A general approach for quantifying the heat-ageing of gaskets

A recurrent concern in the design of packaging for the transportation of radioactive material is to determine the elastomeric gaskets life at high temperature. Most gasket suppliers specify maximum allowable temperatures during “continuous service” and “peak service” (such as “200°C in continuous service” or “250°C in peak”) but they do not specify the definition of “continuous” or “peak” service, what are the acceptance criteria and how these maximum temperatures are determined. Based on this type of data, it is difficult to assess the acceptability of a gasket submitted to fluctuating temperatures. COGEMA LOGISTICS has launched a test program on the different rubber grades used on its casks to determine, for different temperature levels (e.g. 200°C, 210°C,…, 250°C…), the maximum seal life based on clearly defined criteria. The goal is to establish, for each rubber grade, the seal life versus temperature curve. These curves can be used to know if a gasket exposed to any specified temperature profile can guarantee the leaktightness. The principle of the method is to calculate a sum of “elementary damage rates” on the temperature profile (split up into elementary time intervals) and to compare this sum (the “global damage rate”) to a “maximum permissible damage rate”. If the global damage rate is lower than the maximum permissible damage rate, the leaktightness of the packaging can be guaranteed for the given temperature profile.