Inadequacy of margin in qualification tests

Dose-rate effects are now widely known. One nuclear industry response is to use a large margin (i.e., over-dose) in qualification tests to account for this. We have carried out investigations of polymer radiation degradation behaviors which have brought to light a number of reasons why this concept of margin can break down. First of all, we have found that dose-rate effects vary greatly in magnitude. Thus, based on high dose-rate testing, poor materials with large dose-rate effects may be selected over better materials with small effects. Also, in certain cases, material properties have been found to level out (as with PVC) or reverse trend (as with buna-n) at high doses, so that margin may be ineffective, misleading, or counterproductive. For Viton, the material properties were found to change in opposite directions at high and low dose rates, making margin inappropriate. The underlying problem with the concept of margin is that differences in aging conditions can lead to fundamental differences in degradation mechanisms