According to the Nuclear Regulatory Commision, The North Anna Plants went off line when they lost their connection to the grid. That's called a Load Rejection, when you suddenly have no place to send the electricity. The reactor shuts down within seconds, diesel generators come on to power backup cooling systems, and some steam may be released to relieve pressure until the cooling systems are up to speed. The same thing happened in Ohio in 1998 when a tornado wiped out the three power lines connecting Davis-Besse to the grid.

I didn't feel anything today, but I think I was driving at the time of the quake.