The real question about nuclear safety is not "can nuclear accidents be avoided," but "do we want to do what ever is required to avoid nuclear accidents." As it turns out avoiding and mitigating nuclear accidents is not terribly expensive, nor does it make nuclear power impractical, but does require the nuclear industry to change the way it does business. The current nuclear safety philosophy centers on what is called "Defense in Depth." "Defense in Depth:"
Defense in Depth can refer to a system of barriers which serve to prevent the exposure of people to radioactive materials that originate in the reactor core and which might for a variety of reasons, escape from the reactor. This is the central fear for nuclear accident. At one time it was believed that everything that was inside the reactor was fair game for escape, but some materials are a whole lot more likely to escape than others. One way to prevent the escape of radioactive materials is to erect a system of barriers that are intended to block the paths taken by radioactive materials out of reactor cores. The history of major reactor accidents suggests that in the event of a major reactor accident, blocking the paths taken out of the reactor core by some materials may prove difficult. Indeed it might prove a better safety approach to capture some nuclear materials and remove them to a safe places outside the core, rather than preventing their escape.

One reason for doing this is that the escape of some radioactive materials particularly gases and materials that are likely to turn into gases in a serious nuclear accident may be difficult to prevent, if an accident leads to core overheating and meltdown. The conventional defense system of core meltdown prevention is to back up the core coolant system with secondary coolants systems, and back up the secondary systems with emergency coolant systems. Passive emergency coolant circulation is more reliable as well as less expensive than emergency coolant circulation by pumps, as well as more reliable. The Fukushima Dai-ichi reactors, were designed for emergency coolant water circulation by use of electrical powered pumps. The pumps were powered in the event of a grid shutdown by fossil fuel powered generators. Those generators were vulnerable to tsunami at Fukushima Dai-ichi. The Westinghouse AP-1000 is designed with a more advanced safety system. A large tank of emergency coolant water is located above the AP-1000 core. In the event of an emergency shut down, the loss of electricity automatically releases valves that allow the flow of emergency coolant water from the tank to the core. The flow itself is powered by gravity and the coolant lines lead directly from the tank to the core. Such a system provided superior nuclear safety at Fukushima Dai-ichi, In addition to the emergency passive water coolant system, the Westinghouse AP-1000 has a passive air cooling system.
The passive containment cooling system (PCS), provides the safety-related ultimate heat sink for the plant. The PCS cools the containment following an accident so that design pressure is not exceeded and pressure is rapidly reduced. The steel containment vessel provides the heat transfer surface that removes heat from inside the containment and transfers it to the atmosphere. Heat is removed from the containment vessel by the continuous, natural circulation of air. During an accident, air cooling is supplemented by water evaporation. The water drains by gravity from a tank located on top of the containment shield building.
In addition a more primary emergency water cooling system relies on natural water circulation to remove decay heat from the AP-1000 core in the event of an accident.
A literature survey reveals that there have been many experimental and numerical investigations on the characteristics of different PRHRSs. The Westinghouse advanced passive PWRs, AP-600, AP-1000, and EP-1000 (IAEA-TECDOC-1391, 2004; Adomaitis et al. [1]; Reyes and Hochreiter [2]; Zhang et al. [3]) adopt passive core cooling system (PXS) to protect the plant against reactor coolant system (RCS) leaks and ruptures of various sizes and locations. The PXS includes a 100% capacity passive residual heat removal heat exchanger (PRHR HX), which satisfies the safety criteria for loss of feedwater, feedwater and steam line breaks. The PRHR HX, immersed in the in-containment refueling water storage tank (IRWST), is connected through the cold leg and hot leg to the core. The IRWST water volume is sufficient to absorb decay heat for more than 1 hour before the water begins to boil. Once boiling starts in the IRWST, the steam passes to the containment and condenses on the inner surface of the steel containment vessel, and then drains by gravity back into the IRWST. The PRHR HX and the passive containment cooling system (PCCS) provide indefinite decay heat removal capability with no operator action required. The theoretical and experimental investigations on the PXS characteristics of AP600 indicate that the design of the PRHRS is feasible and rational.
The GE/Hitachi ESBWR offers significant advances in passive safety. For example, Coolant flow no longer relies on pumps. Rather the boiling water reactor design allows for the natural circulation of coolant water through the core. This protects the core against loss of coolant in the event of a power shutdown and loss of grid connection. Only a major loss of coolant accident would prevent the primary coolant system from functioning. Only if the primary coolant system of the ESBWR breaks down does emergency coolant system take over. The ESBWR safety system includes many passive safety features.
Despite the use of sophisticated passive safety features which greatly limit the likelihood of an accident that could lead to a core meltdown, both the AP-1000 and ESBWR employ the standard defense in depth barriers for the prevention of the release of radioactive materials in the event of nuclear accidents. The operation of advanced cooling system and emergency cooling system technologies, tend to make the breakdown of fission product release barriers even less likely than would be the case in older reactor designs.
This reactor manufacturers continue to make impressive advances in reactor safety designs. Yet critics of nuclear power seem totally unwilling to acknowledge any improvement in nuclear safety. Michael Collins, a self styled liberal, and "Joiquin" of the Agonist, are implacable enemies of nuclear power. "Joiquin" thinks that nuclear power is so dangerous that the nuclear power industry and the media are afraid to tell the truth about its dangers. Joiquin says,
The truth is, there is a big fat lie that the nuclear power industry and the media are foisting on the public and that has not changed. We are supposed to believe that this hydrogen explosion is no biggie; course it isn't; it's just a direct hit
Of course Joiquin did not go into a similar tizzy when a natural gas fired power plant exploded in Connecticut last year. The fact that the Dai-ichi explosions killed six fewer people than the single Klean Energy Systems explosion. Of course if you get killed in a nuclear plant accident, you are much more dead than if you are killed in a natural gas plant accident. Even if no one is actually killed in a nuclear plant accident it is much more deadly and dangerous than an accident involving fossil fuels that produces real casualties. Joiquin tells
So, back to the big lie; what is it? This lie has to do with the nature of nuclear power in the future. Everyone is asking, can we make nuclear technology, the current, nuclear technology safe? In truth, the current risks with the nuclear fuel cycle i.e., the risks of contaminating the environment, are not the risks of the future because the current nuclear fuel cycle is not the fuel cycle that will be used in the future.
Note, that Joiquin completely ignores the Improvement in reactor design, and focuses on the fuel cycle, as if the fuel cycle alone makes reactors unsafe.
U.S. government intend to use more exotic fuel cycles in the future power plants including, . . . Thorium, and breeder reactors of various types.
All of this is hush, hush because,
the industry and their government and media proxies don't want to talk about this fact too much because the waste from these future fuel cycles is far more dangerous than most of the stuff slowly making a large part of Japan uninhabitable for the next few dozen millennium. In other words, the discussion in the media about future nuclear safety is completely dishonest.
Well somebody is being dishonest. but I would not say it is the industry and the government. Claims such as a "large part of Japan uninhabitable for the next few dozen millennium," are quite dishonest, but all too typical of the sensationalist exaggerations of the anti-nuclear lobby.
Michael Collins basically reposts the previous post.

How did Joiquinfind his material? He references the Wikipedia on thorium and tells us,
Thorium could theoretically be used to fuel future reactors but probably nothing like what we have now; they would be cooled with liquid salt. The advantage is the Thorium is much more naturally abundant that Uranium. Another potential bonanza! Except of course for a few minor problems: doesn't work yet, creates a contaminant that is a gamma ray emitter U 232 which decays into many more alpha and beta emitters making the spent fuel very difficult to handle and very toxic for hundreds of years.
In a note on sources Joiquin adds Arjun Makhijani and Michele Boyd's "Thorium Fuel: No Panacea for Nuclear Power." Makhijani is usually one of the more careful of the nuclear critics but in his thorium fuel essay he makes a number of large errors, including a committing the fallasy of composition when he claims,
Using thorium in a nuclear reactor creates radioactive waste that proponents claim would only have to be isolated from the environment for 500 years, as opposed to the irradiated uranium‐only fuel that remains dangerous for hundreds of thousands of years. This claim is wrong. The fission of thorium creates long‐lived fission products like technetium‐99 (half‐life over 200,000 years). While the mix of fission products is somewhat different than with uranium fuel, the same range of fission products is created.
What Makhijani failed to recognize is that a mixed group of fission products come out of the reactor when the thorium fuel cycle is used. Once they leave the reactor they began to go through decay process that lead toward stability. After 300 - not 500 - years the dacay process has gone far enough that the mixed group of fission products is no more radiactive than thorium ore was when it was dug out of the ground. The fact that technetium‐99 has a half‐life of over 200,000 years means it was not very radioactive to behin with. Technetium‐99 is so safe that it is used in medical tests.
Collins, as I noted, simply quotes Joiquin. The blind leading the blind. It is clear that neithr Collins nor Joiquin knows anything about either the nuclear fuel cycle nor nuclear safety, but they both pose as nuclear safety experts, as if total ignorance was not a hazard to telling truth from lies.

Figures like Collins and "Joiquin" and organizations like the Sierra Club and Greenpeace are enemies of nuclear safety because they deny the very possibility that nuclear power can be made safe or even safer. As long as the public listens to such arguments, it runs the risk that nuclear power may be less safe than it could be.