The nuclear incidents in Japan are described as Level 6 INES events (International Nuclear and Radiological Event Scale). Three Mile Island was a Level 5 event. Chernobyl was a Level 7 event, and that is the top of the event scale [source: Reuters]. Obviously, it's a serious situation.
Japan has lost a significant portion of its electrical generating capacity. Approximately a third of Japan's electricity comes from nuclear power plants, and about half of that capacity has been lost (approximately 20 percent of total generating capacity) [source: Izzo]. That capacity will need to be replaced in some way.
At 40 years old, these reactors are nearing the end of their design lifespans anyway. One alternative is to simply rebuild the plants. The two problems with this approach are that it will be a very lengthy process -- possibly taking a decade or more -- and the general public in Japan may have no appetite for new nuclear reactors. It is still too early to tell.
There are a number of Mark 1 reactors in the United States. It is certain that they will be decommissioned or altered to take advantage of the lessons learned in Japan. Other reactors may also be altered as needed.
The nuclear industry was hoping for a renaissance of nuclear power in the United States now that more than three decades have elapsed since the Three Mile Island incident shut down new nuclear plant construction in the United States. The events in Japan may stop this renaissance. Or they may spur research in other, possibly safer, nuclear technologies.