Nuclear and Industrial Safety Agency has said in the press conference at 7:00PM on April 8 that it may have been a faulty measurement.
This is from the plant parameters as of 6:00AM JST 4/8/2011:
D/W [dry well] 1.00 x 10^2 Sv/h [That's 100 sievert/hour.]
(as of 4/8 12:00AM)
So, right after the April 7 aftershock, it measured 100 sievert/hour.
But not to worry. The plant parameters as of 4:00PM JST 4/8/2011:
D/W 6.83 x 10^1 Sv/h [6.83 sievert/hour.]
(as of 4/8 1:00PM)
The radiation level of the dry well of the Reactor 1 Container Vessel has been rather high, and fluctuating, indicating that the reactor is far from stable.
I will let you know if I hear more squeak from NISA.