In such a case, autonomous sources of onsite power such as diesel generators or “passive systems” driven by gravity would constitute the next line of defence (see also Section 2.3). The general conclusion that the size and the quality of the electricity grid are especially important issues for nuclear power remains thus unaffected. In an industry with only limited means of economical storage this poses a challenge in ensuring continuous supply security for as vital a service as electricity. In particular, it means that back-up capacity from alternative technologies needs to be available for the periods when a given plant is unavailable.