相对来说核事故的危险性要小的多，但是一旦发生，就是非常严重的事故。
The risks of a nuclear accident are small in comparison. But when one of them happens, it does so big time.
鉴于2003年发生的哥伦比亚事件的惨剧仍记忆犹新，NASA预备了奋进号航天飞机作为后援，一周内一旦亚特兰蒂斯号发生任何危险，奋进号将立刻进行救援。
With the 2003 Columbia tragedy in mind, NASA will have the shuttle Endeavour prepared to rescue the Atlantis crew from space on a week's notice should any mishaps occur.
这种情况很少见，但是一旦发生，对于母亲和婴儿来讲是非常危险的。
This is very rare, but when it does happen, it can be very serious for both the mother and the baby.
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