Ageing Analysis of Power Cable used in Nuclear Power Plant

The aim of nuclear power plant ageing management and assessment is to maintain an adequate safety level throughout the life time of the plant and helps in determining the useful operation. Various factors involved in the insulation of polymeric cables such as continuous heat, radiation and atmosphere result in the alteration of chemical and physical properties as well as lowers the service life of material. PVC (Poly Vinyl Chloride) is the most commonly used insulation material in NPP (Nuclear Power Plant). Purpose of this work is to predict the service life of PVC insulation cable for which extrapolation ageing has been performed. Ageing mechanism proceeds in laboratory scale rig to provide temperature and radiation sources. The variation in temperature is regulated and monitored through thermocouple. At higher temperature, semi crystalline free radical may escape out resulting in the degradation of polymer. Different tests have been carried out like shore hardness testing, tensile testing, Differential Scanning Calorimeter and Thermo gravimetric Analysis before and after ageing to analyze the Ageing effect. The impact of ageing days upon yield strength of single stress ageing has been carried out in this research work and it is found that aging has significant effect on service life of cables used in power plants.


INTRODUCTION
N PP works by fission reactions to produce heat and radiation. This nuclear process may cause adverse effect on cables located near NPP. Therefore, the insulation is highly affected by temperature, radiations and moisture. Insulation cable may damage severely, for instance due to current leakage and breakage of power. Insulation material when exposed to high temperature, heat, radiation and moisture may affect its chemical, mechanical and physical properties. Polymeric insulation must have enhanced performance and improved lifespan. Insulation material have advantages such as light weight, high strength to weight ratio, longer spans, better contamination performance, improved transmission line aesthetics and easy of processing and fabrication. Polymeric material also has a low surface energy which means it has good hydrophobic or water repellent properties [1][2][3]. Varieties of cables are used in NPP which includes Power Cable.

Accelerated Ageing
A process of exposing higher temperature, radiation dose and moisture content to the material for a short span is called accelerated ageing. Furthermore, a rig is designed to precede experiment time by time to artificially aged material. This rig consists of a temperature source in which a thermostat is used to set a required temperature or thermocouple may also be used to check the temperature in between. Radiation dose is given through source and moisture content also takes part to react chemically with the insulation material. This rig covers with a Lead shell to avoid radiations leakage. These radiations are injurious and harmful to the living organism, exposed material and apparatus has to be dumped soon after the testing and use [4][5][6].

1 Material:
The major material used in this research is PVC. The properties of PVC are shown in Table 1. The process of PVC compounding starts with initiation, then propagation into long chain with a radical, which join with another long chain radical for bond formation called termination.
The stages of these formations are presented in Fig. 1.

Ageing Method
Artificial ageing will be carried out in to a rig containing heating coils and radiation source is required. Radiation source may check the strength of insulation that serves in a severe environment at the power plant. Lead shielding will be provided for artificial ageing. Ageing is divided into two parts; Insulation Material (PVC) will be kept in the rig for different time duration mentioned in Table 2. This

Apparatus
Two Rigs are made to precede the (a) Only temperature

Testing
Hardness, tensile strength and DSC (Differential Scanning Calorimetric) tests were performed on the un-aged and aged samples and results were analyzed and compared.
DSC curve for un-aged sample is given in Figs Table 3. Tensile testing of 10 days aged multiple stress sample is not possible due to radiations. It shrinks with respect to dose and become out of tensile test parameters as shown in Fig. 8. The parameters values of multi stress aged samples for tensile testing are given in  Table 5.
DSC results of Single Stress Aged Samples are presented in Figs. 9-11 and for single stress ageing a tabular summary is given inside Table 6 i.e. single stress ageing Table 6 FIG

RESULTS AND DISCUSSION
The impact of ageing days upon yield strength of single stress ageing is shown in graph in Fig. 15 Fig. 19 represents heat flow rate with increasing ageing days with respect to temperature [9][10][11][12].
It has been observed that the shift of the baseline accompanying the glass transition for each PVC sample.

CONCLUSIONS
Power plant consists of several segments which are