单一故障是指一种导致系统或者系统组件在执行其预期安全功能时失效的故障，和任何随之引发的故障。(a failure which results in the loss of capability of a system or component to perform its intended safety function(s),and any consequential failure(s) which result from it.(IAEA Safety Glossary,Ed.2.0,2006))