Behaviour of elastomeric seals in transport packaging for radioactive material

The mechanical and sealing behavior of elastomeric seals is a crucial issue for the safety of transport packaging for radioactive materials [1], [2]. The seals must ensure the containment enclosure's tightness over a wide temperature range to guarantee the confinement of gases and radioactive materials, even under accidental conditions.
At -40°C, the compression rate of the O-ring seal ensuring the tightness of the cover must remain sufficient, implying that its diameter should be large compared to the groove height [3]–[6]. Conversely, at high temperatures, efforts are made to prevent the seal's volume from exceeding that of the groove to avoid potential extrusion. Additionally, the residual deformation after compression (RDC), or the inability of the seal to return to its initial position after compression, must also be considered [7], [8].
These two criteria are challenging to reconcile, and the sizing of the groove/seal assembly can only result from a compromise since these requirements conflict. It is sometimes impossible to prevent the seal's volume from exceeding that of the groove, especially if the seal is subjected to high temperatures (e.g., 250°C). In such cases, as elastomers are considered incompressible materials, extrusion will occur if clearance is present, providing the seal with a volume to expand [9]–[12]. This phenomenon generally leads to the loss of the seal's physical integrity.
However, in transport packaging, the assembly between the flange and the containment cover generally does not allow clearance for the seal to expand, preventing extrusion. Yet, the high-temperature behavior of a seal in a constrained volume is poorly documented in the scientific literature [7]. Therefore, it is unknown whether the elastomer can become compressible or if it is, on the contrary, capable of lifting or deforming the groove and/or the metallic cover of the assembly, thereby compromising the packaging's integrity.
In this context, this thesis aims to advance our understanding of elastomeric seal properties with a thermomechanical approach (high and low temperatures), with a particular focus on two aspects: (a) better understanding the extrusion phenomenon when clearance is present and (b) better grasping the maintenance or loss of the concept of incompressibility of the elastomer material in a constrained volume.

The planned work is divided into several parts:
In collaboration with DTEL/SGPE, the study program will be defined, representative of the issues encountered in CEA's transport packaging: different seal and steel grades, dimensions, groove shapes, clamping forces, temperature range, etc.
An experimental protocol will be developed to characterize the behavior of seals in situ in representative screwed assemblies, providing either extrusion clearance or, conversely, a constrained volume. These experimental tests will be conducted at CETIM, benefiting from temperature control facilities, the design and creation of study-appropriate models with instrumentation, metrological control means, and sealing aspects linked to mechanics. CETIM also has extensive knowledge in the nuclear field, particularly with significant experience in studying issues related to the transport of radioactive material [2], [13].

After tests, the structure of the seals will be characterized at the atomic, microscopic, and mechanical scales using various techniques (SEM, microhardness, reaction force...).
These tests will be designed and interpreted using numerical simulations based on functional and material databases to model the extrusion phenomenon. With this method, and knowing the geometry and properties of the materials, it is possible to predict the extrusion pressure limit at different temperatures.
The experimental results will be compared to the calculations to optimize the design and parameters of the test devices.

The results obtained in this thesis will ultimately advance our understanding of elastomeric seal behavior and their ability to maintain tightness under extreme conditions. They will contribute to adopting a more innovative approach in designing transport packaging for radioactive materials, making them safer. Finally, the coupling between experiments and simulations will advance the numerical codes used to model the extrusion phenomenon.

This entire thesis work will be carried out through several collaborations:
• CETIM in Nantes,
• DES/DDSD/DTEL/SGPE at CEA Cadarache, and
• Gabriel Lamé Mechanics Laboratory (LaMé - EA 7494) - University of Tours
The doctoral student will be primarily based at CETIM in Nantes but will regularly visit CEA Cadarache and LaMé in Tours depending on the progress of each part of the work.