Post-irradiation effect analysis on XLPE-insulated LV cables used in nuclear power plants

This paper investigates aging of low voltage cables used in nuclear power plants by the means of the dielectric spectroscopy technique. Aging has been performed on XLPE cables through high temperature and different dose rates in order to evaluate the electrical response under different aging conditions. Test have been carried out immediately after aging and years after the radiation source has been turned off. Significant changes in dielectric response have been observed due to postirradiation effects, suggesting that degradation continues even after the aging source has been removed

Ageing Assessment of XLPE LV Cables for Nuclear Applications through Physico-Chemical and Electrical Measurements

This paper investigates the changes in electrical and physico-chemical properties of low-voltage power cables for nuclear application when subjected to the combined effects of gamma radiation and temperature. Electrical response is evaluated by means of the dielectric spectroscopy, while the physico-chemical changes are analyzed at different structural scales through five complementary techniques (OIT measurements, FTIR spectroscopy, swelling measurements, DSC analysis and micro-indentation). The dielectric spectroscopy and the first two chemical techniques are shown to be appropriate for evaluating the development of radio-thermal ageing in low-voltage cables. Hence, the results reported in this article suggest the effectiveness of dielectric spectroscopy as a non-destructive technique for on-site cable diagnosis

Degradation assessment of polyethylene-based material through electrical and chemical-physical analyses

The usability of any material hinges upon its stability over time. One of the major concerns, focusing on polymeric materials, is the degradation they face during their service life. The degradation mechanisms are deeply influenced by the aging temperature to which the material is subjected. In this paper, low-density polyethylene (LDPE) flat specimens were thermally aged under two different temperatures (90 °C and 110 °C) and analyzed. Specimens were characterized through both the most common mechanical and chemical measurements techniques (e.g., tensile stress, thermal analyses, oxidation induction time) and electrical measurements (dielectric spectroscopy, in particular), which are examples of non-destructive techniques. As a result, a very spread characterization of the polyethylene-based materials was obtained and a very good correlation was found to exist between these different techniques, highlighting the possibility of following the aging degradation development of polymers through electrical non-destructive techniques

Multi scale aging assessment of low-voltage cables subjected to radio-chemical aging: Towards an electrical diagnostic technique

In this article, the aging behavior of nuclear-grade low voltage cables, characterized by different geometries and insulation compositions, is investigated. Cables were subjected to radio-chemical aging at different dose rates (7 Gy/h, 66 Gy/h and 400 Gy/h), in order to simulate typical aging environments inside nuclear plants. The changes of insulation properties due to aging are investigated at different scales, aiming at highlighting possible correlations between molecular-scale properties and global macroscopic material behavior (e.g., mechanical and electrical ones). Microscale material behavior is investigated by means of FTIR spectroscopy and oxidation induction time (OIT) measurements, in order to evaluate material composition changes and material resistance to oxidation, respectively. On the other side, mechanical and electrical macroscopical properties are examined through tensile stress and dielectric spectroscopy measurements. It is found that aging is deeply influenced by the effect of additives (e.g. antioxidants) inside the insulation. In particular, the presence of antioxidants delays oxidation process allowing material modifications during the early aging states to be evaluated. Dielectric spectroscopy is demonstrated to properly follow all the stages of the degradation process, confirming its appropriateness as a non-destructive condition monitoring technique for cables. Finally, the evolution with aging of the dielectric response is associated with the variations of the considered chemical and mechanical properties, allowing the derivation of correlation master curves

Piezoelectric Nanofibers for Integration in Multifunctional Materials

This paper deals with realization of multifunctional composite materials, having piezoelectric effect. First of all polymeric mats of electrospun piezoelectric nanofibers were realized with different geometries. Such effect has been maximized by designing properly the electrospinning apparatus in order to enhance the electric field in the interelectrodic space which polarize the dipolar moments. The mats are then integrated in a silicon rubber matrix and measurements of the electromechanical response of the composite materials thus manufactured are performed. A good integration of nanofibers inside the host material is evidenced by electron microscopy images, allowing delaminations, which could occur using piezoelectric films, to be avoided. A large electrical response to both impact and vibration stimuli has been finally demonstrated

A methodology to investigate heterogeneous oxidation of thermally aged cross-linked polyethylene by ToF-SIMS

Artificial ageing of polymeric insulation jackets is routinely performed in order to assess end-of-life material characteristics. Practical constraints including high temperatures/ short times ageing treatments lead to strong influence of diffusion-limited oxidation (DLO) resulting in unreliable life-time predictions. This study proposes a new experimental approach to the investigation of cable insulation ageing, exploiting analytical techniques capable of resolving chemistry at the length scale relevant for DLO (nano-microscale). When studying the potential effects of DLO using time of flight secondary ion mass spectrometry (ToF-SIMS) sample preparation becomes crucial. This paper presents the development of a methodology to generate suitable specimens to investigate the DLO effect using ToF-SIMS. A reference polymeric material has been thermally aged in various DLO conditions. Cross sections of aged samples were generated using three different methods. In order to assess the most suitable approach for this study, cross-section topography were scanned using a profilometer and the surface chemistry was investigated using ToF-SIMS together with multivariate analysis methods