Causes of nuclear accidents?

Reactor designers try to anticipate every possible fault route and ensure the design is robust enough. Risks such as earthquakes and aircraft crashes are assessed. Earthquakes are particularly likely in some countries, Japan for example, and design will be aimed at ensuring the reactor primary circuit is strong enough to withstand the forecast forces developed, in an earthquake of an agreed level.

Generally however accidents have been caused by non standard operations (Chernobyl), or poor instrumentation (Three Mile Island). Avoidance of accidents does require well thought out operating procedures, and staff well trained who can work out what is happening in an unusual event, combined with reliable instruments.