Modeling the effect of dose rate and time on crosslinking and scission in irradiated polyethylene

The insulation around the electrical cabling in nuclear power plants is frequently made of ethylene-propylene rubber and crosslinked polyethylene that is subjected to low levels of environmental stressors and radiation over the duration of their decades long service life. For the purpose of maintenance and reactor recertification, it is necessary to develop a non-destructive approach to determine the degree of damage the insulation has sustained. Accelerated aging experiments are used to develop these methods, although it is unclear how to relate these specimens to in situ aged ones. Here a kinetic rate model is used to investigate the impact of radiation dose rate and total dose on the crosslinking and chain scission of polyethylene. Analytical expressions for the concentration of crosslinking and scission sites as a function of time and dose rate, both during and post irradiation, are presented. During irradiation the concentration of crosslinking sites increases linearly with time, and when the dosing ends the crosslinking reaction terminates. The scission reaction begins slower, due to the necessity of forming intermediate species, but increases in rate, and eventually the concentration of scission sites overtakes crosslinking. However, at low dose rates, less than 10 Gy/hr, scission damage is always the primary form of damage. Unlike the crosslinking reaction, scission continues for months and years due to the slow decomposition of the intermediate species.