Different plastics, rubber and other polymer based materials are widely used in various applications in nuclear power plants and inside containments, e.g. cable jacketing/insulators, sealants, paint coatings, lubricants and greases. As any other material or component, polymers are susceptible to ageing. Ele-vated temperature, ionizing radiation and moisture are considered to be the most important ageing stressors and they tend to interact with the polymer structure in different ways. In addition to these ageing stressors, properties of polymer composition, e.g. crystallinity degree, amount of fillers and antioxidants, has an effect to the ageing behavior. Thus, the degradation mechanism can be quite complex.

Proper ageing management procedures are based on knowledge on the ageing behavior and how to set correct requirements for the polymer components in use that they will endure their designed life-time. The ageing behavior needs to be known when polymer components are qualified. Accelerated ageing is used as part of the artificial ageing of the qualified polymer and the ageing mechanism should be the same as in the real service environment in order to yield in identical ageing conditions. Thus, the effects of dose rate and temperature to the ageing mechanism must be known as well as the synergistic effects rising from the simultaneous and/or sequential exposure to ionizing radiation and excess heat.

In a previous feasibility study [2015:157]1 the acceptance criteria for functional properties for different polymers in system components was studied. The components were selected based on interviews with the five Nordic nuclear power plants. A need to study the polymer degradation, using low dose rates (20-50 Gy/h to total dose of 14 kGy) was identified since previous work described in literature often focused on using a high dose rate to achieve the life time dose during a short period of time to achieve a life time dose2. This may cause different degradation, compared to that obtained with a long exposure at a low dose rate, which is the case for many O-rings inside a Nuclear Power plant (NPP). Like the previous feasibility study this research project is performed in collaboration with the Nordic nuclear industry for in a longer perspective implementing the results.

Rubber materials were tested for different properties such as compression set, stress relaxation, elon-gation at break, Oxidation Induction Temperature (T

ox) and hardness after heat ageing and gamma irradiation. These properties were then correlated to the function of the O-ring i.e. tightness verified by a leakage test in a specially designed test rig. To be able to better compare the degradation effect caused by the irradiation, a parallel test on samples aged in heat only was performed. By testing the correlation between material data and tightness of an O-ring an understanding of the function (tight-ness) based on a material property (compression set) can be made. The aim was to be able to use this to set acceptance criteria for an O-ring using compression set as a property. A model using Finite Elements (FE) based on the relaxation data and compression set data from the experiment was also done but is only described briefly in this paper.