The explosion at the Japanese Fukushima Dai-ichi nuclear plant, will undoubtedly and other events that are still unfolding will no doubt lead to further debates about nuclear safety. We know that the explosion was probably a hydrogen explosion inside the containment building rather than a nuclear explosion inside the reactor core. The core vessel appears to still be intact. What we know is that the cooling systems of two reactors in the Fukushima Dai-ichi nuclear plant failed and that heat began to build up un their cores, along with pressure inside the air tight containment structure. In the event of an emergency reactor shutdown, electricity from back-up electrical sources operates the emergency cooling system, but the earthquake appears to have damaged Fukushima Dai-ichi back-up electrical sources. The pressure inside the containment structure probably was the result of the venting of radioactive gases as well as other volatile fission products and hydrogen from the reactor core. The fission products would have been released following the rupturing of uranium fuel pellets inside the reactor core. Hydrogen can be produced by a high temperature chemical process involving the disassociation of hydrogen and oxygen in coolant water and steam, that would in close proximity to the overheated nuclear fuel pellets inside the core. 

The air tight containment structure is designed to prevent the escape of volatile fission products and radioactive gasses in the event of a nuclear accident. The airtight barrier also contains any hydrogen gas released from the nuclear core, as well as steam. It appears that air leaked into the containment structure, and an explosive oxygen-hydrogen reaction was triggered destroying the outer containment building. The inner steel containment vessel remains in tact, at least for now.

While the explosion of the containment building was impressive, it pales to insignificance compared to the disaster that has fallen on Japan due to the magnitude 8.9 earthquake, and subsequent tsunami. So far there have been no deaths or injuries associated with the reactor accident, and no one is known to have received a high level radiation exposure. (Update: We now have reports of at least 4 injuries to plant workers.) The Radioactive gasses are quickly dissipated as the mix with air in he atmosphere, and pose little threat to people. Volatile fission products tend to precipitate out of air as they cool, and much of them will fall within a few hundred yards of their source reactor. Reportedly radioactive materials released by the Fukushima Dai-ichi blast are currently blowing out to sea, but there is no reason to assume that this will continue to be te case.

As I have indicated the Fukushima Dai-ichi explosion and other possible further events at the nuclear plant are relatively minor events compared to the enormous property damage and loss of life in Japan. Much of the Japanese energy structure has been subject to catastrophic failure, Natural gas pipelines have ruptured, and escaping gas has caught on fire, triggering fires in homes and other buildings. Oil and oil product tanks have ruptured and been set on fire. Many power lines are down, and the damage to hydroelectric dams has yet to be assessed. No one has any idea yet of the extent of the loss of life and property damage, but the dead are likely to number in the tens of thousands, and property damage could run into the trillion dollar range.

Could this accident been avoided? The answer is "probably yes," with the best recent reactor designs, the AP-1000 and the ESBWR, and certainly yes with advanced Molten Salt Reactor (MSR) technology. With the Integral Fast Reactor (IFR) the safety issues in the even of a large scale earthquake are less clear.

The AP-1000 and the ESBWR rely on passive emergency cooling features, that simply rely on the laws of nature rather than outside power sources. Had the Fukushima Dai-ichi reactors been AP-1000, or ESBWRs, they would have survived the Earthquake event intact with no emergency cooling failure.

In the case of molten salt technology, for example the Liquid Fluoride Thorium Reactor, as core temperature rises above operational range, a solid salt plug in a reactor core drainage system will automatically melt and the core fluid will automatically drain into passively cooled tanks. Thus the entire MSR/LFTR emergency shutdown and coolant system is dependent on the unfailing laws of nature.

The Fukushima Dai-ichi accident, as it now stands has set a new standard for thinking about nuclear safety. It is clear that some new nuclear plant designs could have successfully withstood the 8.9 magnitude earthquake, other reactors, similar to the Fukushima Dai-ichi nuclear plants would be less likely to do so. Safety standards should undoubtedly be reevaluated, and the consequences of the fission product escape from the Fukushima Dai-ichi reactor carefully and rationally assessed.

UPDATE: News sources are now reporting that up to 4 Fukushima Dai-ichi nuclear plant workers have been injured.
UPDATE 2: The Nature of the explosion is now open to question, as far as I am concerned.
UPDATE 3: CBS News reports:
Eleven reactors shut down automatically when the earthquake hit. And with most of those, the cooling has been reasonably straightforward, because there's been a power supply to the plant, either from the grid, or from the backup generators. But with the first two units of the Fukushima Dai-ichi plant, the generators cut in and ran for about an hour and then stopped. And we understand the reason they were stopped is because they were overwhelmed by the tsunami. And that precipitated the crisis, really, and then the challenge of keeping the reactor cool, because they then have to default to their battery power. And as far as I understand it, that may not have been sufficient to do everything that was fully required."
We still hack, as far as I am concerned a satisfactory account of how the explosion occurred, although CBS news continues to describe it as a hydrogen explosion.
UPDATE 4: Reports from Japan indicate that radioactivity at the boundary of the Fukushima-Dai ichi nuclear power plant in northern Japan has exceeded statutory limits and the incident has been rated as level 4 on the International Atomic Energy Agency’ International Nuclear and Radiological Event Scale.  Japanese authorities are reportedly passing out iodine to civilians living near the reactor who have not beem evacuated.  Iodine is administered inorder to prevent the release of radioactive iodine, a fission byproduct that would be anticipated to ne released in the event of a core melt down.  These and other developments suggest that we may not be through with Dai-ichi events.