One factor that might determine how serious the situation becomes is whether the uranium oxide pellets in the rods stay vertical even if the cladding burns off. This is possible because pellets sometimes become fused together while in the reactor. If the pellets stay standing up, then even with the water and zirconium gone, nuclear fission will not take place, Mr. Albrecht said.

¶But Tokyo Electric said this week that there was a chance of “recriticality” in the storage pools — that is, the uranium in the fuel rods could resume the fission that previously took place inside the reactor, spewing out radioactive byproducts.

¶Mr. Albrecht said this was very unlikely, but could happen if the stacks of pellets slumped over and became jumbled together on the floor of the storage pool.

¶Plant workers would then need to add water with lots ofboron because the boron absorbs neutrons and interrupts nuclear chain reactions.

¶If a lot of fission occurs, which may happen only in an extreme case, the uranium would melt through anything underneath it. If it encounters water as it descends, a steam explosion could then scatter the molten uranium.