. These seals are basic components to meet the leak tightness criteria for these kind of packages. An overview over ongoing research and development concerning metal and elastomer seals is given in the presentation. Introductions in the fundamental functionality of elastomer and metal seals are presented. Ageing processes are shown for both components regarding to different ageing effects.
Derzeit existieren für Gefahrgutverpackungen in den internationalen Gefahrgutvorschriften keine Festlegungen für quantitative Grenzleckageraten, die sich an Sicherheitsbetrachtungen während der Beförderung orientieren. Für die Dichtheitsprüfung im Rahmen der Bauartzulassung von Gefahrgutverpackungen für flüssige Füllgüter ist das Standardprüfverfahren das Eintauchverfahren in Wasser („Bubble Test“). Hierbei handelt es sich um ein lokalisierendes Prüfverfahren. Seine Anwendung lässt keine quantitative Aussage darüber zu, ob unter Beförderungsbedingungen aufgrund von strömungsbedingter Stofffreisetzung durch Leckstellen der Gefahrgutverpackungen die Gefahr der Bildung einer explosionsfähigen Atmosphäre besteht. Zentrales Ziel der vorliegenden Arbeit ist daher, zunächst quantitative Dichtheitsanforderungen an Gefahrgutverpackungen im Hinblick auf die Entstehung explosionsfähiger Dampf-Luft-Gemische während des Transports zu entwickeln. Im Anschluss werden strömungsbedingte Leckageraten der Verschlüsse verschiedener Bauarten von Gefahrgutverpackungen gemessen. Der Vergleich der Messwerte mit den berechneten Grenzwerten ermöglicht die Einschätzung hinsichtlich der Bildung einer explosionsfähigen Atmosphäre. Dieser quantitative Ansatz zur Beurteilung der Dichtheit ist für Gefahrgutverpackungen derzeit noch nicht etabliert. Die Grenzleckageraten werden für das Szenario des interkontinentalen Transports von Gefahrgutverpackungen in einem 20-Fuß-Frachtcontainer im Hinblick auf die untere Explosionsgrenze abgeleitet. Dies geschieht unter Annahme einer Worst-Case-Betrachtung für Beförderungsdauer, Beladung und Luftwechselrate. Als mittlere Beförderungstemperatur wird 30 °C angesetzt. Eine vollständige Durchmischung im freien Luftraum des Containers wird angenommen. Es werden drei repräsentative Baugrößen von Gefahrgutverpackungen gewählt, mit einem Volumen von ca. 6 L, ca. 60 L und ca. 220 L. Als Füllgüter werden die 23 meistbeförderten flüssigen Gefahrgüter betrachtet. Die treibende Kraft für die Strömung durch Leckstellen ist der sich in der Verpackung ausbildende Überdruck. Die Berechnung des Überdrucks erfolgt durch analytische Modellgleichungen in Abhängigkeit der spezifischen Stoffdaten, Füllgrad, Befülltemperatur, Transporttemperatur und Nachgiebigkeit der Verpackungsbauart. Die quantitative Leckageratenmessung der Gefahrgutverpackungen wird mit dem Überdruckverfahren mit Ansammlung (Akkumulationsverfahren) unter Verwendung von Helium als Prüfgas vorgenommen. Zusätzlich erfolgt die Detektion weiterer potentieller Leckstellen außerhalb des Verschlussbereiches mit dem Schnüffelverfahren. Bei allen untersuchten Bauarten, mit Ausnahme des 6 L-Feinstblechkanisters, ist der Verschluss die einzige systematische Leckstelle der Verpackung. Die Messung der Helium-Leckageraten und der anschließende Vergleich mit den berechneten Helium-Grenzleckageraten zeigt, dass folgende Bauarten hinsichtlich des Erreichens der unteren Explosionsgrenze (UEG) durch eine Leckageströmung als kritisch einzuschätzen sind: Kunststoffverpackungen mit Schraubverschlüssen mit Flachdichtung, wenn bei diesen bestimmte Schädigungsmuster im Verschlussbereich vorliegen; Feinstblechkanister, da bei ihnen nicht nur der Verschlussbereich eine Leckstelle darstellt; Kunststoffverpackungen mit Schraubverschlüssen mit Flachdichtung, wenn diese auch für Füllgüter der Verpackungsgruppe I zugelassen sind. Als Konsequenz sollten bei diesen kritischen Bauarten entweder Modifikationen in Bezug auf die Verpackung selbst oder auf die Transportbedingungen im Frachtcontainer vorgenommen werden. Bei Kunststoffverpackungen ist auch die Füllgutpermeation als Freisetzungsmechanismus relevant. Es wird der prinzipielle Rechenweg zur Berücksichtigung dieses Quellterms exemplarisch gezeigt. Diese Arbeit leistet einen grundlegenden Beitrag für die Etablierung einer systematischen quantitativen Dichtheitsbetrachtung von Gefahrgutverpackungen mit dem Ziel der Verbesserung der Sicherheit beim interkontinentalen Gefahrguttransport im Frachtcontainer.
The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of the containment system of transport casks for spent fuel and high radioactive waste. The containment systems of such transport casks usually include bolted lids with metallic or elastomeric seals.
Scaled cask models are often used for providing the required mechanical and thermal tests series. Leak tests have been conducted on those models.
It is common practice to use scaled component tests to investigate the influence of deformations or displacements of the lids and the seals on the standard leakage rate as well as to study the temperature and time depending alteration of the seals.
In this paper questions and open points of the transferability of scaled test results to the fullscale design of the containment system will be discussed.
The containment systems of transport casks for spent fuel and high radioactive waste usually include bolted lids with metallic or elastomeric seals.
The mechanical and thermal loadings associated with the routine, normal and accident conditions of transport can have a significant effect on the leak tightness of such containment system.
Scaled cask models are often used for providing the required mechanical and thermal tests series. Leak tests have been conducted on those models.
It is also common practice to use scaled component tests to investigate the influence of deformations or displacements of the lids and the seals on the standard leakage rate as well as to study the temperature and time depending alteration of the seals.
In this paper questions of the transferability of scaled test results to the full size design of the containment system will be discussed.