Apparently officials with Tokyo Electric Power Company (which has recently been nationalized by the Japanese government, because TEPCo is broke) never thought to protect all the exposed pipes from freezing weather.  Maybe they thought global warming would protect them?

Japan is being hit with their usual freezing winter temps, along with record snow fall in some areas, and that has caused cooling pipes to freeze and burst at the General Electric designed Fukushima Daiichi nuclear plant.

TEPCo said at least 40 liters (10.5 U.S. gallons) of water leaked from pipes in Reactor 4.  However, they said 7 tons of water have leaked from Reactor 6!

Matsumoto Junichi, a TEPCo rep,  admitted they failed to protect the exposed pipes from the freezing weather!