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Effect of combined gamma and neutron irradiation on EPDM and FPM elastomers
Abstract
The effect of mixed gamma and neutron radiation on the properties of O-rings made of elastomeric materials differing for composition and declared radiation resistance, a fluoroelastomer and an ethylene-propylene diene rubber, was investigated experimentally. The O-rings were subject to mixed gamma and neutron irradiation for various hours, leading to significant changes in the mechanical behaviour. These variations were further explored with tests that allowed highlighting the changes in the macromolecular architecture: solvent swelling tests, to evaluate the occurrence of two competitive phenomena of crosslinking and cleavage, and dynamic mechanical thermal analysis, to better interpret the mechanical response as due to the evolution of the crosslinked structure and of the temperature dependence.
... Rather expensive. More information is reported in [5] and [15]. ...
... In particular, elongation at break degrade significantly as a function of the dose for all the selected products. Solvent swelling tests performed on FPM-based product and on EPDM-based product by Dichtomatik, evidence an important and progressive decrease of the swelling ratio as a function of the absorbed dose [15]. This is an indicator of an increased cross-linking of the polymer at the structural level. ...
The SPES Radioactive Ion Beam (RIB) facility at INFN-LNL is in the construction phase. It is based on the ISOL method with an UCx Direct Target able to sustain a power of 10 kW. The primary proton beam is delivered by a high current Cyclotron accelerator, with energy 35-70 MeV and a beam current of 0.2-0.5 mA. Neutron-rich radioactive ions will be produced by proton induced Uranium fission in the UCx target at an expected fission rate in the order of 10 13 fissions per second. The exotic isotopes will be re-accelerated by the ALPI superconducting LINAC at energies of 10A MeV and higher, for masses in the region A=130 amu at expected rate on the secondary target of 10 7 – 10 9 pps. The SPES project has the aim to provide high intensity and high-quality beams of neutron-rich nuclei as well as to develop an interdisciplinary research center based on the cyclotron proton beam.
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.
The SPES Radioactive Ion Beam (RIB) facility is in the construction phase at INFN-LNL. It is based on a dual exit high current Cyclotron, with tunable proton beam energy 35-70MeV and 0.2-0.5 mA, used as proton driver to supply an ISOL system with an UCx Direct Target able to sustain a power of 10 kW. The second exit will be used for applied physics: radioisotope production for medicine and neutrons for material study. The ISOL system will produce neutron-rich radioactive ions by proton induced fission in the UCx target. The expected fission rate in the target is in the order of 1013 fissions per second. The exotic isotopes will be re-accelerated by the ALPI superconducting LINAC at energies of 10AMeV and higher, for masses around A = 130amu, with an expected beam intensity around 107-109 pps. Fast neutron spectra will be produced by the proton beam interaction with a conversion target. A production rate in excess of 1014 n/s is expected. The SPES project has the aim to provide high intensity and high-quality beams of neutron-rich nuclei as well as to develop an interdisciplinary research center based on the cyclotron proton beam. The status of the project will be presented.
Effect of fast neutron irradiation on polyethylene, ethylene-propylene copolymer, and tetrafluoroethylene-propylene copolymer was compared with those of Co-60 γ-ray by the measurements of gel fraction and of swelling ratio after irradiation under vacuum. Difference in the effect between fast neutron and Co-60 γ-ray for the three polymers was not detected by those methods in spite of a large difference of linear energy transfer (LET). The G values of crosslink and chain scission determined from gel fraction data were the same for both fast neutron and Co-60 γ-ray for each polymer within the experimental error. It is thought that energy absorbed in a local region of polymer by fast neutron irradiation generates radicals in a more spread region and the radicals move in the polymer matrix until their recombination to form crosslink.
Around particle accelerators used for fundamental research on the basic structure of matter, materials and components are exposed to ionizing radiation caused by beam losses in the proton machines and by synchrotron radiation in the lepton machines. Furthermore, with the high-energy and high-intensity collisions produced from future colliders, radiation damage is also to be expected in particle-physics detectors. Therefore, for a safe and reliable operation, the radiation aging of most of the components has to be assessed prior to their selection. An extensive radiation-damage test program has been carried out at CERN for decades on a routine basis and many results have been published. The tests have mainly concentrated on magnet-coil insulations and cable-insulating materials; they are carried out in accordance with the IEC 544 standard which defines the mechanical tests to be performed and the methods of degradation evaluation. The mechanical tests are also used to assess the degradation of composite structural materials. Moreover, electrical properties of high-voltage insulations and optical properties of organic scintillators and wave guides have also been studied. Our long-term experience has pointed out many parameters to be taken into account for the estimate of the lifetime of components in the radiation environment of our accelerators. One of the main parameters is the dose-rate effect, but the influence of other parameters has sometimes to be taken into account.
We present a general approach for more confidently correlating accelerated aging results with aging under service conditions using the Arrhenius methodology. We first show that, as a result of complex diffusion-limited oxidation effects, time/temperature correlations may occur for some properties but not for others. To rigorously extrapolate high temperature results to low temperatures, we sought an ultrasensitive technique correlated to macroscopic degradation and capable of measurements at or near service temperatures. We achieved this objective by monitoring oxygen consumption rates at high (accelerated) temperatures, to establish the necessary correlation, and at low temperatures (down to 23 °C), to determine their temperature-dependence in the extrapolation region. Because easily measurable oxygen consumption rates of 10−13 mol/g s correspond to decades of predicted lifetime for most elastomers, this approach increases confidence in long-term predictions and therefore provides a means of testing Arrhenius extrapolations.
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- T Seguchi
- N Hayakawa
- K Yoshida
- N Tamura
- Y Katsumura
- Y Tabata