What are the implications of the blast at Fukushima Daiichi plant?

Japan's nuclear crisis worsened on Wednesday, with workers ordered to withdraw briefly from the stricken power plant after radiation levels spiked, just hours after smoke was seen rising from the quake-crippled nuclear facility.

Q: What does the smoke mean?

A: The smoke is most likely to be steam, a natural byproduct of pouring water into the reactors to cool them down and keep the fuel rods covered. Authorities are also trying to maintain water levels at a spent-fuel storage pool at the plant's No 4 reactor, which experts now view as the real threat.