Can't they just deactivate this nuclear plant in Japan?

It is my understanding that the plant has been “deactivated”, but it’s not possible to simply close the doors and walk away from a nuclear power plant, or any other type of power plant, for that matter, without pretty severe ecological consequences. In the case of the nuclear plant, of course, one also has to consider radiological environmental consequences.

Nuke plants have a lot of highly contaminated and high-radiation materials leftover even when the plant is shut down. The high-level radiation materials (primarily, in a light-water reactor system, the reactor vessel, steam generators and primary loop piping between the reactor and steam generators) have to be kept in the reactor pool, which is a flooded pool (think of a giant swimming pool) which helps to contain the heat of the nuclear reaction. In the “cold” plant, the pool also contains the residual now-radioactive metal of the system. That water also becomes radioactively contaminated. In the normal course of events in an operating plant, that’s not a problem, but now… the water has to be contained or disposed of.

If the water is going to be disposed of, then the primary system: reactor, steam generators and primary piping also have to be cut up and carefully disposed of.

Then there is the likely problem of spent fuel. “Exhausted” fuel still has high radioactive content, though it’s not commercially usable in an operating reactor. Those fuel bundles are typically stored in a “spent fuel pool” (more water) which allows the residual heat of continuing radioactive decay to be contained and dissipated. If the leak is from that pool, then that spent fuel needs to also find a new home.

I’m sure that all of this is in the cards, anyway: there’s no point to maintaining a “dead” nuke plant, after all. But it’s a complicated, expensive and very time-consuming project to undertake. I’m sure the first priorities of the Japanese have been to complete the recovery of the residents of the affected areas. They will certainly decommission and dismantle the plant, eventually, but I doubt that they wanted that to be a high priority item. It may become so.