I am currently taking a nuclear engineering class, and we asked my professor (a top nuclear expert in the country) for his thoughts. Thought I would post them in case anyone is interested....
"This elevates this disaster beyond TMI. Two issues are of concern.
First, like TMI, hydrogen explosions have stressed the primary containment vessel, but in this case, it appears that there may be a permanent leak. The reactor vessel, a strong tight concrete structure which contains the suppression pool is now the primary containment. It is good that the pressure has reached ambient - if I read it correctly as NOT being vented directly to atmosphere - as that means that the suppression pool is condensing heated gasses. The problems with the pressure operated relief valve (PORV) is similar to that of TMI when the operators attempted to reflood the reactor - seems like a common problem in a LOCA/overheat situation - the gas bubble acted as a vapor barrier to incoming liquids.
Second, by pushing sea water into the reactor, there is much more activation of products (primarily minerals) that can become radioactive. Since the reactor is in shutdown, this is minor, given the low flux of the reactor. Sea water is, however, corrosive, which will lead to more containment issues as time goes on. Additionally, many of the minerals that are injected tend to oxidize. In an oxygen deficient high temperature and high pressure environment, this further separates hydrogen from the water - increasing the potential for explosions.
The primary work must still be to cool the core. After this, the crisis will be averted and the long process of removing the core will be at hand.
Some news correspondents have discussed a lot about radiation coming to the US. I have two comments. Firstly, the primary gases of concern (if venting instead of severe explosion) are I-135 to Xe-135. Their half lives are 6.7 and 9.5hr respectively. Thus with dispersion and decay as it traverses the Pacific it is unlikely that radiation levels in the US will increase beyond a slightly measurably amount (remember we can measure picocuries - there are 520 picocuries of radiation in a banana - giving us the silly measurement of a BED (banana equivalent dose))
In the case of an explosion (like Chernobyl - which is a different type of reactor, or from nuclear bombs) the high temperature of the explosion (much more energy than the hydrogen gases that seems to be the cause of the recent explosions) put the debris into higher elevations and they are carried by the jet stream. This is a very unlikely scenario.
This said, it is not a very good week for nuclear energy and more so the Japanese people. The nuclear crisis at present is a bad diversion for a country that should be at task searching for survivors and sadly burying their dead. It will be interesting in the months to come to see what the overall reaction of the US will be."
