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It's a very different situation. Chernobyl had no reactor containment building and was a sodium cooled reactor (highly volatile). What happened at Chernobyl was a steam explosion that ruptured the primary system. Remember the high school chemistry class when a piece of sodium was put into a beaker of water and it became highly reactive? Well, at Chernobyl, the explosion resulted in emptying the reactor into the atmosphere. Add water to the fire that ensued and it really became a mess. That cloud went around the world several times depositing its radioactive contents everywhere. I remember being on the submarine pier in Charleston, SC and took an air sample. It was "hot". I had to "finger print" the nuclides to verify that it was Chernobyl and not a naval reactor from the subs.

I don't think it was a sodium moderator, but a graphite or sodium graphite moderator, which is highly flammable when exposed to air (or oxygen). I'm assuming your 'fingerprinting' only confirmed that it was not typical nucleides and half-life of those emitted by our naval reactor types.

What APPEARS to be happening in Japan is that the reactor containment building(s) have been breached. I read that there is an estimated 26 foot hole in one. That was caused by the hydrogen explosion. But the reactor itself which contains the fuel rods is intact. Those rods have to be "melting down" based upon the radiation levels I have read about close to the reactor. This is further confirmed by reports that they are now using seawater to try to keep the reactor cooled. There is a loss of all AC power and their backup systems must have had failures, as well. Seawater is a last resort and they are pumping like crazy. So, in this case, it is more like Three Mile Island...only worse.

The Zirconium alloy plating is likely melting (emission of FP) and uncovering them will emit higher levels of radiation still, since water is also reflective and absorbs various radiation emissions (reflect neutrons, absorb/slow down high energy photons, for example). Backup generators failed, but battery backup seems to have taken over, but I'm not sure if they are still have power there or if they managed to restore any of the backup means. Boric acid was included with the seawater - neither is really desirable, but the decay heat is significant - in the MW heat generation range still and you want to keep the neutron generation well into the sub-critical range.

What is complicating things even further is that their spent fuel pool which contains the "old" fuel rods appears to be damaged. This huge "swimming pool" typically will have 20 feet of water over the spent fuel rods and is circulating to remove the decay heat. The 20 foot of water is necessary for shieldind the radiation. Their rods appear to be uncovered due to the pool being damaged. There is a fire in the Fuel building and that is what is sending radioactive contamination into the air. So, they not only have a meltdown occurring within the multiple reactors (which seem to be contained at this point), but they have spent fuel rods that are experiencing a meltdown...which is not contained. It is not all going into the atmosphere as at Chernobyl, but the fire is sending some amount into the air. They are trying to keep water (seawater) on the spent fuel rods with various levels of success/failure.

That seems to be the larger concern. Uncovering spent fuel rods which seems to be the larger issue. They are not retained in a containment vessel and burning will definitely disperse radioactive contaminants. As you stated previously, the amount of emission from this is significantly lower than Chernobyl, which had a breached containment vessel spewing radioactive element far and wide.

This is just an opinion as I have mentioned that I am trying to piece together how this that was reported can happen and that can happen and so on. Trying to connect the dots. My guess is that the site will eventually have to be entombed as dismantelling the reactors and associated systems will be cost prohibitive, not to mention the radiological environment being prohibitive to working. How much of the site will have to be entombed is yet to be seen.

I don't think the site needs to be entombed. The containment vessels (as reported now) have not been breached. Decon efforts will have to happen and portions of the site will have to be dismantled and removed due to high half-life irradiation. However, due to the extreme stresses the RV has been under, it's like the reactors will be decommissioned and dismantled. Replacement...? Perhaps with a new design or a new site, but that's beyond my ken on what they would plan.

My guess is that in comparison to Chernobyl, there will be far less radioactive material leaving the site as it appears to be better contained. In comparison to TMI, this will be several times worse since the various containment buildings and fuel buildings have been breached.

I think the severity will depend on whether the RV is breached or not. TMI released fission products into the environment. If they are able to maintain RV containment, they may be able to prevent FP release. There's too little information to determine that without more real data (that the news doesn't know how to provide, I think). As cooling continues and if they can keep the cores covered, the decay heat generation will decrease over time to a manageable level where restoration of more normal cooling means can occur and assessment of the damage can be evaluated.

It's important to remember than it was a natural disaster of a large scale and multiplicative effect - 9.0 earthquake followed by a tsunami. TMI was more or less operator error/component failure. At Chernobyl, it's the positive temperature coefficient design of the reactor (along with cooling test parameters that put the plant in a non-normal lineup) that caused an uncontrolled power increase.

Sorry to jump in on the discussion, but as a submariner, I find this a very interesting topic to follow. Thanks for letting me participate.
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