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.