Radiation resistance of elastomeric O-rings in mixed neutron and gamma fields: Testing methodology and experimental results

Materials and components employed in the presence of intense neutron and gamma fields are expected
to absorb high dose levels that may induce deep modifications of their physical and mechanical
properties, possibly causing loss of their function. A protocol for irradiating elastomeric materials
in reactor mixed neutron and gamma fields and for testing the evolution of their main mechanical
and physical properties with absorbed dose has been developed. Four elastomeric compounds used
for vacuum O-rings, one fluoroelastomer polymer (FPM) based and three ethylene propylene diene
monomer rubber (EPDM) based, presently available on the market have been selected for the test.
One EPDM is rated as radiation resistant in gamma fields, while the other elastomers are general
purpose products. Particular care has been devoted to dosimetry calculations, since absorbed dose
in neutron fields, unlike pure gamma fields, is strongly dependent on the material composition and,
in particular, on the hydrogen content. The products have been tested up to about 2 MGy absorbed
dose. The FPM based elastomer, in spite of its lower dose absorption in fast neutron fields, features
the largest variations of properties, with a dramatic increase in stiffness and brittleness. Out of the
three EPDM based compounds, one shows large and rapid changes in the main mechanical properties,
whereas the other two feature more stable behaviors. The performance of the EPDM rated as radiation
resistant in pure gamma fields does not appear significantly better than that of the standard product. The
predictive capability of the accelerated irradiation tests performed as well as the applicable concepts of
threshold of radiation damage is discussed in view of the use of the examined products in the selective
production of exotic species facility, now under construction at the Legnaro National Laboratories of
the Italian Istituto Nazionale di Fisica Nucleare. It results that a careful account of dose rate effects and
oxygen penetration in the material, both during test irradiations and in operating conditions, is needed
to obtain reliable predictions. Published by AIP Publishing. https://doi.org/10.1063/1.5011035