Effect of UV and gamma radiation on Rn222 permeation through polyvinyl chloride (PVC). Application to the packaging of radium sources for the purpose of storage

Abstract

Mining of uranium and thorium, as well as the use of radioactive sources in radiotherapy, have caused, during the 20th century, the production of waste with a varying concentration in long-lived radionucleides (particularly radium). This waste cannot be stored in traditional storage sites which will return to the public domain after 300 years, due to, on the one hand, the radium period (1620 years) and, on the other hand, the build-up of radon, its gaseous daughter. One solution, in order to optimize the packaging and storage of such products, could be to use successive barriers, made of polymer membranes, intended to limit radon emission. Laboratory tests have shown that polyvinyl chloride (PVC) greatly reduces radon emission from a radium source. However one should take into account the damage of the polymer in time, due to radioactive waste storage itself over long periods of time. Therefore, in order to check the durability of such barriers, PVC samples have been subjected to different accelerated ageing processes by exposure to ultra-violet (UV) radiations or gamma rays. We have determined the effect of such radiation of the samples using two approaches: — demonstration of structural changes using analytical method (Infra-red with Fourier Transform or IRTF), — study of variations in the radon permeation factor. In the first analysis, it seems that the UV irradiation, causes structural changes in the PVC as a function of the irradiation length of time. This leads first to an increase in the efficiency of the polymer as a “radon barrier”, i.e. a reduction in its permeation factor (2.41 × 10 −12m 2s −1 for a non exposed membrane, against 3.30 × 10 −13m 2.s −1 for a membrane exposed during 284 hours, with an exposure rate of 62.5 W.m −2, thus a reduction by a factor 10 of the radon emission) then to a long-term weakness. The gamma irradiation (dose rate: 1.05 kGy.h −1, dose: 0.71 MGy) also causes a reduction in the permeation factor of PVC, but only by a factor 2.7.