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

Abstract

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.

Degradation of EPDM and FPM elastomers irradiated at very high dose rates in mixed gamma and neutron fields

“…Finally, elemental analyses were carried out to determine the content of the various elements, which was necessary to perform dosimetry calculations .The measured elemental composition of EPDM and FPM is reported in Table .…”

“…Dose components and dose rates were calculated taking into account the elemental composition of materials and the irradiation conditions, as previously described in Ref. . Using the same methodology, dose rates for the two materials were calculated in the LS facility.…”

“…This means that, in the investigated conditions, D 50EB remains stable irrespectively of the prevailing mechanism between crosslinking and chain cleavage. In addition to the EPDM tested in the present work, the D 50EB for two other EPDM‐based elastomeric O‐rings can be interpolated from the data published by Zenoni et al . Elastomers were irradiated in the CT at nominal power of 250 kW.…”

“…A case study dedicated to the lifetime prediction of these O‐rings under mixed irradiation was published . In this study, the foreseen evolution of the O‐rings during the facility operation was based on data collected in CT at maximum power and was correlated to the total absorbed dose parameter only, under the unproved assumption that dose rate effects were negligible. The dose rate results here reported allow more accurate and steady conclusions to be drawn on the expected evolution with dose of such O‐rings.…”

“…In the proximity of its 238 UC x target, materials will be subjected to intense neutron and gamma fields, with fluxes around the target estimated in the order of 10 10 particle cm −2 s −1 . In order to gather information on the resistance of polymeric O‐rings in such extreme conditions, in a previous work, two rubbers, namely ethylene propylene diene monomer (EPDM) rubber and a fluoroelastomer (FPM), were irradiated in reactor mixed neutron and gamma fields to investigate the effect of the adsorbed dose on their tensile properties and compression set values . EPDM O‐rings were then studied to assess their sealing performance both during the facility service time and the postservice storage phase .…”