The operator of the crippled Fukushima Daiichi nuclear plant says the March 15th explosion at the No.4 reactor building may have been caused by hydrogen from the No. 3 reactor. Tokyo Electric Power Company has been investigating the cause of the explosion and fires.

It was initially thought that the March 15th explosion was triggered by hydrogen produced by damaged spent fuel rods in a pool inside the No.4 reactor building. But photographs of the pool taken in April show no damage to the rods.

TEPCO focused on ducts from the No.4 and neighboring No.3 reactor buildings that join into a single duct before an exhaust pipe. The company says that when it vented gas from the No.3 reactor through the duct, hydrogen may have seeped into the No.4 reactor building. Hydrogen that accumulated in the upper part of the No.4 reactor building may have caused the explosion.

The ducts from 3 and 4 are joined and hydrogen leaked from 3 to 4? Who could have known? Guess they weren't monitoring the atmosphere above #4 spent fuel pool for hydrogen. It only had the biggest and hottest fuel load on the entire site.

"It is not necessary to have hope in order to persevere."