RADIATION levels that can prove fatal were detected outside reactor buildings at Japan’s Fukushima No.1 plant for the first time, complicating efforts to contain the worst disaster since Chernobyl in 1986.

Water in an underground trench outside the No.2 reactor had levels exceeding one sievert an hour, a spokesman for plant operator Tokyo Electric Power Company said.

Thirty minutes’ exposure to that dose would trigger nausea and four hours might lead to death within two months, according to the US Environmental Protection Agency.

Preventing the most-contaminated water from leaking into the ground or air is key to containing the spread of radiation beyond the plant.

A partial meltdown of fuel rods in the No.2 reactor probably caused a jump in the readings, Japan’s chief government spokesman said yesterday.