Complex sentence: Specifically, after 9/11, he pointed out problems with contingency plans to protect US nuclear plants from terrorist attacks, namely that the plans assumed attackers would both enter and exit from facilities, whereas suicide terrorists would not need to exit.
Split into simpler sentences: Specifically, after 9/11, he identified problems with contingency planning to protect US nuclear plants from terrorist attacks. He said that the assumption that attackers would both enter and exit from facilities was not valid, since suicide terrorists would not need to exit.