The determination of mechanical properties of materials provides the basis for the fundamental understanding of the behaviour of components that can experience degradation in operation and/or even during storage. A very representative example of this is the thermal aging mechanism that severely affects materials that are ultimately intended to operate in a harsh operating environment as that of a nuclear reactor.

Polymers, and especially elastomers, play a key role as part of the many mechanical, electrical and electronic components found in nuclear power generation plants. The degradation of polymeric materials is a frequent phenomenon that is accelerated, in many cases, by arduous operating conditions. Elastomers, especially rubbers - such as acrylonitrile butadiene, NBR - experience degradation that is favoured by contact with oxygen [1]. This type of reaction -which triggers the irreversible damage of the component- is favoured by an increase in the operating temperature. Therefore, it is of interest to analyse how their intrinsic properties influence their thermal aging.

One of most usual parts with relevant safety-related function in nuclear equipment is the NBR O-rings or gaskets that are used as mechanical sealing elements, since their safety function is being capable of preventing any leakage (whether internal or external) throughout the useful life of the equipment [2].

The objective of this work is the development of a methodology to determine the useful life -based on the storage temperature- of NBR O-rings using a reliability-based approach that allows to obtain the health condition at different supposed storage scenarios, considering the required in-service performance.

For the study, NBR has been selected as a gasket material, since a previous work has shown that acrylonitrile is the best option [3] to withstand moderate levels of radiation thresholds extracted from databases [4-5] as well as its recyclability providing a sustainable life cycle. The evaluated parameter has been the Shore A hardness in accordance with ISO 868 [6] during a period of five years (between 2014 and 2019). Thus, the thermal embrittlement is quantified based on an adaptation of Arrhenius model-based correlation between hardness and temperature and storage time. The study incorporates a comparison between the results obtained for newly manufactured and existing O-rings in the warehouse, considering several statistical scenarios.

REFERENCES

1. Azura A., Thomas A (2006): “Effect of heat ageing on crosslinking scission and mechanical properties. elastomer and components. service life prediction–progress and challenges”. In Coveney, V. (Ed.). Elastomer and components: Service life prediction - progress and challenges, 27–38, Cambridge: Woodhead Publishing.
   Available from: https://www.researchgate.net/publication/281852920_Effect_of_Curing_Systems_on_Thermal_Degradation_Behavior_ofNatural_Rubber_SMR_CV_60 [accessed Sep 08 2020].
2. EPRI CGI-OR02 (1992): “Commercial grade item evaluation for national O-Rings”. Electrical Power Research Institute, Palo Alto-CA (USA).
3. Rodríguez-Prieto A., Camacho A.M., Sebastián M.A., Yanguas-Gil A. (2019): “Analysis of mechanical and thermal properties of elastomers for manufacturing of components in the nuclear industry”. Procedia Manufacturing 41, 177–184.
4. IAEA-TECDOC-1551 (2007): “Implementation strategies and tools for condition based on maintenance at nuclear power plants”. International Atomic Energy Agency, Vienna, Austria.
5. Van de Voorde M.H., Restat C. (1972): “Selection guide to organic materials for nuclear engineering”. European Organization for Nuclear research, CERN, Geneva, Switzerland.
6. ISO 868 (2003): “Plastics and ebonite — Determination of indentation hardness by means of a durometer (Shore hardness)”. International Standardization Organization (ISO), Geneva, Switzerland.