Summary Simulation of thermal ageing is an important part of qualification of materials designed for the use in nuclear power plants (NPP). According to standards, the simulation of long-term service thermal ageing is performed isothermally at elevated temperature using Arrhenius methodology. The samples or equipment are aged in thermal chamber, to bring them to the same state as after long-term service time. To proceed a reliable simulation, the testing parameters should be taken very carefully and the accelerator factors should not be too high. The testing temperature and time and the activation energy are the most important parameters. Determination of these factors and the limitations of their use in practice are discussed.
Degradation of polymeric materials used in nuclear power plants (NPP), especially polymeric cable insulation materials, in
the course of their service can be monitored by measuring their properties by DSC, mainly oxidative induction time — OIT.
The studied materials were in-laboratory aged by applying main stressors that act in NPP — ionising radiation and temperature.
The dependence of OIT on radiation and thermal degradation of polymeric material was determined. The OIT values have been
compared to elongation at break as a property that directly reflects the functionality of the studied material. The comparison
of monitored OIT of real cable samples taken from NPP with dependencies on how the OIT values change with the elongation at
break, makes possible to establish the extent of cable degradation. This method can be considered as a suitable and effective
technique for lifetime assessment not only of cable insulations but also of many other plastics.
The results of determination of activation energies (EA) of polymeric cable insulations obtained by conventional methods (usually based on the evaluation of changes of mechanical
properties of insulations after their ageing in thermal chamber at different temperatures) have been compared with results
obtained by methods employing the differential scanning calorimetry (DSC). Three DSC methods have been tested: the method
according the ASTM E 698; measuring of DSC characteristics in the isothermal mode at several different temperatures; and the
method based on evaluation of DSC characteristics of insulations after their thermal ageing in thermal chamber. The last method
— which can be called as a modified conventional method, because instead of mechanical properties, the DSC characteristics
are determined — has been found as most acceptable and giving similar values of EA as the other conventional methods.
Authors:M. Beneš, N. Milanov, G. Matuschek, A. Kettrup, V. Plaček, and V. Balek
Thermogravimetry (TG/DTG) coupled with evolved gas analysis (MS detection) of volatiles was used to characterize the thermal
behavior of commercial PVC cable insulation material during heating in the range 20-800C in air and nitrogen, respectively.
In addition, simultaneous TG/FTIR was used to elucidate chemical processes that caused the thermal degradation of the sample.
A good agreement between results of the methods was found. The thermal degradation of the sample took place in three temperature
ranges, namely 200-340, 360-530 and 530-770C. The degradation of PVC backbone started in the range 200-340C accompanied
by the release of HCl, H2O, CO2 and benzene. The non-isothermal kinetics of thermal degradation of the PVC cable insulation in the temperature range 200-340C
was determined from TG results measured at heating rates of 1.5, 5, 10, 15 and 20 K min-1 in nitrogen and air, respectively. The activation energy values of the thermal degradation process in the range 200-340C
of the PVC cable insulation sample were determined from TG results by ASTM method.