Sheehan said one of the four reactor coolant pumps appears to have stopped working.

CNN: Pump triggers Three Mile Island reactor shutdown, NRC says

Electricity is needed to keep the reactor cores covered with coolant and to keep safe temperatures in spent fuel pools.

WSJ: Virginia Nuclear Power Plant Loses Power After Quake

The tsunami destroyed the diesel generators that provide power to drive the pumps that circulate the water coolant through the reactor that removes decay heat.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi

Some of the suggestions include using molten lead as the coolant for the system, but the reactor would run at a temperature above 700 degrees C. when the material becomes corrosive.

FORBES: Thorium Nuclear Power -- A Lesson From Norway

Integral reactor systems -- including the reactor vessel itself, six steam generators, five primary coolant pumps, 12 isolation valves and more -- were stored outside for up to 18 months, exposed to highly corrosive tropical sea air and weather.

CENTERFORSECURITYPOLICY: A Chernobyl reminder just off our shores

The reactor also lacks a back-up mechanism to stop it overheating and exploding should the helium coolant escape.

ECONOMIST: Prospects for a new type of nuclear reactor look mixed