Olli Heinonen is now a senior fellow at Harvard, but for 25 years, he was an inspector for the International Atomic Energy Agency.
He lived in Japan and visited the crippled nuclear reactors many times.
He says, the plants are built to withstand an earthquake.
"It's part of the design. It's part of the design. But then, there is an upper limit, you know."
And then -- in this case -- there was the tsunami, which likely caused more damage to the reactors than the quake itself.
"It throws the water and kicks some of the auxiliary equipment away. Probably plugs all the water intakes is my guess."
So with the normal power shut down, and water intakes plugged... It becomes difficult to cool the reactor.
A build up of hydrogen gas can cause explosions like the one Saturday morning.
But Heinonen says, with the reactors in such a fragile state, the biggest worry is aftershocks.
"You have these aftershocks. And now you are working with equipment that is improvised, temporary, not that well fixed. And I think we really need to hope there are not that big ones."
Even with all the problems at the plants right now, Heinonen says, this likely won't be as bad as the Chernobyl disaster.
Instead, it will probably be more comparable to what happened at Three Mile Island.
Although, in this case, the Japanese are now flooding the reactors with sea water, an extraordinary step, because the sea water will damage the reactors beyond repair.