They’ve been telling us from day one that a meltdown wouldn’t happen, but at this point it seems inevitable.

TOKYO — A Japanese utility says fuel rods at a troubled nuclear reactor were once again fully exposed hours after authorities were able to stabilize a similar emergency.

Tokyo Electric Power Co. says the exposure happened at Unit 2 of the Fukushima Dai-ichi plant because a steam vent wouldn’t open Monday, causing a sudden drop of water.

That reactor and two others at the plant are dangerously overheating and authorities are racing to prevent meltdown.

The second hydrogen explosion in three days rocked a Japanese nuclear plant Monday, devastating the structure housing one reactor and injuring 11 workers. Water levels dropped precipitously at another reactor, completely exposing the fuel rods and raising the threat of a meltdown.

The morning explosion in Unit 3 of the Fukushima Dai-ichi plant was felt 25 miles away, but the plant’s operator said radiation levels at the reactor remained within legal limits. Hours later, officials reported that fuel rods at Unit 2 were fully exposed at some point and may have been damaged.

Most attention, though, has been focused on Dai-ichi units 1 and 3, where operators have been funneling in sea water in a last-ditch measure to cool the reactors. A complete meltdown — the melting of the radioactive core — could release radioactive contaminants into the environment and pose major, widespread health risks.